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OpenGL Fundamental problem with smooth normals

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Hello from Germany :) I created 2 triangles with different vertex order (counter clockwise and clockwise). This orders are user data and may not be changed. For the flat shading the view is ok. But for the smooth shading I get partial or full black triangles for the clockwise orders :( The problem is that I have only 1 averaged normal for a vertex. Is there a openGl statement to correct this mistake? Are the normals to be set principle after the "right hand rule"? Thanks in advance Greetings hoon Here's is a pic: black triangle And here's is the code with jogl (executable):
import javax.swing.*;
import javax.media.opengl.GLEventListener;
import javax.media.opengl.GLCanvas;
import javax.media.opengl.GLAutoDrawable;
import javax.media.opengl.GL;
import javax.media.opengl.glu.GLU;
import java.nio.FloatBuffer;

public class NormalProblem
        extends JFrame
        implements GLEventListener
{
    private static final boolean SMOOTH = true;
    private int angle = 0;

    public static void main(final String[] args)
    {
        new NormalProblem();
    }

    public NormalProblem()
    {
        final GLCanvas canvas = new GLCanvas();
        canvas.addGLEventListener(this);
        add(canvas);

        setSize(500, 500);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        while (isWoken())
            canvas.display();
    }

    public void display(final GLAutoDrawable drawable)
    {
        final GL gl = drawable.getGL();

        gl.glClear(GL.GL_COLOR_BUFFER_BIT | GL.GL_DEPTH_BUFFER_BIT);
        gl.glLoadIdentity();

        gl.glTranslatef(0, 0, -6);
        gl.glRotated(angle, 1, 1, 1);
        gl.glRotated(angle++, 0, 1, 0);

        /*           3
         *          /          *         / 1 \    triangle 1 [1, 2, 3] (counter clockwise vertex order)
         *       1/_____\ 2
         *        \     /
         *  y      \ 2 /    triangle 2 [1, 2, 4] (clockwise vertex order)
         *  ^       \ /
         *  |        4
         *  '---> x
         */

        if (SMOOTH)
            gl.glShadeModel(GL.GL_SMOOTH);

        else
            gl.glShadeModel(GL.GL_FLAT);

        drawTriangles(gl);
        drawNormals(gl);
    }

    private void drawTriangles(final GL gl)
    {
        gl.glBegin(GL.GL_TRIANGLES);

        if (SMOOTH)
        {
            // triangle 1 [1, 2, 3]
            gl.glNormal3f(0, 0, 1);
            gl.glVertex3f(-2, 0, 0);

            gl.glNormal3f(0, 0, 1);
            gl.glVertex3f(2, 0, 0);

            gl.glNormal3f(0, 0, 1);
            gl.glVertex3f(0, 2, 0);

            // triangle 2 [1, 2, 4]
            gl.glNormal3f(0, 0, 1); // (0, 0, -1) is correct but I have only "one" vertex normal
            gl.glVertex3f(-2, 0, 0);

            gl.glNormal3f(0, 0, 1); // (0, 0, -1) is correct but I have only "one" vertex normal
            gl.glVertex3f(2, 0, 0);

            gl.glNormal3f(0, 0, -1);
            gl.glVertex3f(0, -2, 0);
        }

        else
        {
            // triangle 1 [1, 2, 3]
            gl.glNormal3f(0, 0, 1);
            gl.glVertex3f(-2, 0, 0);
            gl.glVertex3f(2, 0, 0);
            gl.glVertex3f(0, 2, 0);

            // triangle 2 [1, 2, 4]
            gl.glNormal3f(0, 0, -1);
            gl.glVertex3f(-2, 0, 0);
            gl.glVertex3f(2, 0, 0);
            gl.glVertex3f(0, -2, 0);
        }

        gl.glEnd();
    }

    private void drawNormals(final GL gl)
    {
        gl.glDisable(GL.GL_LIGHTING);
        gl.glBegin(GL.GL_LINES);

        if (SMOOTH)
        {
            gl.glColor3f(1, 1, 0);
            // vertex normals of triangle 1 [1, 2, 3]
            // common normal on vertex 1 for triangle 1 and 2
            gl.glVertex3f(-2, 0, 0);
            gl.glVertex3f(-2, 0, 1);
            // common normal on vertex 2 for triangle 1 and 2
            gl.glVertex3f(2, 0, 0);
            gl.glVertex3f(2, 0, 1);
            gl.glColor3f(0, 1, 0);
            gl.glVertex3f(0, 2, 0);
            gl.glVertex3f(0, 2, 1);

            gl.glColor3f(1, 0, 0);
            // vertex normals of triangle 2 [1, 2, 4]
            /* normals for vertex 1 and 2 already exist
            gl.glVertex3f(-2, 0, 0);
            gl.glVertex3f(-2, 0, -1);
            gl.glVertex3f(2, 0, 0);
            gl.glVertex3f(2, 0, -1);
            */
            gl.glVertex3f(0, -2, 0);
            gl.glVertex3f(0, -2, -1);
        }

        else
        {
            gl.glColor3f(0, 1, 0);
            // normal of triangle 1 [1, 2, 3]
            gl.glVertex3f(0, 1, 0);
            gl.glVertex3f(0, 1, 1);

            gl.glColor3f(1, 0, 0);
            // normal of triangle 2 [1, 2, 4]
            gl.glVertex3f(0, -1, 0);
            gl.glVertex3f(0, -1, -1);
        }

        gl.glEnd();
        gl.glEnable(GL.GL_LIGHTING);
    }

    public void init(final GLAutoDrawable drawable)
    {
        final GL gl = drawable.getGL();

        gl.glEnable(GL.GL_DEPTH_TEST);

        gl.glLightModeli(GL.GL_LIGHT_MODEL_TWO_SIDE, GL.GL_TRUE);

