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

Fragment Processing Functions

Fragment processing functions are available only in fragment shaders.

Derivative Functions

OpenGL implementations typically approximate derivatives by comparing a computed expression’s value at neighboring fragments. Hence, when this is requested within nonuniform control flow (lines of the shader that are conditionally executed, where the condition varies from pixel to pixel), the derivative is undefined.

genType dFdx(genType p);

Returns the derivative in the x direction, based on the value of p at neighboring fragments.

genType dFdy(genType p);

Returns the derivative in the y direction, based on the value of p at neighboring fragments.

genType fwidth(genType p);

Returns the sum of the absolute derivative ...