Texture Functions
Texture lookup functions are available in all shading stages. However, level of detail is implicitly computed only for fragment shaders, so that OpenGL can automatically perform mipmap filtering. Other shading stages use a base level of detail of zero or use the texture directly if it is not mipmapped. When texture functions require implicit derivatives, they must be called outside of nonuniform flow control. That is, if they are called within flow control that varies from fragment to fragment, there is not enough information to properly compute the level of detail, giving undefined implicit derivatives and hence undefined results from the texture lookup.
Texture data can be stored by the GL as single-precision floating point, ...
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