        gl.glEnable(GL.GL_LIGHT0);
        gl.glEnable(GL.GL_LIGHTING);
        gl.glLightfv(GL.GL_LIGHT0, GL.GL_AMBIENT, FloatBuffer.wrap(new float[]{0, 0, 0, 1}));
        gl.glLightfv(GL.GL_LIGHT0, GL.GL_DIFFUSE, FloatBuffer.wrap(new float[]{1, 1, 1, 1}));
        gl.glLightfv(GL.GL_LIGHT0, GL.GL_SPECULAR, FloatBuffer.wrap(new float[]{1, 1, 1, 1}));
        gl.glLightfv(GL.GL_LIGHT0, GL.GL_POSITION, FloatBuffer.wrap(new float[]{0, 0, 6}));

        gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_AMBIENT, FloatBuffer.wrap(new float[]{0.2f, 0.1f, 0, 1}));
        gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_DIFFUSE, FloatBuffer.wrap(new float[]{0.6f, 0.2f, 0.1f, 1}));
        gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_SPECULAR, FloatBuffer.wrap(new float[]{0.6f, 0.2f, 0.1f, 1}));
        gl.glMaterialfv(GL.GL_FRONT_AND_BACK, GL.GL_EMISSION, FloatBuffer.wrap(new float[]{0, 0, 0, 1}));
        gl.glMaterialf(GL.GL_FRONT_AND_BACK, GL.GL_SHININESS, 50);

        gl.glMatrixMode(GL.GL_PROJECTION);
        gl.glLoadIdentity();
        new GLU().gluPerspective(50, drawable.getWidth() / drawable.getHeight(), 1, 1000);
        gl.glMatrixMode(GL.GL_MODELVIEW);
        gl.glLoadIdentity();

        gl.glLineWidth(3);
    }

    private boolean isWoken()
    {
        try
        {
            Thread.sleep(10);
        }

        catch (InterruptedException e)
        {
            e.printStackTrace();
        }

        return true;
    }

    public void reshape(
            final GLAutoDrawable drawable,
            final int x,
            final int y,
            final int width,
            final int height)
    {
    }

    public void displayChanged(
            final GLAutoDrawable drawable,
            final boolean mode,
            final boolean device)
    {
    }
}






[Edited by - hoonsworld on January 28, 2008 3:01:33 PM]

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Yes, this is a fundamental problem in OpenGL. In order for the lighting to look good the polygon winding and normals have to be consistent. I haven't tested this, but could probably solve the problem like this:

Draw front facing polygons:
- Enable back face culling
- Draw the scene

Draw back facing polygons:
- Flip the normals
- Enable front face culling
- Draw the scene again

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Quote:

Is there a openGl statement to correct this mistake?


there is also:

glFrontFace(GLenum mode);

(for mode: GL_CCW, GL_CW i.e. counterclockwise and clockwise respectively)
to match the simple, hardcoded and constrained nature of your sample code.

Editorial comment:It is a fundamental issue the programmer (or modeller?) should/may account for with regard to 3d geometry in both Opengl and DirectX.

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Quote:
Original post by deathkrush
Yes, this is a fundamental problem in OpenGL.


I won't say this is a problem in OpenGL. Polygon winding and normals need not to be consistant. In fact, if the normals are correct, your 3D model can still look good even its polygons have a lousy winding.

For hoon: When we use averaged normals, we are expecting a smooth surface. If you are not, why are you averaging your normals in the first place? The polygons share the same vertices doesn't mean they need to share normals. And, if your artist give you a 3D model that share/not-share normals, just let it be.

For more details about how and when to average the normals, please refer to
http://www.xmission.com/~nate/smooth.html

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Hi

Thanks for your answers :)

Quote:
Original post by ma_hty
The polygons share the same vertices doesn't mean they need to share normals.


Yes, I tried it with a second inverse normal on the shared vertices and it works correct :)
I thought it exist an OpenGL statement for this problem :(

OK, now I write a net algorithm to recognize the opposite wound triangles over the adjacent edges and their vertex order. A wound triangle obtains a boolean flag for usage the inverse normal.

I think it is a good idea :)
Or are there other ideas?

Here is our mini project:
gui3d.org

Best regards from Hamburg
hoon

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The correct way is to obtain a 3D model with a consistant winding, instead of struggling about the winding problem. In general, I can easily construct a 3D model with a winding so ambiguous that impossible to be fixed. Therefore, no matter how much effort you spend on your routine, you cannot handle all cases.

By the way, any 3D model designing software or 3D model scanner will, try their best to, produce a 3D model with consistant winding. Why bother?

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Arr......

In case your 3D model have its back-face visible, you can command OpenGL to flip the normal whenever the face is backward facing via

glLightModelf(GL_LIGHT_MODEL_TWO_SIDE, 1.0);

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Hi ma_hty :)

In the example I set up the statement:

gl.glLightModeli(GL.GL_LIGHT_MODEL_TWO_SIDE, GL.GL_TRUE);

It works correct but it is not the solution for the problem :(


Yes, you are right! Any 3D modeler will try to produce consistant windings. But our program/engine works with simulation data too :) This simulation data based on finite elements. The user has the full control about the winding for each element (polygon) or element groups. Here are 2 links for the background and a little simulation video:
en.wikipedia.org/wiki/Finite_element_analysis
ncac.gwu.edu

This is the reason for our effort.
The inconsistent winding is a feature in our engine but I have to visualize it correctly.

OK, Thanks again :)

Regards from Germany
hoon

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May I ask you what is the role of winding in your application? How is it get related to user's interaction? If you can give more information, may be I can help.

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Hi :)

In our program we have to manage the windings. The windings are important for the
most finite element solvers. These solvers are very sophisticated programs in
view of mathematics and physics.

