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Asked: 2026-05-25T23:02:35+00:00

I’m working with shaders just now and I’m observing some very strange behavior. I’ve

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I’m working with shaders just now and I’m observing some very strange behavior. I’ve loaded this vertex shader:

#version 150

uniform float r;
uniform float g;
uniform float b;
varying float retVal;
attribute float dummyAttrib;

void main(){
    retVal = dummyAttrib+r+g+b; //deleting dummyAttrib = corruption
    gl_Position = gl_ModelViewProjectionMatrix*vec4(100,100,0,1);
}

First of all I render with glDrawArrays(GL_POINTS,0,1000) with this shader without nothing special, just using shader program. If you run this shader and set point size to something visible, you should see white square in middle of screen (I’m using glOrtho2d(0,200,0,200)). DummyAttrib is just some attrib – my shaders won’t run if there’s none. Also I need to actually use that attribute so normally I do something like float c = dummyAttrib.That is also first question I would like to ask why it is that way.

However this would be fine but when you change the line with comment (retval=...) to retVal = r+g+b; and add that mentioned line to use attrib (float c = dummyAttrib), strange things happen. First of all you won’t see that square anymore, so I had to set up transform feedback to watch what’s happening.

I’ve set the dummyAttrib to 5 in each element of field and r=g=b=1. With current code the result of transform feedback is 8 – exactly what you’d expect. However changing it like above gives strange values like 250.128 and every time I modify the code somehow (just reorder calls), this value changes. As soon as I return that dummyAttrib to calculation of retVal everything is magically fixed.

This is why I think there’s some sort of shader corruption. I’m using the same loading interface for shaders as I did in projects before and these were flawless, however they were using attributes in normal way, not just dummy for actually running shader.

These 2 problems can have connecion. To sum up – shader won’t run without any attribute and shader is corrupted if that attribute isn’t used for setting varying that is used either in fragment shader or for transform feedback.

PS: It came to my mind when I was writing this that it looks like every variable that isn’t used for passing into next stage is opt out. This could opt out attribute as well and then this shader would be without attribute and wouldn’t work properly. Could this be a driver fault? I have Radeon 3870HD with current catalyst version 2010.1105.19.41785.

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1 Answer

  1. Editorial Team

    In case of your artificial usage (float c = dummyAttrib) the attribute will be optimized out. The question is what your mesh preparing logic does in this case. If it queries used attributes from GL it will get nothing. And with no vertex attributes passed the primitive will not be drawn (behavior of my Radeon 2400HD on any Catalyst).

    So, basically, you should pass an artificial non-used attribute (something like 1 byte per vertex on some un-initialized buffer) if GL reports attributes are not at all.

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