# OpenGL Problems with rotations

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Hi, i'm new here. I was looking for a good opengl forum, and i think this would be. I want to rotate the axes first 90 degree around the x axe, then 90 degree around y axe and then 90 degree around z axe. This is easy, and works fine in my program. But if I want to rotate 90 degree around the x axe, then 90 degree around z axe and then 90 degree around y axe, I have a problem. I use this: glRotatef(90,1,0,0); glRotatef(90,0,1,0); glRotatef(90,0,0,1); So the problem is the order in the matrix multiplication (always multiply the y axe matrix before z axe matrix). But I don't know how to do this only using glRotatef. (I have read about quaternions, euler angle, ... , but I want to do, if it is possible, only with glRotatef). Thanks

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Actually, if I'm not much mistaken, you've posted the code for rotations about the z, y and x-axes in that order, rather than the other way around. Since the matrices are post-multiplied, the last operation is essentially performed first on the vertices. That is, in your example, if, before your glRotate calls are performed, the transformation matrix is A, and we call your rotations B, C and D respectively, then the result after making the glRotate calls should be:

ABCD (A multiplied by B multiplied by C multiplied by D)

In matrix mathematics, this results in the operation D "taking effect" first when the matrix is applied to a vertex, followed by C, B and A, in that order.

(A note: this is the result of the matrix mathematics, as I recall, rather than OpenGL doing anything funny with the actual calls.)

PS: The English singular of "axes" is "axis", by the way - one axis, two axes. ^_^

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I know.

So you say that I have to change the order of glRotatef. I have tried and this is not the main problem.

I have a picture to show you the problem, but i don't know how to attach a picture here.

Thanks and thanks for the english correction.

[Edited by - zorro68 on May 7, 2008 8:18:48 AM]

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glRotatef(90,1,0,0);
glRotatef(90,0,1,0);
glRotatef(90,0,0,1);

This is doing what you wanted to do. It rotates X then Y then Z if you want X Z Y

This will do X,Z,Y
glRotatef(90,1,0,0);
glRotatef(90,0,0,1);
glRotatef(90,0,1,0);

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Quote:
 Original post by ThaumaturgeThat is, in your example, if, before your glRotate calls are performed, the transformation matrix is A, and we call your rotations B, C and D respectively, then the result after making the glRotate calls should be:ABCD (A multiplied by B multiplied by C multiplied by D)

No, that's not the order OpenGL uses. If A is the original matrix and you call a matrix function with a matrix B, the resulting matrix is BA, not AB. Your sequence will be DCBA.

Zorro68: I'm not quite sure what your problem is, but are trying to say that you wonder why the resulting rotation is different when you change the order? Or are you experiencing gimbal lock, which is what happens when you rotate so, that rotating around one axis no longer is possible?

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Quote:
Original post by SnotBob
Quote:
 Original post by ThaumaturgeThat is, in your example, if, before your glRotate calls are performed, the transformation matrix is A, and we call your rotations B, C and D respectively, then the result after making the glRotate calls should be:ABCD (A multiplied by B multiplied by C multiplied by D)

No, that's not the order OpenGL uses. If A is the original matrix and you call a matrix function with a matrix B, the resulting matrix is BA, not AB. Your sequence will be DCBA.

Err, well, Thaumaturge isn't wrong but right! OpenGL uses column vectors, and OpenGL does post-multiplication (i.e. on the right side) when glTranslate, glRotate, glScale, or glMultMatrix is invoked. Hence, in an abstract form, the sequence
glAnyTransformation(A);
glAnyTransformation(B);
glAnyTransformation(C);
glVertex(v);
means mathematically
v' := A * B * C * v
Although OpenGL computes that (due to the order matrices are supplied) as
v' := ( ( A * B ) * C ) * v
it is true that the parantheses have no mathematical effect. E.g. the form
v' := A * ( B * ( C * v ) )
will give the identical result! I've chosen this form because it shows best what's "logically" happens: It is C that is applied to v, and B that is applied to the already transformed vertex, and A that is applied to those already twice transformed vertex.

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You made me check the Spec! Serves me right for not refreshing my memory earlier.

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Thank you, Haegarr. ^_^

Zorro, in order to display an image, first upload it to an image host and then include the relevant code in html <img> tags.

If you use ImageShack, then I believe that the code given for an image under "sites" should work (without any additional tags).