For example: An airbag has internal pressure!
But in which direction does the pressure have an effect?
The user has to set up this information over the windings or the finite
element normals. The following video shows an airbag simulation:
airbag deployment

This is not the only reason for the role of winding. There are contact problems, and many of much other ...
In our program the user can select one or several polygons or groups and can invert the winding direction.
The selecting is possible over picking or an intelligent fence mechanism with box or polygon selecting.

For flat shading it is not a problem but the smooth shading is difficult :(

I think, I write a net algorithm to recognize the opposite wound triangles or quads!?

Regards
hoon

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If you are talking about pressure simulation, I think only specifying normals is enough. You have no reason to control winding in this case.

I don't quite sure what you mean by "contact problems". However, if you are talking about the relations between faces, it is definately a bad idea for you to allow users to control winding.

For the video you shown above, I don't see any necessity for abitrary winding. You can do exactly the same with consistant winding and varying normals. And very likely, it will also simplify your program and your calculation by a huge extend.

The vertices normals usually come from the average of faces normals. To get the vertices normals of each frame, you can keep the contributing faces of each vertex and average the face normals after each deformation.

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The simulation programs calculates the normal direction from winding (vertex order). It happens over the "Right hand rule" and the cross

For the airbag we have consistance normals. It was a bad example :( But for other simulations the normals (vertex orders) can vary. The requests of the simulation programs are very hard.

Here is a worst case:
I have 2 adjacent triangles with the following polygon normals:
triangle 1: [0, 0, 1]
triangle 2: [0, 0, -1]
The averaged vertex normal is [0, 0, 0] :(

Only the information about the polygon winding can help. The winding information is important for the vertex normal calculation and for rendering.
For rendering the opposite wound triangles use the inverted vertex normal for smooth shading. This is my idea for the net algorithm ...

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Quote:

In our program the user can select one or several polygons or groups and can invert the winding direction.
The selecting is possible over picking or an intelligent fence mechanism with box or polygon selecting.


Looks like the definition of the problem is coming along
(it often takes a long time to simply describe an actual problem
on a forum)

It still looks a little hazy:
So...lets see if my (sort of) paraphrasing of the problem is close:

In the program the user can select one or several polygons or groups and can invert the winding direction. For physics/physical simulation purposes only?
[i.e. to actually generate different normals(based on winding or even user defined)] to experiment
with say... physical simulations/'whatif' scenarios?.

But you want the 'real life' visual appearance of the structure/object in question to remain... err.. realistic (as it would appear in real life?)
when smooth shaded?

If so:

I would 'conceptually' devise 2 systems:
The physical simulation system and the visual/rendering system.
Maintain separate normals for each system. Or is that too simple?

Another (less appealing) option (with the same aim, but different means) would be to store a flag(with face or vertex.. that's up to you) indicating wether the orginal normal(winding?)has been flipped by the user or not. When it comes time to do physical simulation processing, check the flag to see if is flipped: and use the flipped normal(I can only presume this is the reason it is flipped). When it comes time to do the visual rendering use the orginal normal data( I presume you aim is to maintain realistic visuals). Or visa versa.

Is there another way?

Let me be a bit more presumptuous:

I bet you knew all this, but you just wanted to see if there are other better ways to go about it?

Or your system does not allow this?

[Edited by - steven katic on January 31, 2008 5:26:48 PM]

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Arr......

I think you are abusing the winding information.

For a smooth surface, the triangles of it should have consistnat winding. You shouldn't allow user to change it in the first place. That's why you have your problem. The correct way to fix your problem is to fix the fault design of the user interface. Otherwise, you will have strange results for complicated 3D model regardless of how much effort you spend on your correction routine, as some vital information of the geometry had lost.

And, the vertices normals is for display only. In general, they are not very useful for simulation. Just, there is nothing to stop you to run your simulation with face normal and display your 3D model with vertex normal.

And, to average face normal for vertex normals, you can refer to
http://www.xmission.com/~nate/smooth.html
for more information. The way to avoid the problems from averaging has been discussed there. The basic idea is simple, i.e. don't do the averaging if it is not appropriate.

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Quote:

I think you are abusing the winding information.


who me or hoonsworld?

why does/would the user change the winding?
(I am just assuming the user would want to be able to do something it:
I can't imagine what, but a non existent physics/physical simulation system is as good as any other place to use it, or at the very least it seems like a system requirement?)

Look further back in the replies and reread the reasons hoonsworld gives for allowing the user to change the winding.



You there hoonsworld?
(EDIT: hoonsworld must have what he needs now; another happy customer
RE: abuse of winding info; it's ok isn't, so long as it is not illegal abuse :))


[Edited by - steven katic on February 1, 2008 4:31:57 PM]

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Hello steven and ma_hty

yesterday, I was on party and I drank a lot of yummy beer :)


OK, for example the winding control (node/vertex order) is useful for difficult contact definitions in a crash simulation. Some contact definitions are dependent on the normal directions. The mesh generators does not know in which direction the user needs the normals :(
See the following crash video:
Pedestiran impacting

Quote:
But you want the 'real life' visual appearance of the structure/object in question to remain... err.. realistic (as it would appear in real life?)
when smooth shaded?

If so:

I would 'conceptually' devise 2 systems:
The physical simulation system and the visual/rendering system.
Maintain separate normals for each system. Or is that too simple?

Another (less appealing) option (with the same aim, but different means) would be to store a flag(with face or vertex.. that's up to you) indicating wether the orginal normal(winding?)has been flipped by the user or not. When it comes time to do physical simulation processing, check the flag to see if is flipped: and use the flipped normal(I can only presume this is the reason it is flipped). When it comes time to do the visual rendering use the orginal normal data( I presume you aim is to maintain realistic visuals). Or visa versa.

