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xtapol

OpenGL GLSL lighting performance

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xtapol    100
I'm not a newbie to OpenGL, but I am a newbie to *modern* OpenGL, and I'm seeing some bizarre performance characteristics in my specular lighting shader. I have a multipass renderer, and a test scene with a plane, a sphere, and 8 lights each rendered individually in two passes - one for diffuse lighting, one for specular lighting. I recently reworked the renderer, but my problem also occurred in exactly the same way when I was rendering all lighting in one pass, so I know the multipass structure is not the cause. The problem occurs when viewing the sphere from only certain angles and only when zoomed in on one or more specular highlights. Suddenly the framerate drops from ~8000 FPS (vsync disabled) to 50 or 60. This is 100% reproducible and I can have the camera in one position and be getting 8000 FPS, and then rotate a few degrees and hit the problem, even though it's still rendering the same number of specular highlights. I've tracked it down to this specular component in my fragment shader. When I comment this out, the framerate no longer drops: -- halfV = normalize(halfVector); NdotHV = max(dot(n,halfV),0.0); color = att * gl_FrontMaterial.specular * light.specular * pow(NdotHV, gl_FrontMaterial.shininess); -- The really strange part is that the performance drop seems to come from RETURNING a value, not from the calculations themselves. If I don't do the calculations but simply return vec4(1, 1, 1, 1), I still get the performance drop. If I do the calculations but return vec4(0, 0, 0, 0) instead of the result, there is no problem. Here are two screenshots. The first runs at a framerate of ~8000 FPS, the second, different only in camera rotation, drops to 50-60. Again, this is reliably reproducible (GeForce 9600M GT on OS X). http://img532.imageshack.us/i/screenshot20100424at507.png/ http://img519.imageshack.us/i/screenshot20100424at507.png/ Can somebody help me track this down? What is happening here? Are there any good OS X tools to help debugging/profiling GLSL code?

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xtapol    100
Sure, here it is. There are some extraneous varying parameters from when this was doing 4 lights at a time, but they are not populated by the vertex shader.

As a side note, I've tried removing all the branching statements, but it didn't help.

--
varying vec3 normal, lightDir[4], halfVector[4];
varying float dist[4];
varying vec4 ambient;

vec4 pointLightContribution (int lightNum, gl_LightSourceParameters light, vec3 n)
{
float NdotL = max (dot (n, normalize(lightDir[lightNum])), 0.0);
vec4 color = vec4(0, 0, 0, 0);

if (NdotL > 0.0)
{
vec3 halfV;
float NdotHV;
float att;

att = 1.0 / (light.constantAttenuation +
light.linearAttenuation * dist[lightNum] +
light.quadraticAttenuation * dist[lightNum] * dist[lightNum]);

if (gl_FrontMaterial.shininess > 0.0)
{
halfV = normalize(halfVector[lightNum]);
NdotHV = max(dot(n,halfV),0.0);
float p = pow(NdotHV, gl_FrontMaterial.shininess);

color = att * gl_FrontMaterial.specular * light.specular * p;
}
}

return color;
}


void main()
{
vec3 n = normalize(normal);
vec4 color = vec4(0, 0, 0, 0);

color += pointLightContribution (0, gl_LightSource[0], n);
gl_FragColor = color;
}

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karwosts    840
Quote:
The really strange part is that the performance drop seems to come from RETURNING a value, not from the calculations themselves.


If you're concerned that it is returning from your function that is the issue, can you just cut/paste inline the function to prove it?

Quote:
Are there any good OS X tools to help debugging/profiling GLSL code?


If you've got deep pockets you could spring for Mac version of gDEBugger, which is an awesome debugger yet costs like $800 dollars :(
You can play with the free trial version for a few weeks if you want to use it to look at your problem.



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xtapol    100
I just tried inlining the whole function (by putting it in main()... I assume that's what you meant). Same results.

I didn't mean returning the value from the function was the problem, but returning a non-zero vec4 from the shader itself into gl_FragColor.

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karwosts    840
Its possible that by setting the final fragcolor to 0,0,0,0 that GLSL is just optimizing out your entire shader and not doing anything (as having 0 alpha would do nothing to the image), so this might not be a very good idea for a test.

I'm having trouble viewing your first image (it loads halfway and then the image dissappears and says "The image “http://img532.imageshack.us/img532/2552/screenshot20100424at507.png” cannot be displayed, because it contains errors." So I'm not sure whats happening there.

Can I ask what resolution you are running this test at?

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xtapol    100
Hmm, yeah. Seems like something broke during the image upload. I uploaded another copy, this one looks ok:

http://img227.imageshack.us/i/screenshot20100424at507.png/

My display is 1440x900, but the renderer is not quite fullscreen - there's a sidebar with some Cocoa controls and things. The renderer resolution is probably somewhere in the ballpark of 1200x900.

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karwosts    840
Does your slowdown always correspond to when looking at the top of that sphere? It looks kind of funky, like the texture is super-compressed or something.

Also, try shrinking the size of the window when you're getting your slowdown and see if that helps speed things up. Could identify if you're really fill limited (pixel shader) or if there's something else going on.

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xtapol    100
No, it doesn't correspond to the top of the sphere, and it happens when the weird part of the texture isn't visible as well. That was just a quick Maya export for test purposes, I think the texturing issue is just funky UV coords. The same issue happens with other models, including simple cubes, textured or not.

I just did a test - the problem does still occur with a much smaller window (400x400 maybe), and the frame rate drops to the same 50-60 FPS level.

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