How does texture lookup in non fragment shaders works?

Question

The following is an excerpt from GLSL spec:

"Texture lookup functions are available in all shading stages. However, automatic level of detail is computed only for fragment shaders. Other shaders operate as though the base level of detail were computed as zero."

So this is how I see it:

Vertex shader:

vec4 texel = texture(SamplerObj, texCoord);   
// since this is vertex shader, sampling will always take place
// from 0th Mipmap level of the texture.

Fragment shader:

vec4 texel = texture(SamplerObj, texCoord);   
// since this is fragment shader, sampling will take place
// from Nth Mipmap level of the texture, where N is decided
// based on the distance of object on which texture is applied from camera.

Is my understanding correct?

Answer 1

That sounds right. You can specify an explicit LOD by using textureLod() instead of texture() in the vertex shader.

I believe you could also make it use a higher LOD by setting the GL_TEXTURE_MIN_LOD parameter on the texture. If you call e.g.:

glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_LOD, 2.0f);

while the texture is bound, it should use mipmap level 2 when you sample the texture in the vertex shader. I have never tried this, but this is my understanding of how the behavior is defined.

Answer 2

// since this is fragment shader, sampling will take place
// from Nth Mipmap level of the texture, where N is decided
// based on the distance of object on which texture is applied from camera.

I think the bit about the distance isn't correct. The mipmap level to use is determined using the derivation of the texture coordinates for the neighbouring pixels. The sampler hardware can determine this because the generated code for the fragment shader typically uses SIMD instructions and generates values for multiple pixels simultaneously. For example, on Intel hardware a single thread usually operates on a 4x4 grid of pixels. That means that whenever a message is sent to the sampler hardware it is given a set 16 of texture coordinates and 16 texels are expected in reply. The sampler hardware can determine the derivation by looking at the difference between those 16 texture coordinates. That is probably why further down in the GLSL spec it says:

Implicit derivatives are undefined within non-uniform control flow and for non-fragment-shader texture fetches.

Non-uniform control flow would mess up the implicit derivatives because potentially not all of the fragments being processed in the thread would be sampling at the same time.