Yes, this is correct :)


Here is a example with the commercial program Patran (pics). This program managed the
pre- and post-processing for the nastran code.

The first pic shows the opposite wound triangles 7, 8 and 9 and the normal wound
triangles 16, 17, 18, 25, 26 and 27. For example the triangle 9 has the vertex
order [9, 19, 20, 10] and the triangle 18 has the order [19, 20, 30, 29]. They are opposed :)

The second pic shows the polygon normals and the smooth shading. There are also two systems!




Quote:
http://www.xmission.com/~nate/smooth.html

Thanks for the link ma_hty :)

That is not the winding solution, but very helpful for the next step and the quality of appearance :)


OK, thanks for all - now I start the implementation with the flipped flag and the different normal treatment
(physical/visual)

Best regards from Hamburg :)
hoon

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Quote:
Original post by hoonsworld
OK, for example the winding control (node/vertex order) is useful for difficult contact definitions in a crash simulation. Some contact definitions are dependent on the normal directions. The mesh generators does not know in which direction the user needs the normals :(


Oh... people refer your "contact problem"/"contact definitions" as collision. And, I think I have repeated myself too many times. Your application doesn't require an abitrary defined winding. Everything you claimed that are going to require winding information, are indeed, require normal instead.

The problems you have are probably came from your problematic user interface implementation. If you wanted to solve your problem, that's the way to go.

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Yes this is the way! We design a very flexible engine for multi-disciplinary fields of application in general for simulation, animation and cyber space :)

Yes in the world of simulation the people say "contact problem" for collision :)
The simulation engineers calculates with spring forces when a vertex (node) permeates a triangle or quad (finite element). Keep in your mind: "A simulation is an abstract physical calculation to portray the reality". A lot of products in our life are designed on computers. Here are two example:
Toy
Helmet

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Quote:
Original post by hoonsworld
...
The simulation engineers calculates with spring forces when a vertex (node) permeates a triangle or quad (finite element).
...


It don't require an arbitrary defined winding neither. What's your point?

PS : Please tell me your need of arbitrary defined winding is not a lousy way of recording normal information.

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Quote:

It don't require an arbitrary defined winding neither. What's your point?

PS : Please tell me your need of arbitrary defined winding is not a lousy way of recording normal information.


ewwh.. the level of decorum seems to have slipped a notch or two?

Hi ma_hty,

The important point you should try and see here is that the visual normal is treated the way you expect it to be treated visually. The other 'normal' or 'flipped normal' or 'user or arbitrarily defined winding' that we have made mention of is a completely different animal. These terms refer to data related to the physics/physical simulation system used to simulate (among other things) the behaviour of the particular type of material (that the 3D object may be made of) during contact/collision.

hoonsworld is the expert here though, not me. Sadly I suspect your tone may have put him off from responding to your questions (understandably).

Hopefully you can grasp more from the link(s) hoons provided here:

en.wikipedia.org/wiki/Finite_element_analysis

It's mandatory reading to understand what hoons is doing:
It's all there (and more) in a much more verbose form (than my distilled summary), so you may need to synthesize the data into useful info pertaining to you specific question(s) and concerns if you are really interested.
Or if you ask nicely, hoons may be interested in discussing FEA examples with you, it is ,after all, what he does (as far as this post is concerned).

I hope that helps you.

PS. I remember you now ma_hty: The Intraobject Transparency post and the dreaded old "cull-mode sorting trick" hack. I hope you found the info you needed on that.

Cheers

[Edited by - steven katic on February 6, 2008 1:02:16 AM]

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Steven Katic,

Be frank, I will not believe in something just because the name is big (let alone some strangers claim himself an expert). I just believe in evidence. That's why I wrote you a demo program in the thread "The Intraobject Transparency".

Anyway, as you don't like my post, I will stop posting further reply to this thread simply. Then, you are happy and I save my valuable time. Everyone win, isn't excellent?

Yours sincerely,
Gary

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Quote:

Be frank, I will not believe in something just because the name is big (let alone some strangers claim himself an expert). I just believe in evidence.


As do we all?

The evidence is in the authoritative information you find and research, not in anything anyone tries to convince you of, that's why references to relevant/background information (that is hopefully legitimate and valid) is provided often (as hoons has done with FEA and as I think I did with the "cull-mode sorting trick").


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      4. Are resources destroyed with commands?
      Likewise with initializing and assuming #3 is correct, removing an item from the scene would mean removing it from the job queue, no? Would I need to send a onetime command to the renderer to cleanup?
    • By Finalspace
      I am starting to get into linux X11/GLX programming, but from every C example i found - there is this XVisualInfo thing parameter passed to XCreateWindow always.
      Can i control this parameter later on - when the window is already created? What i want it to change my own non GLX window to be a GLX window - without recreating. Is that possible?
       
      On win32 this works just fine to create a rendering context later on, i simply find and setup the pixel format from a pixel format descriptor and create the context and are ready to go.
       
