What is the difference between Mipmap arrays and Level of Detail?

Question

The textureQueryLod call returns two components:

1. x - Tells in which proportion are mipmap levels used.
2. y - Level of detail.

Are they both same?

Answer 1

When you have a problem like this and find it cannot be sufficiently answered by looking at the basic API documentation for a class of GLSL functions, you should consult the formal GLSL specification.

If you read the specification, specifically Section 8.9.1 - Texture Query Functions, you will come across a very detailed explanation of the "used" mipmap level. In short, this value is the fractional component that would be used to select the nearest mipmap level(s) during during minification+mipmap filtering.

Recall that there are two possible ways to resolve a mipmap sample:

1. Interpolate between the two nearest mipmap LODs (linear mip filter)
   
   
   • This is the 3^rd linear in a tri-linear filter
     
     
2. Sample the closest mipmap LOD (nearest mip filter)

With that out of the way, you should have the theoretical basis to understand the pseudo-code presented in the specification on pp. 155:

float ComputeAccessedLod(float computedLod)
{
    // Clamp the computed LOD according to the texture LOD clamps.
    if (computedLod < TEXTURE_MIN_LOD) computedLod = TEXTURE_MIN_LOD;
    if (computedLod > TEXTURE_MAX_LOD) computedLod = TEXTURE_MAX_LOD;
    // Clamp the computed LOD to the range of accessible levels.
    if (computedLod < 0.0)
        computedLod = 0.0;
    if (computedLod > (float)
        maxAccessibleLevel) computedLod = (float) maxAccessibleLevel;
    // Return a value according to the min filter.
    if (TEXTURE_MIN_FILTER is LINEAR or NEAREST) {
        return 0.0;
    } else if (TEXTURE_MIN_FILTER is NEAREST_MIPMAP_NEAREST
               or LINEAR_MIPMAP_NEAREST) {
        return ceil(computedLod + 0.5) - 1.0;
    } else {
        return computedLod;
    }
}

There are three branches in this pseudo-code:

1. Handles the situation where a sampler does not use mipmapping at all
   • The accessed LOD is always 0.0
2. Handles the situation where a sampler uses a nearest mip filter
   • The accessed LOD is the closest integer
3. Handles the situation where a sampler uses a linear mip filter
   • The accessed LOD is between the two nearest integer LODs
   • Two LODs would be sampled in this case, and the single returned sample is interpolated between both based on the fractional part of this value

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The second component returned by textureQueryLod (...) is the unclamped integer LOD.