Sampling Textures with Missing Derivatives

A key part of rendering high-quality imagery without aliasing has been to use first-order differentials of texture coordinates to determine spatially varying filter kernels for texture map lookups. With traditional rasterization, a variety of techniques have been developed to do this, but the most common one used today is to compute differences among adjacent pixels in a 2 2 quad of pixels. However, doing so is not always possible with modern rendering techniques, including both some forms of deferred shading as well as ray tracing.