OpenGL Programming Guide: The Official Guide to Learning OpenGL, Version 4.3, Eighth Edition

Shadow Samplers

A special type of sampler is provided in GLSL called a shadow sampler. A shadow sampler takes an additional component in the texture coordinate that is used as a reference against which to compare the fetched texel values. When using a shadow sampler, the value returned from the texture function is a floating-point value between 0.0 and 1.0, indicating the fraction of fetched texel values that passed the comparison operator. For texture accesses that sample only a single texel value (using the GL_NEAREST filtering mode, no mipmaps, and one sample per texel), the returned value will be either 0.0 or 1.0, depending on whether the texel passes the comparison or not. If more than one texel would normally be used to construct the value ...