My question is specifically in regards to Metal, since I don't know if the answer would change for another API.
What I believe I understand so far is this:
A mipmapped texture has precomputed "levels of detail", where lower levels of detail are created by downsampling the original texture in some meaningful way.
Mipmap levels are referred to in descending level of detail, where level
0is the original texture, and higher-levels are power-of-two reductions of it.
Most GPUs implement trilinear filtering, which picks two neighboring mipmap levels for each sample, samples from each level using bilinear filtering, and then linearly blends those samples.
What I don't quite understand is how these mipmap levels are selected. In the documentation for the Metal standard library, I see that samples can be taken, with or without specifying an instance of a
lod_options type. I would assume that this argument changes how the mipmap levels are selected, and there are apparently three kinds of
lod_options for 2D textures:
gradient2d(float2 dPdx, float2 dPdy)
Unfortunately, the documentation doesn't bother explaining what any of these options do. I can guess that
bias() biases some automatically chosen level of detail, but then what does the bias
value mean? What scale does it operate on? Similarly, how is the
level() translated into discrete mipmap levels? And, operating under the assumption that
gradient2d() uses the gradient of the texture coordinate, how does it use that gradient to select the mipmap level?
More importantly, if I omit the
lod_options, how are the mipmap levels selected then? Does this differ depending on the type of function being executed?
And, if the default no-lod-options-specified operation of the
sample() function is to do something like
gradient2D() (at least in a fragment shader), does it utilize simple screen-space derivatives, or does it work directly with rasterizer and interpolated texture coordinates to calculate a precise gradient?
And finally, how consistent is any of this behavior from device to device? An old article (old as in DirectX 9) I read referred to complex device-specific mipmap selection, but I don't know if mipmap selection is better-defined on newer architectures.