In a GLSL fragment shader, how to access to texel at a specific mipmap level?

Question

I am using OpenGL to do some GPGPU computations through the combination of one vertex shader and one fragment shader. I need to do computations on a image at different scale. I would like to use mipmaps since their generation can be automatic and hardware accelerated. However I can't manage to get access to the mipmap textures in the fragment shader.

I enabled automatic mipmap generation: glTexParameteri(GL_TEXTURE_2D, GL_GENERATE_MIPMAP, GL_TRUE);

I tried using texture2DLod in the shader with no luck, it simply kept giving the normal texture. I also tried using glTextureParameteri(GL_BASE_LEVEL, X) in the main program and it did not change anything.

How would you do that?

I am using Linux. My graphic card is a Nvidia Quadro quite old. Here is my glxinfo output with all the supported extensions.

Answer 1

gvec4 textureLod (gsampler1D sampler, float P, float lod)
gvec4 textureLod (gsampler2D sampler, vec2 P, float lod)
gvec4 textureLod (gsampler3D sampler, vec3 P, float lod)
gvec4 textureLod (gsamplerCube sampler, vec3 P, float lod)
float textureLod (sampler1DShadow sampler, vec3 P, float lod)
float textureLod (sampler2DShadow sampler, vec3 P, float lod)
gvec4 textureLod (gsampler1DArray sampler, vec2 P, float lod)
gvec4 textureLod (gsampler2DArray sampler, vec3 P, float lod)
float textureLod (sampler1DArrayShadow sampler, vec3 P, float lod)

Did you try one of those built-in? Also lod has to be a float type. What errors/warning is reporting GLSL compiler?

Answer 2

The GLSL 1.20 specification (section 8.7) states that fragment shaders cannot choose their own mipmap level (and that the texture*Lod function are only available in vertex shaders). If anything, you may be able to use the bias parameter to the non-Lod variants to change the mipmap level, but it can only change it relative to what that the card has already calculated for you.

I don't know if latter versions of GLSL may have changed that.

Answer 3

try:

glGenerateMipmapEXT(GL_TEXTURE_2D);

after you bind the texture. (And before doing the rendering of course)

The glTexParameteri-GL_GENERATE_MIPMAP is deprecated I think... MfG Digi