      I am asking, because if that doesent work - i need to change a few things to support both worlds (Create a context from a existing window, create a context for a new window).
    • By DiligentDev
      This article uses material originally posted on Diligent Graphics web site.
      Introduction
      Graphics APIs have come a long way from small set of basic commands allowing limited control of configurable stages of early 3D accelerators to very low-level programming interfaces exposing almost every aspect of the underlying graphics hardware. Next-generation APIs, Direct3D12 by Microsoft and Vulkan by Khronos are relatively new and have only started getting widespread adoption and support from hardware vendors, while Direct3D11 and OpenGL are still considered industry standard. New APIs can provide substantial performance and functional improvements, but may not be supported by older hardware. An application targeting wide range of platforms needs to support Direct3D11 and OpenGL. New APIs will not give any advantage when used with old paradigms. It is totally possible to add Direct3D12 support to an existing renderer by implementing Direct3D11 interface through Direct3D12, but this will give zero benefits. Instead, new approaches and rendering architectures that leverage flexibility provided by the next-generation APIs are expected to be developed.
      There are at least four APIs (Direct3D11, Direct3D12, OpenGL/GLES, Vulkan, plus Apple's Metal for iOS and osX platforms) that a cross-platform 3D application may need to support. Writing separate code paths for all APIs is clearly not an option for any real-world application and the need for a cross-platform graphics abstraction layer is evident. The following is the list of requirements that I believe such layer needs to satisfy:
      Lightweight abstractions: the API should be as close to the underlying native APIs as possible to allow an application leverage all available low-level functionality. In many cases this requirement is difficult to achieve because specific features exposed by different APIs may vary considerably. Low performance overhead: the abstraction layer needs to be efficient from performance point of view. If it introduces considerable amount of overhead, there is no point in using it. Convenience: the API needs to be convenient to use. It needs to assist developers in achieving their goals not limiting their control of the graphics hardware. Multithreading: ability to efficiently parallelize work is in the core of Direct3D12 and Vulkan and one of the main selling points of the new APIs. Support for multithreading in a cross-platform layer is a must. Extensibility: no matter how well the API is designed, it still introduces some level of abstraction. In some cases the most efficient way to implement certain functionality is to directly use native API. The abstraction layer needs to provide seamless interoperability with the underlying native APIs to provide a way for the app to add features that may be missing. Diligent Engine is designed to solve these problems. Its main goal is to take advantages of the next-generation APIs such as Direct3D12 and Vulkan, but at the same time provide support for older platforms via Direct3D11, OpenGL and OpenGLES. Diligent Engine exposes common C++ front-end for all supported platforms and provides interoperability with underlying native APIs. It also supports integration with Unity and is designed to be used as graphics subsystem in a standalone game engine, Unity native plugin or any other 3D application. Full source code is available for download at GitHub and is free to use.
      Overview
      Diligent Engine API takes some features from Direct3D11 and Direct3D12 as well as introduces new concepts to hide certain platform-specific details and make the system easy to use. It contains the following main components:
      Render device (IRenderDevice  interface) is responsible for creating all other objects (textures, buffers, shaders, pipeline states, etc.).
      Device context (IDeviceContext interface) is the main interface for recording rendering commands. Similar to Direct3D11, there are immediate context and deferred contexts (which in Direct3D11 implementation map directly to the corresponding context types). Immediate context combines command queue and command list recording functionality. It records commands and submits the command list for execution when it contains sufficient number of commands. Deferred contexts are designed to only record command lists that can be submitted for execution through the immediate context.
      An alternative way to design the API would be to expose command queue and command lists directly. This approach however does not map well to Direct3D11 and OpenGL. Besides, some functionality (such as dynamic descriptor allocation) can be much more efficiently implemented when it is known that a command list is recorded by a certain deferred context from some thread.
      The approach taken in the engine does not limit scalability as the application is expected to create one deferred context per thread, and internally every deferred context records a command list in lock-free fashion. At the same time this approach maps well to older APIs.
      In current implementation, only one immediate context that uses default graphics command queue is created. To support multiple GPUs or multiple command queue types (compute, copy, etc.), it is natural to have one immediate contexts per queue. Cross-context synchronization utilities will be necessary.
      Swap Chain (ISwapChain interface). Swap chain interface represents a chain of back buffers and is responsible for showing the final rendered image on the screen.
      Render device, device contexts and swap chain are created during the engine initialization.
      Resources (ITexture and IBuffer interfaces). There are two types of resources - textures and buffers. There are many different texture types (2D textures, 3D textures, texture array, cubmepas, etc.) that can all be represented by ITexture interface.
      Resources Views (ITextureView and IBufferView interfaces). While textures and buffers are mere data containers, texture views and buffer views describe how the data should be interpreted. For instance, a 2D texture can be used as a render target for rendering commands or as a shader resource.
      Pipeline State (IPipelineState interface). GPU pipeline contains many configurable stages (depth-stencil, rasterizer and blend states, different shader stage, etc.). Direct3D11 uses coarse-grain objects to set all stage parameters at once (for instance, a rasterizer object encompasses all rasterizer attributes), while OpenGL contains myriad functions to fine-grain control every individual attribute of every stage. Both methods do not map very well to modern graphics hardware that combines all states into one monolithic state under the hood. Direct3D12 directly exposes pipeline state object in the API, and Diligent Engine uses the same approach.