Quote:
 Originally posted by MARS_999glRotatef(90,1,0,0);glRotatef(90,0,1,0);glRotatef(90,0,0,1);This is doing what you wanted to do. It rotates X then Y then Z if you want X Z YThis will do X,Z,YglRotatef(90,1,0,0);glRotatef(90,0,0,1);glRotatef(90,0,1,0);

I'm pretty sure that the order is the opposite way around - respectively, I believe that those sets of calls would produce rotations about z, x and then y, and y, z, and then x.

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Thanks to all for a quickly answer.

But I (due to my bad english) don't explain you what is my problem. First at all, i say that i know the order of matrix, and what happend if you change this order. I know the gimbal lock problem, so I'm going to try to explain better:

I have 3 slices, one to rotate in the x axis, other to rotate in the y axis, and other for z axis.
If I move the slices in this order, first x , y and z (the program execute glRotatef(90,1,0,0), glRotatef(90,0,1,0), glRotatef(90,0,0,1);) and all works right.
But if I move the slices in this order, first x, z and y (the program execute the same, so the rotation is bad, I have to change the code).

So if I move the slices in different order, I have to change the code, and I need a code with glRotatef that always work.

I don't know if this is possible or not (with glRotatef) or if I need to implement quaternions or Euler angles or ....

Thanks and I hope you understand my problem.

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What I don't understand is that, if your rotating an object about its own local coordinates, which is what I assume your trying to do, those 3 rotations in any order is going to produce a similar affect? The main difference is going to be which faces are facing forward.

For instance if you have a square rotated about x, then y, then z at 90 degrees a piece, your always going to get the same square back, but by switching the order of rotations a different face will be front facing.

From your picture it looks to me that the rotations are working as they should, if you want it to produce a different result then you need to change the rotations.

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The picture lets me assume you want to rotate around local axes, while your code lets me assume you want to rotate around global axes. Perhaps your problem is to distinuish these 2 cases?

For this you must be aware that a transformation happens ever in its global co-ordinate frame. From your picture it seems me that you want to rotate around local axes. If so, you cannot simply use global axes! Instead, you have to figure out how the desired local axis is currently oriented in global space, and use that as argument to the glRotate function.

E.g. the y axis of your co-ordinate cross is
y = [ 0 1 0 ]T
in global space before any transformation. After a rotation by 90° around (global) x axis
y' := Rgx(90°) * y = [ 0 0 1 ]T
the result will point along the global z axis! See the difference: The global y axis is, of course, still [ 0 1 0 ]T w.r.t. the global space, but the local y axis is now [ 0 0 1 ]T w.r.t. the global space.

Now, rotating by glRotatef(90,0,1,0) around the (global) y axis will result in
yg" := Rgy(90°) * y' = [ 1 0 0 ]T
in a vector pointing along global x axis, while rotating around the "local" y axis using glRotate(90,0,0,1) results in
yl" := Rly(90°) * y' = [ 0 0 1 ]T
(doesn't change anything, because y axis rotated around y axis ;) ). Perhaps I've used a wrong sign anywhere, but the principle should became clear.

Using Euler angles means nothing else to use a specific order of a sequence of 3 glRotate invocations, each one with a defined axis. Several Euler triples exist; in principle your XYZ rotation is one of them. Using a quaternion means nothing more than to use a single glRotate, but with a "arbitrary" axis, i.e. where [ x y z ]T will typically differ all from 0. Although you can compute the belonging matrices by yourself, IMHO it doesn't effectively change anything significant here. So go with glRotate, but clarify which axes you want to rotate around.

[Edited by - haegarr on May 8, 2008 9:26:03 AM]

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haegarr, thanks for your explanation. I understand all you say, but there's something that I don't understand. I explain you:

The program always run this sentences continously:

glRotatef(rotx,1,0,0);
glRotatef(roty,0,1,0);
glRotatef(rotz,0,0,1);

so, as you say, when rotx=90, roty=0 and rotz=0, I have the local y axis pointing out of the screen [0 0 1], along the positive global z axis. Is it ok? (I think so)

Then I change the value of roty=90 (rotx=90,roty=90,rotz=0), so when I compute the three sentences glRotatef, I have to obtain the local y axis looking at the direction of positive x axis [1 0 0]. Is it ok? (I think so)

But I don't get this. If you show my picture you can see that I get a rotation around the global z axis (out the screen) so the local y axis doesn't move, and get the local y axis [0 0 1].

