GLSL : common mistakes

Revision as of 11:17, 27 May 2008 by V-man (Talk | contribs) (Binding A Texture)

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The following article discusses common mistakes made in the OpenGL Shading Language, GLSL.

Enable Or Not To Enable

With fixed pipeline, you needed to call glEnable(GL_TEXTURE_2D) to enable 2D texturing. You needed to call glEnable(GL_LIGHTING). Since shaders override these functionalities, you don't need to glEnable/glDisable. If you don't want texturing, you either need to write another shader that doesn't do texturing or you can attach a all white or all black texture, depending on your needs. You can also write one shader that does lighting and one that doesn't.

Things that are not overriden by shaders, like the alpha test, depth test, stencil test... calling glEnable/glDisable will have an effect.

Binding A Texture

When you compile and link your GLSL shader, the next step is to get uniform locations for your samplers (I'm talking about texture samplers) and setup the samplers. Some people do this:

glUniform1i(location, textureID)

You can't send a GL texture ID as your sampler. A sampler should be from 0 to the max number of texture image units.
Once you compile and link your shader, make sure that you setup all the samplers by calling (assuming of course your samplers are named Texture0, Texture1 and Texture2

location=glGetUniformLocation(shaderProgram, "Texture0"); 
glUniform1i(location, 0);
location=glGetUniformLocation(shaderProgram, "Texture1"); 
glUniform1i(location, 1);
location=glGetUniformLocation(shaderProgram, "Texture2"); 
glUniform1i(location, 2);

To bind a texture, always use glBindTexture.

glBindTexture(GL_TEXTURE_2D, textureID[0]);
glBindTexture(GL_TEXTURE_2D, textureID[1]);
glBindTexture(GL_TEXTURE_2D, textureID[2]);


for(i=0; i<3; i++)
  glBindTexture(GL_TEXTURE_2D, textureID[i]);


nVidia drivers are more relaxed. You could do

float myvalue = 0;

but this won't compile on other platforms. Use 0.0 instead. Don't write 0.0f. GLSL is not C or C++.

float texel = texture2D(tex, texcoord);

The above is wrong since texture2D returns a vec4 Do this instead

float texel = float(texture2D(tex, texcoord));


float texel = texture2D(tex, texcoord).r;


float texel = texture2D(tex, texcoord).x;


Functions should look like this

vec4 myfunction(inout float value1, in vec3 value2, in vec4 value3)

instead of

vec4 myfunction(float value1, vec3 value2, vec4 value3)

Not Used

In the vertex shader

gl_TexCoord[0] = gl_MultiTexCoord0;

and in the fragment shader

vec4 texel = texture2D(tex, gl_TexCoord[0].xy);

zw isn't being used in the fs.

Easy Optimization

gl_TexCoord[0].x = gl_MultiTexCoord0.x;
gl_TexCoord[0].y = gl_MultiTexCoord0.y;

turns into

gl_TexCoord[0].xy = gl_MultiTexCoord0.xy;

The MAD instruction

MAD is short for multiply, then add. It is a special floating point circuit. Very fast. Costs 1 GPU cycle.

vec4 result1 = (value / 2.0) + 1.0;
vec4 result2 = (value / 2.0) - 1.0;
vec4 result3 = (value / -2.0) + 1.0;

The above doesn't quite easily turn into a MAD. It might be compiled to a reciprocal, then add. That might cost 2 or more cycles. Below is GLSL code that converts to a single MAD instruction (for each line of code of course)

vec4 result1 = (value * 0.5) + 1.0;
vec4 result2 = (value * 0.5) - 1.0;
vec4 result3 = (value * -0.5) + 1.0;