      Shader Resource Binding (IShaderResourceBinding interface). Shaders are programs that run on the GPU. Shaders may access various resources (textures and buffers), and setting correspondence between shader variables and actual resources is called resource binding. Resource binding implementation varies considerably between different API. Diligent Engine introduces a new object called shader resource binding that encompasses all resources needed by all shaders in a certain pipeline state.
      API Basics
      Creating Resources
      Device resources are created by the render device. The two main resource types are buffers, which represent linear memory, and textures, which use memory layouts optimized for fast filtering. Graphics APIs usually have a native object that represents linear buffer. Diligent Engine uses IBuffer interface as an abstraction for a native buffer. To create a buffer, one needs to populate BufferDesc structure and call IRenderDevice::CreateBuffer() method as in the following example:
      BufferDesc BuffDesc; BufferDesc.Name = "Uniform buffer"; BuffDesc.BindFlags = BIND_UNIFORM_BUFFER; BuffDesc.Usage = USAGE_DYNAMIC; BuffDesc.uiSizeInBytes = sizeof(ShaderConstants); BuffDesc.CPUAccessFlags = CPU_ACCESS_WRITE; m_pDevice->CreateBuffer( BuffDesc, BufferData(), &m_pConstantBuffer ); While there is usually just one buffer object, different APIs use very different approaches to represent textures. For instance, in Direct3D11, there are ID3D11Texture1D, ID3D11Texture2D, and ID3D11Texture3D objects. In OpenGL, there is individual object for every texture dimension (1D, 2D, 3D, Cube), which may be a texture array, which may also be multisampled (i.e. GL_TEXTURE_2D_MULTISAMPLE_ARRAY). As a result there are nine different GL texture types that Diligent Engine may create under the hood. In Direct3D12, there is only one resource interface. Diligent Engine hides all these details in ITexture interface. There is only one  IRenderDevice::CreateTexture() method that is capable of creating all texture types. Dimension, format, array size and all other parameters are specified by the members of the TextureDesc structure:
      TextureDesc TexDesc; TexDesc.Name = "My texture 2D"; TexDesc.Type = TEXTURE_TYPE_2D; TexDesc.Width = 1024; TexDesc.Height = 1024; TexDesc.Format = TEX_FORMAT_RGBA8_UNORM; TexDesc.Usage = USAGE_DEFAULT; TexDesc.BindFlags = BIND_SHADER_RESOURCE | BIND_RENDER_TARGET | BIND_UNORDERED_ACCESS; TexDesc.Name = "Sample 2D Texture"; m_pRenderDevice->CreateTexture( TexDesc, TextureData(), &m_pTestTex ); If native API supports multithreaded resource creation, textures and buffers can be created by multiple threads simultaneously.
      Interoperability with native API provides access to the native buffer/texture objects and also allows creating Diligent Engine objects from native handles. It allows applications seamlessly integrate native API-specific code with Diligent Engine.
      Next-generation APIs allow fine level-control over how resources are allocated. Diligent Engine does not currently expose this functionality, but it can be added by implementing IResourceAllocator interface that encapsulates specifics of resource allocation and providing this interface to CreateBuffer() or CreateTexture() methods. If null is provided, default allocator should be used.
      Initializing the Pipeline State
      As it was mentioned earlier, Diligent Engine follows next-gen APIs to configure the graphics/compute pipeline. One big Pipelines State Object (PSO) encompasses all required states (all shader stages, input layout description, depth stencil, rasterizer and blend state descriptions etc.). This approach maps directly to Direct3D12/Vulkan, but is also beneficial for older APIs as it eliminates pipeline misconfiguration errors. With many individual calls tweaking various GPU pipeline settings it is very easy to forget to set one of the states or assume the stage is already properly configured when in fact it is not. Using pipeline state object helps avoid these problems as all stages are configured at once.
      Creating Shaders
      While in earlier APIs shaders were bound separately, in the next-generation APIs as well as in Diligent Engine shaders are part of the pipeline state object. The biggest challenge when authoring shaders is that Direct3D and OpenGL/Vulkan use different shader languages (while Apple uses yet another language in their Metal API). Maintaining two versions of every shader is not an option for real applications and Diligent Engine implements shader source code converter that allows shaders authored in HLSL to be translated to GLSL. To create a shader, one needs to populate ShaderCreationAttribs structure. SourceLanguage member of this structure tells the system which language the shader is authored in:
      SHADER_SOURCE_LANGUAGE_DEFAULT - The shader source language matches the underlying graphics API: HLSL for Direct3D11/Direct3D12 mode, and GLSL for OpenGL and OpenGLES modes. SHADER_SOURCE_LANGUAGE_HLSL - The shader source is in HLSL. For OpenGL and OpenGLES modes, the source code will be converted to GLSL. SHADER_SOURCE_LANGUAGE_GLSL - The shader source is in GLSL. There is currently no GLSL to HLSL converter, so this value should only be used for OpenGL and OpenGLES modes. There are two ways to provide the shader source code. The first way is to use Source member. The second way is to provide a file path in FilePath member. Since the engine is entirely decoupled from the platform and the host file system is platform-dependent, the structure exposes pShaderSourceStreamFactory member that is intended to provide the engine access to the file system. If FilePath is provided, shader source factory must also be provided. If the shader source contains any #include directives, the source stream factory will also be used to load these files. The engine provides default implementation for every supported platform that should be sufficient in most cases. Custom implementation can be provided when needed.
      When sampling a texture in a shader, the texture sampler was traditionally specified as separate object that was bound to the pipeline at run time or set as part of the texture object itself. However, in most cases it is known beforehand what kind of sampler will be used in the shader. Next-generation APIs expose new type of sampler called static sampler that can be initialized directly in the pipeline state. Diligent Engine exposes this functionality: when creating a shader, textures can be assigned static samplers. If static sampler is assigned, it will always be used instead of the one initialized in the texture shader resource view. To initialize static samplers, prepare an array of StaticSamplerDesc structures and initialize StaticSamplers and NumStaticSamplers members. Static samplers are more efficient and it is highly recommended to use them whenever possible. On older APIs, static samplers are emulated via generic sampler objects.
