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martyj2009

OpenGL
OpenGL Tessellation

6 posts in this topic

I'm having a hard time getting OpenGL Tessellation to work. I have been struggling with it for about a month now. I've looked at a lot of example code and what I am doing doesn't seem that different to what they are doing in their code. I was wondering if anyone could be a second pair of eyes and maybe point out why it isn't working.

My scene is only rendering a black screen.

My shaders are as follows:

 

Vertex Shader

#version 400
#extension GL_ARB_tessellation_shader: enable
#extension GL_ARB_separate_shader_objects: enable

layout(location = 0) in vec3 vertexPosition;

void main()
{
    gl_Position = vec4(vertexPosition, 1.0);
}

Tessellation Control Shader

#version 400
#extension GL_ARB_separate_shader_objects: enable

layout(vertices = 3) out;

void main()
{
    float inLevel = 2;
    float outLevel = 2;
    
    gl_out[gl_InvocationID].gl_Position = gl_in[gl_InvocationID].gl_Position;
    
    if (gl_InvocationID == 0)
    {
        gl_TessLevelOuter[0] = outLevel;
        gl_TessLevelOuter[1] = outLevel;
        gl_TessLevelOuter[2] = outLevel;
        gl_TessLevelOuter[3] = outLevel;

        gl_TessLevelInner[0] = inLevel;
        gl_TessLevelInner[1] = inLevel;
    }
}

Tessellation Eval Shader

#version 400
#extension GL_ARB_tessellation_shader: enable
#extension GL_ARB_separate_shader_objects: enable

layout(triangles, equal_spacing, ccw) in;

uniform mat4 uMVPMatrix;

void main()
{
    vec4 p0 = gl_TessCoord.x * gl_in[0].gl_Position;
    vec4 p1 = gl_TessCoord.y * gl_in[1].gl_Position;
    vec4 p2 = gl_TessCoord.z * gl_in[2].gl_Position;

    vec4 newCoord = normalize(p0 + p1 + p2);
    gl_Position = newCoord;
}

Geometry Shader

#version 400
#extension GL_ARB_separate_shader_objects: enable

layout(triangles) in;
layout(triangle_strip, max_vertices = 3) out;

uniform mat4 uMVPMatrix;

uniform bool UseWaveGeometry;
uniform float GameTime;

layout(location = 3) out vec4 vColor;

float rand(vec2 n)
{
    return 0.5 + 0.5 * fract(sin(dot(n.xy, vec2(12.9898, 78.233)))* 43758.5453);
}

void setUpValues(int index)
{
    vec4 vertPosit = gl_in[index].gl_Position;
    
    if(UseWaveGeometry)
    {
        vertPosit.y = vertPosit.y + (GameTime + ((cos(vertPosit.x)) * GameTime) - ((sin(vertPosit.x))*GameTime)*2.2);
    }
    
    float cval = int(vertPosit.x) % 2 + int(vertPosit.y)%2;
    vColor = vec4(cval, cval, cval, 1.0);

    gl_Position = uMVPMatrix * vertPosit;
}

void main()
{
    for(int i = 0; i < 3; i++)
    {
        setUpValues(i);
        EmitVertex();
    }
    EndPrimitive();
}

Fragment Shader

#version 400
#extension GL_EXT_gpu_shader4: enable
#extension GL_ARB_separate_shader_objects: enable

uniform float ColorAlpha;

layout(location = 3) in vec4 vColor;

layout(location = 0) out vec4 FragColor;

void main()
{
    vec4 color = vec4(vColor.r, vColor.g, vColor.b, ColorAlpha);
    
    FragColor = color;
}

Thank you for your time,
Marty

Edited by martyj2009
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First of all I assume that rendering single triangle with only VS and FS works (e.g. you don't have issues in calling glDrawArrays/glDrawElements).
 
Some points that probably cause issues:
1.) Names with out qualifier in some shader should correspond to names with in qualifier in the shader describing next stage. At least unless you use layout qualifier and separate shader objects extensions... e.g.:
// Standard way of passing "varyings" from one shader to another. The GLSL compliler does pattern
// matching between their names and connects them.
[Vertex Shader]
out vec3 vsVertexPosition;
out vec3 vsVertexNormal;

[Geometry Shader]
in vec3 vsVertexPosition[];
in vec3 vsVertexNormal[];

// Another possible way since ARB_separate_shader_objects - available since OpenGL 4.1, core
// since OpenGL 4.3 - this one explicitly says which "varyings" links together. Note that
// using this approach the corresponding ones can have different names! Otherwise they can't
[Vertex Shader]
layout(location = 0) out vec3 position;
layout(location = 1) out vec3 normal;

[Geometry Shader]
layout(location = 0) in vec3 pos;
layout(location = 1) in vec3 n;

// But this is NOT possible, the GLSL compiler can't tell which variables can be connected
// together.
[Vertex Shader]
out vec3 pos;
out vec3 normal;

[Geometry Shader]
in vec3 mPosition;
in vec3 mNormal;
2.) Version definition should be same in all shaders! On AMD this can result in error and compilation will fail, thus the shaders won't run.

3.) Don't mix gl_in/gl_out and in/out. This is bad and results in (very) messy shaders.

4.) Tessellation shaders should have
#extension GL_ARB_tessellation_shader : enable
These are just few points by quickly going through your code. I can post you shader code of very basic tessellation example that works - if you can't make it work. It's really very basic example that needs just 2 parameters - view and projection matrices. Edited by Vilem Otte
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Thank you for the reply. Sorry for the late response. I read your message a few days ago.

 

The reason I didn't use the layout location identifiers is that I am dynamically getting the location in my C code by the name of the attribute. I also keep my variables named the same. Although it would make for easier to understand shader code if I used them as well. That way users can see at a glance what output matches up with what input.

 

I will implemented this and post my results.

 

Thank you very much for the feedback. It is greatly appreciated.

 

Edit

--------------------------------------------

 

I modified my previous post with the updated shaders. I still get the same issues with tessellation.

Edited by martyj2009
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Anyone have any idea? Anything at all I could try? Any feedback would be greatly appreciated.

 

 

Edit:

---------------------------

I simplefied the code. I still get a black screen when the tessellation shaders are enabled. It renders a scene with them commented out.

Edited by martyj2009
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I ran all the shaders through AMD's GPU shaderAnalyzer and shaders 2,3, and 5 all have reported errors.  Most notable, 2 is using variables that were not declared.

 

Also, there is usually a huge problem with shader tech that is very new.  A lot of it is not tested against multiple GPU's and it can take a while for the GPU manufacturers to play catch-up with one another. Often you will run into compatibility issues if you have a different brand than what the code was tested on.  

 

Here's another site that you can cross-reference against.  

 

http://prideout.net/blog/?p=48

 

ShaderAnalyzer only has a problem with the tess eval shader in this example. 

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I have been using that reference as a tessellation example. 

 

The biggest problem is that my code base is so huge. I have texture arrays for height maps, FBO for shaows, reflections, and a bunch of other stuff which complicate code slightly.

 

I will check out the AMD GPU shader analyzer. Thanks for the info. I will post an update when I test it

Edited by martyj2009
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I finally figured out my issue.

 

It wasn't shaders at all. My problem with tessellation was that I was drawing my objects using GL_TRIANGLES. Tessellation only works on GL_PATCHES.

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