What's happend, or what is what i don't understand?

Pds: I understand that glRotatef always rotate around global axis, as you say.

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I think you still don't consider all aspects. Let's see ...
Still using column vectors and right handed co-ordinate systems:

Quote:
 Original post by zorro68so, as you say, when rotx=90, roty=0 and rotz=0, I have the local y axis pointing out of the screen [0 0 1], along the positive global z axis. Is it ok? (I think so)

Yes, its okay, because the mathematical equivalent is
v' := Rx(90°) * Ry(0°) * Rz(0°) * [ 0 1 0 ]T
== Rx(90°) * I * I * [ 0 1 0 ]T
== Rx(90°) * [ 0 1 0 ]T
== [ 0 0 1 ]T

Quote:
 Original post by zorro68Then I change the value of roty=90 (rotx=90,roty=90,rotz=0), so when I compute the three sentences glRotatef, I have to obtain the local y axis looking at the direction of positive x axis [1 0 0]. Is it ok? (I think so)

Nope. Because the mathematical equivalent is
v' := Rx(90°) * Ry(90°) * Rz(0°) * [ 0 1 0 ]T
== Rx(90°) * Ry(90°) * I * [ 0 1 0 ]T
== Rx(90°) * Ry(90°) * [ 0 1 0 ]T
== Rx(90°) * [ 0 1 0 ]T
== [ 0 0 1 ]T

Keep in mind that (as already written in one of the previous answers) mathematically
A * B * C == ( A * B ) * C == A * ( B * C )
is all the same.

I suggest you: Don't think in code with stuff like this! Think in mathematical terms, and then translate that to code.

[Edited by - haegarr on May 8, 2008 11:14:14 AM]

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I see what you say (I see better with maths).

So if I think in math terms, the solution of my problem is to multiply the three matrix before apply to the scene. (ok?)

But how can I do this with glRotatef function only?

PD: I have to do this in my paintGL() functions that is always running.

Thanks very much.

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Quote:
 Original post by zorro68So if I think in math terms, the solution of my problem is to multiply the three matrix before apply to the scene. (ok?)

Err, not primarily; I'm not sure whether I understood you correctly here, so my following explanation may be superfluous, but what should I do ;) Make yourself clear what happens step-by-step. Assume you use the pseudecode sequence
glRotate(A);
glRotate(B);
glBegin(...);
glVertex3(v);

Now, glLoadIdentity() prepares OpenGL's matrix stack:
M := I

The 1st rotation glRotate(A) then causes
M := I * A
being on the stack.

The 2nd rotation glRotate(B) then causes
M := I * A * B
being on the stack.

Applying that to the vertex glVertex3(v) then means
v' := M * v = I * A * B * v

Now, the sequence of invocations has computed this actually as
( ( I * A ) * B ) * v
but, as already said several times, it is mathematically absolutely the same as
I * ( A * ( B * v ) )
or any other (syntactically correct) arrangement of parantheses!

In other words, using glRotate has definitely build up the matrix as a whole internally before the first vertex is transformed by it, but it plays no role for the result (only for the performance). You could use quaternions, multiply them together, convert the result to a matrix, and use glMultMatrix; whatever ... it'll change nothing.

Unfortunately, I still have no real clue what you are desired to reach. So I tell you again what's going wrong, but am not able to hint in the correct direction. I'm pretty sure that the results you want to reach are possible using glRotate (it must be since glRotate does nothing more than converting into a rotation matrix and multiply that). Could you perhaps illustrate in detail what should happen? And why are you forced to use that specific sequence of rotations?

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I have no clue what he the OP is trying to do, but I have tried the rotations you asked for and found the cube will end up back at the starting point after you move the axes in the order you wanted.

To check this yourself grab a dice, mark all 6 sides as front, back, left, right, bottom, top. Now keep the front facing you to start with, rotate X,Z,Y mind you you need to rotate in a CW direction when you do this. You will be back at the front facing you after all three have moved. Is this what you want?

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If you are trying to implement a rotation user interface, do not use Euler angle.

Using Euler angle is a bad idea.

By the way, this is going to help...
http://www.gamedev.net/community/forums/topic.asp?topic_id=463800

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u can use eular angles no problems, what u shouldnt do though is perform several glRotatef(..) commands in a row as this will often end in gimbal lock
see the matrix faq

http://www.j3d.org/matrix_faq/matrfaq_latest.html

Q33. How do I combine rotation matrices?
Q34. What is Gimbal Lock?