      The following is an example of shader initialization:
      ShaderCreationAttribs Attrs; Attrs.Desc.Name = "MyPixelShader"; Attrs.FilePath = "MyShaderFile.fx"; Attrs.SearchDirectories = "shaders;shaders\\inc;"; Attrs.EntryPoint = "MyPixelShader"; Attrs.Desc.ShaderType = SHADER_TYPE_PIXEL; Attrs.SourceLanguage = SHADER_SOURCE_LANGUAGE_HLSL; BasicShaderSourceStreamFactory BasicSSSFactory(Attrs.SearchDirectories); Attrs.pShaderSourceStreamFactory = &BasicSSSFactory; ShaderVariableDesc ShaderVars[] = {     {"g_StaticTexture", SHADER_VARIABLE_TYPE_STATIC},     {"g_MutableTexture", SHADER_VARIABLE_TYPE_MUTABLE},     {"g_DynamicTexture", SHADER_VARIABLE_TYPE_DYNAMIC} }; Attrs.Desc.VariableDesc = ShaderVars; Attrs.Desc.NumVariables = _countof(ShaderVars); Attrs.Desc.DefaultVariableType = SHADER_VARIABLE_TYPE_STATIC; StaticSamplerDesc StaticSampler; StaticSampler.Desc.MinFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MagFilter = FILTER_TYPE_LINEAR; StaticSampler.Desc.MipFilter = FILTER_TYPE_LINEAR; StaticSampler.TextureName = "g_MutableTexture"; Attrs.Desc.NumStaticSamplers = 1; Attrs.Desc.StaticSamplers = &StaticSampler; ShaderMacroHelper Macros; Macros.AddShaderMacro("USE_SHADOWS", 1); Macros.AddShaderMacro("NUM_SHADOW_SAMPLES", 4); Macros.Finalize(); Attrs.Macros = Macros; RefCntAutoPtr<IShader> pShader; m_pDevice->CreateShader( Attrs, &pShader );
      Creating the Pipeline State Object
      After all required shaders are created, the rest of the fields of the PipelineStateDesc structure provide depth-stencil, rasterizer, and blend state descriptions, the number and format of render targets, input layout format, etc. For instance, rasterizer state can be described as follows:
      PipelineStateDesc PSODesc; RasterizerStateDesc &RasterizerDesc = PSODesc.GraphicsPipeline.RasterizerDesc; RasterizerDesc.FillMode = FILL_MODE_SOLID; RasterizerDesc.CullMode = CULL_MODE_NONE; RasterizerDesc.FrontCounterClockwise = True; RasterizerDesc.ScissorEnable = True; RasterizerDesc.AntialiasedLineEnable = False; Depth-stencil and blend states are defined in a similar fashion.
      Another important thing that pipeline state object encompasses is the input layout description that defines how inputs to the vertex shader, which is the very first shader stage, should be read from the memory. Input layout may define several vertex streams that contain values of different formats and sizes:
      // Define input layout InputLayoutDesc &Layout = PSODesc.GraphicsPipeline.InputLayout; LayoutElement TextLayoutElems[] = {     LayoutElement( 0, 0, 3, VT_FLOAT32, False ),     LayoutElement( 1, 0, 4, VT_UINT8, True ),     LayoutElement( 2, 0, 2, VT_FLOAT32, False ), }; Layout.LayoutElements = TextLayoutElems; Layout.NumElements = _countof( TextLayoutElems ); Finally, pipeline state defines primitive topology type. When all required members are initialized, a pipeline state object can be created by IRenderDevice::CreatePipelineState() method:
      // Define shader and primitive topology PSODesc.GraphicsPipeline.PrimitiveTopologyType = PRIMITIVE_TOPOLOGY_TYPE_TRIANGLE; PSODesc.GraphicsPipeline.pVS = pVertexShader; PSODesc.GraphicsPipeline.pPS = pPixelShader; PSODesc.Name = "My pipeline state"; m_pDev->CreatePipelineState(PSODesc, &m_pPSO); When PSO object is bound to the pipeline, the engine invokes all API-specific commands to set all states specified by the object. In case of Direct3D12 this maps directly to setting the D3D12 PSO object. In case of Direct3D11, this involves setting individual state objects (such as rasterizer and blend states), shaders, input layout etc. In case of OpenGL, this requires a number of fine-grain state tweaking calls. Diligent Engine keeps track of currently bound states and only calls functions to update these states that have actually changed.
      Binding Shader Resources
      Direct3D11 and OpenGL utilize fine-grain resource binding models, where an application binds individual buffers and textures to certain shader or program resource binding slots. Direct3D12 uses a very different approach, where resource descriptors are grouped into tables, and an application can bind all resources in the table at once by setting the table in the command list. Resource binding model in Diligent Engine is designed to leverage this new method. It introduces a new object called shader resource binding that encapsulates all resource bindings required for all shaders in a certain pipeline state. It also introduces the classification of shader variables based on the frequency of expected change that helps the engine group them into tables under the hood:
      Static variables (SHADER_VARIABLE_TYPE_STATIC) are variables that are expected to be set only once. They may not be changed once a resource is bound to the variable. Such variables are intended to hold global constants such as camera attributes or global light attributes constant buffers. Mutable variables (SHADER_VARIABLE_TYPE_MUTABLE) define resources that are expected to change on a per-material frequency. Examples may include diffuse textures, normal maps etc. Dynamic variables (SHADER_VARIABLE_TYPE_DYNAMIC) are expected to change frequently and randomly. Shader variable type must be specified during shader creation by populating an array of ShaderVariableDesc structures and initializing ShaderCreationAttribs::Desc::VariableDesc and ShaderCreationAttribs::Desc::NumVariables members (see example of shader creation above).
      Static variables cannot be changed once a resource is bound to the variable. They are bound directly to the shader object. For instance, a shadow map texture is not expected to change after it is created, so it can be bound directly to the shader:
      PixelShader->GetShaderVariable( "g_tex2DShadowMap" )->Set( pShadowMapSRV ); Mutable and dynamic variables are bound via a new Shader Resource Binding object (SRB) that is created by the pipeline state (IPipelineState::CreateShaderResourceBinding()):
      m_pPSO->CreateShaderResourceBinding(&m_pSRB); Note that an SRB is only compatible with the pipeline state it was created from. SRB object inherits all static bindings from shaders in the pipeline, but is not allowed to change them.
      Mutable resources can only be set once for every instance of a shader resource binding. Such resources are intended to define specific material properties. For instance, a diffuse texture for a specific material is not expected to change once the material is defined and can be set right after the SRB object has been created:
      m_pSRB->GetVariable(SHADER_TYPE_PIXEL, "tex2DDiffuse")->Set(pDiffuseTexSRV); In some cases it is necessary to bind a new resource to a variable every time a draw command is invoked. Such variables should be labeled as dynamic, which will allow setting them multiple times through the same SRB object:
      m_pSRB->GetVariable(SHADER_TYPE_VERTEX, "cbRandomAttribs")->Set(pRandomAttrsCB); Under the hood, the engine pre-allocates descriptor tables for static and mutable resources when an SRB objcet is created. Space for dynamic resources is dynamically allocated at run time. Static and mutable resources are thus more efficient and should be used whenever possible.
      As you can see, Diligent Engine does not expose low-level details of how resources are bound to shader variables. One reason for this is that these details are very different for various APIs. The other reason is that using low-level binding methods is extremely error-prone: it is very easy to forget to bind some resource, or bind incorrect resource such as bind a buffer to the variable that is in fact a texture, especially during shader development when everything changes fast. Diligent Engine instead relies on shader reflection system to automatically query the list of all shader variables. Grouping variables based on three types mentioned above allows the engine to create optimized layout and take heavy lifting of matching resources to API-specific resource location, register or descriptor in the table.
      This post gives more details about the resource binding model in Diligent Engine.
      Setting the Pipeline State and Committing Shader Resources
      Before any draw or compute command can be invoked, the pipeline state needs to be bound to the context:
      m_pContext->SetPipelineState(m_pPSO); Under the hood, the engine sets the internal PSO object in the command list or calls all the required native API functions to properly configure all pipeline stages.
      The next step is to bind all required shader resources to the GPU pipeline, which is accomplished by IDeviceContext::CommitShaderResources() method:
      m_pContext->CommitShaderResources(m_pSRB, COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES); The method takes a pointer to the shader resource binding object and makes all resources the object holds available for the shaders. In the case of D3D12, this only requires setting appropriate descriptor tables in the command list. For older APIs, this typically requires setting all resources individually.
      Next-generation APIs require the application to track the state of every resource and explicitly inform the system about all state transitions. For instance, if a texture was used as render target before, while the next draw command is going to use it as shader resource, a transition barrier needs to be executed. Diligent Engine does the heavy lifting of state tracking.  When CommitShaderResources() method is called with COMMIT_SHADER_RESOURCES_FLAG_TRANSITION_RESOURCES flag, the engine commits and transitions resources to correct states at the same time. Note that transitioning resources does introduce some overhead. The engine tracks state of every resource and it will not issue the barrier if the state is already correct. But checking resource state is an overhead that can sometimes be avoided. The engine provides IDeviceContext::TransitionShaderResources() method that only transitions resources:
      m_pContext->TransitionShaderResources(m_pPSO, m_pSRB); In some scenarios it is more efficient to transition resources once and then only commit them.
      Invoking Draw Command
      The final step is to set states that are not part of the PSO, such as render targets, vertex and index buffers. Diligent Engine uses Direct3D11-syle API that is translated to other native API calls under the hood:
      ITextureView *pRTVs[] = {m_pRTV}; m_pContext->SetRenderTargets(_countof( pRTVs ), pRTVs, m_pDSV); // Clear render target and depth buffer const float zero[4] = {0, 0, 0, 0}; m_pContext->ClearRenderTarget(nullptr, zero); m_pContext->ClearDepthStencil(nullptr, CLEAR_DEPTH_FLAG, 1.f); // Set vertex and index buffers IBuffer *buffer[] = {m_pVertexBuffer}; Uint32 offsets[] = {0}; Uint32 strides[] = {sizeof(MyVertex)}; m_pContext->SetVertexBuffers(0, 1, buffer, strides, offsets, SET_VERTEX_BUFFERS_FLAG_RESET); m_pContext->SetIndexBuffer(m_pIndexBuffer, 0); Different native APIs use various set of function to execute draw commands depending on command details (if the command is indexed, instanced or both, what offsets in the source buffers are used etc.). For instance, there are 5 draw commands in Direct3D11 and more than 9 commands in OpenGL with something like glDrawElementsInstancedBaseVertexBaseInstance not uncommon. Diligent Engine hides all details with single IDeviceContext::Draw() method that takes takes DrawAttribs structure as an argument. The structure members define all attributes required to perform the command (primitive topology, number of vertices or indices, if draw call is indexed or not, if draw call is instanced or not, if draw call is indirect or not, etc.). For example:
      DrawAttribs attrs; attrs.IsIndexed = true; attrs.IndexType = VT_UINT16; attrs.NumIndices = 36; attrs.Topology = PRIMITIVE_TOPOLOGY_TRIANGLE_LIST; pContext->Draw(attrs); For compute commands, there is IDeviceContext::DispatchCompute() method that takes DispatchComputeAttribs structure that defines compute grid dimension.
      Source Code
      Full engine source code is available on GitHub and is free to use. The repository contains tutorials, sample applications, asteroids performance benchmark and an example Unity project that uses Diligent Engine in native plugin.
      Atmospheric scattering sample demonstrates how Diligent Engine can be used to implement various rendering tasks: loading textures from files, using complex shaders, rendering to multiple render targets, using compute shaders and unordered access views, etc.

      Asteroids performance benchmark is based on this demo developed by Intel. It renders 50,000 unique textured asteroids and allows comparing performance of Direct3D11 and Direct3D12 implementations. Every asteroid is a combination of one of 1000 unique meshes and one of 10 unique textures.

      Finally, there is an example project that shows how Diligent Engine can be integrated with Unity.

      Future Work
      The engine is under active development. It currently supports Windows desktop, Universal Windows, Linux, Android, MacOS, and iOS platforms. Direct3D11, Direct3D12, OpenGL/GLES backends are now feature complete. Vulkan backend is coming next, and Metal backend is in the plan.
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