The OES_standard_derivatives extension is part of the WebGL API and adds the GLSL derivative functions dFdx, dFdy, and fwidth.
Documentation OES_standard_derivatives by Mozilla Contributors, licensed under CC-BY-SA 2.5.
See also:
Static variables
staticinlineread onlyFRAGMENT_SHADER_DERIVATIVE_HINT_OES:Int = 35723
A Glenum indicating the accuracy of the derivative calculation for the GLSL built-in functions: dFdx, dFdy, and fwidth.