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Quote:
 Original post by zedzu can use eular angles no problems, what u shouldnt do though is perform several glRotatef(..) commands in a row as this will often end in gimbal lock

How are you using Euler angles w/o performing several glRotates in row (or, as an equivalent, compose the matrix by yourself)? Euler angles means ever a composition of consecutive rotations.

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Quote:
 Original post by haegarrIn other words, using glRotate has definitely build up the matrix as a whole internally before the first vertex is transformed by it, but it plays no role for the result (only for the performance). You could use quaternions, multiply them together, convert the result to a matrix, and use glMultMatrix; whatever ... it'll change nothing.

Yes you are right.

Quote:
 Original post by MARS_999To check this yourself grab a dice, mark all 6 sides as front, back, left, right, bottom, top. Now keep the front facing you to start with, rotate X,Z,Y mind you you need to rotate in a CW direction when you do this. You will be back at the front facing you after all three have moved. Is this what you want?

I'm going to explain you what I need?

Quote:
 Original post by ma_htyIf you are trying to implement a rotation user interface, do not use Euler angle.Using Euler angle is a bad idea.

I have read this. By the way, thanks for the gimbal lock discussion.

Quote:

It's a very good summarized in maths.

I thought the rotation problem was a closed theme (in maths and in opengl), but I can see that there's a lots of doubts.

I'm going to explain what I am doing and what is my problem. I'm programming a molecular viewer with opengl. I have seen other molecular viewer and when you click with the mouse into the scene and move the mouse you rotate all the scene (in my case, the molecule). But I have seen that all of them has a problem, if you rotate the scene with the mouse you can see the rotation of the scene and it seems that is all ok. But you are rotating the scene sometimes around global axis and sometime around local axes. This is the same effect that I am having when I use, three times, glRotatef function. Due to the matrix order multiplication, it seems that you are rotating the scene sometime around its local axes and sometime around its global axes. (Show what I'm trying to say in the picture)

What I am trying to program is that when I click and move the mouse (left click + left-right move around x axis, left click + top-down move around y axis, right click + left-right move around z axes, and right click + top-down zoom the scene) or move the slices the scene seems to rotate ALWAYS around the global axes, or seems to rotate ALWAYS around the local axes. But not a mix of them.

I hope you understand me. If not, show me and I'll try again.

Pd: My main problem is that I cannot explain in English like I would like.

Thanks to all of you.

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ATM I think your desire is not 100% possible to be realized (but perhaps someone else from the cummunity will contradict). However, I furthur think you've several more-or-less good options to work around:

(a) Make the user clear that s/he is varying an Euler angle like set-up with the 3 sliders. This solution doesn't touch the math, but is IMHO user unfriendly, since the user shouldn't be burdened with such stuff.

(b) Rotate the molecule by the currently dragged slider always as an incremental rotation around the local axis, but decompose the resulting orientation back into its 3 angles, and adapt the sliders accordingly. This allows you to still show the absolute angles (but only w.r.t. the equivalent rotations of a defined order). DCC packages often offer this way.

(c) Don't use absolute angles but relative ones. I.e. remake the GUI so that pulling a slider's knob changes its belonging angle incrementally around the local axis, but releasing the knob lets it snap back to the neutral position. The molecule, on the other hand, is left as is. So at most one slider knob is outside of 0, and that only temporarily. This simulates a kind of turn table but with sliders, so restricting the kinds of rotation axes (see below).

(d) Drop the usage of slides totally, and go with the tool approach (e.g. turn table like). I.e. manipulate the model directly inside the view, not indirectly and beneath it. Besides the tool idea, this is similar to (c), but also allows for more or less arbitrary rotation axes.

(e) One of (b) or (c) above but using global axes.

Other options may exist, too. I personally would prefer the tool approach. But that is the opinion of a programmer of a 3D DCC editor, not of a chemist :)

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Quote:
 Original post by zorro68...But I have seen that all of them has a problem, if you rotate the scene with the mouse you can see the rotation of the scene and it seems that is all ok. But you are rotating the scene sometimes around global axis and sometime around local axes....

This is a typical defect introduced by mapping Euler angle to the user interface directly. As long as you are using Euler angle honestly, there is no way you can solve this problem.

