Gathering Texels
The textureGather function is a special function that allows your shader to read the four samples that would have been used to create a bilinearly filtered texel from a 2D texture (or cube map, rectangle texture, or array of these types). Typically used with single-channel textures, the optional comp component of the function allows you to select a channel other than the x or r component of the underlying data. This function can provide significant performance advantages when you need to sample many times from a single channel of a texture because, depending on the desired access pattern, it is possible to use this function to cut the number of texture lookups by three quarters.
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