# 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: 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();

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.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)

else

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

gl.glLineWidth(3);
}

private boolean isWoken()
{
try
{
}

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 deathkrushYes, 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

Quote:
 Original post by ma_htyThe 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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By the way, the video is lovely. ^^

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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?)

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 hoonsworldOK, 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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