Why don't you give up Euler angle and use Axis-angle instead? If you are using Axis-angle correctly, there will not be such a problem. Well well, you are probably going to make a lengthy statements to defend your idea (that's what had happened repeatedly whenever there is discussion about Euler angle user interface).

I have been participated in those discussions too many times. Therefore, even if you are decided to defend your idea anyway, I am not going to join your discussion. Instead, I give you the source code of what a correctly designed Axis-angle user interface should look like (i.e. almost the ArcBall).
/////////////////////////// glutTest13.cpp//// Created by Gary Ho, ma_hty@hotmail.com, 2007//#include <stdio.h>#include <stdlib.h>#include <math.h>#include <GL/glut.h>#define PI 3.14159265358979323846ffloat v0[3], v1[3];float mo[16] = { 1,0,0,0, 0,1,0,0, 0,0,1,0, 0,0,0,1 };float clamp( float x, float a, float b );float dot( const float *a, const float *b );float norm( const float *a );void vassign( float *a, float x, float y, float z );void vassign( float *a, const float *b );void cross( float *a, const float *b, const float *c );void normalize( float *a );void display();void mousebutton(int button, int state, int x, int y );void mousemove(int x, int y);void main( int argc, char **argv ){  glutInitDisplayMode( GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH );  glutInitWindowSize( 512, 512 );  glutCreateWindow( "test09" );  glutDisplayFunc( display );  glutMouseFunc( mousebutton );  glutMotionFunc( mousemove );  glutMainLoop();}void display(){  GLint viewport[4];    glGetIntegerv( GL_VIEWPORT, viewport );  glEnable( GL_DEPTH_TEST );  glEnable( GL_LIGHTING );  glEnable( GL_LIGHT0 );  glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );  glMatrixMode(GL_PROJECTION);    glLoadIdentity();    gluPerspective( 45, double(viewport[2])/viewport[3], 0.1, 10 );    glMatrixMode(GL_MODELVIEW);    glLoadIdentity();    gluLookAt( 0,0,3, 0,0,0, 0,1,0 );    glMultMatrixf( mo );    glutSolidTeapot(1);  glutSwapBuffers();}void mousebutton(int button, int state, int x, int y ){  vassign( v0, 2.0*x/512-1, -2.0*y/512+1, 1 );  normalize(v0);}void mousemove(int x, int y){  float axis[3], angle;  vassign( v1, 2.0*x/512-1, -2.0*y/512+1, 1 );  normalize(v1);  if( v0[0]==v1[0] && v0[1]==v1[1] && v0[2]==v1[2] )    return;  cross(axis,v0,v1);  normalize(axis);  angle = acosf( clamp(dot(v0,v1),-1,1) );  vassign( v0, v1 );  glPushMatrix();    glLoadIdentity();    glRotatef( angle*180/PI, axis[0], axis[1], axis[2] );    glMultMatrixf( mo );    glGetFloatv( GL_MODELVIEW_MATRIX, mo );  glPopMatrix();  glutPostRedisplay();}float clamp( float x, float a, float b ){ return x<a ? a : (x<b?x:b); }float dot( const float *a, const float *b ){ return a[0]*b[0]+a[1]*b[1]+a[2]*b[2]; }float norm( const float *a ){ return sqrtf(dot(a,a)); }void vassign( float *a, float x, float y, float z ){ a[0]=x; a[1]=y; a[2]=z; }void vassign( float *a, const float *b ){ a[0]=b[0]; a[1]=b[1]; a[2]=b[2]; }void cross( float *a, const float *b, const float *c ){  a[0] = b[1]*c[2] - c[1]*b[2];  a[1] = -b[0]*c[2] + c[0]*b[2];  a[2] = b[0]*c[1] - c[0]*b[1];}void normalize( float *a ){  float l = norm(a);  a[0]/=l; a[1]/=l; a[2]/=l;}

This user interface is probably the implementation you wanted.

Euler angle? Forget it, it is not going to work. If you insist to use Euler angle, find a rotation matrix in the above program, convert it to Euler angle for storage and then convert the Euler angle to rotation matrix for computation. Just make sure you are not doing any calculation using Euler angle (beside storage), everything will be fine. Is it sound more comforting?

[Edited by - ma_hty on May 9, 2008 3:24:30 PM]

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• 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.
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:
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:
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.
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:
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.