Built-In Functions - Praise for OpenGL ES 30 Programming Guide - page 484

Other properties that are desirable, but not required, are

• Functions should be evaluated within the interior of a primitive
(interpolated, not extrapolated).

• Functions for dFdx should be evaluated while holding y constant.
Functions for dFdy should be evaluated while holding x constant.
However, mixed higher-order derivatives, like dFdx ( dFdy ( y )) and
dFdy ( dFdx ( x )), are undefined.

• Derivatives of constant arguments should be 0.

In some implementations, varying degrees of derivative accuracy
can be obtained by providing hints using glHint
(GL_FRAGMENT_SHADER_DERIVATIVE_HINT) , allowing a user to make an
image quality versus speed trade-off.

Table B-10 describes the fragment processing functions.

Table B-10 Fragment Processing Functions

Syntax Description

genType dFdx (genType p ) Returns the derivative in x using local differencing

for the input argument p .

genType dFdy (genType p ) Returns the derivative in y using local differencing

for the input argument p .

genType fwidth (genType p ) Returns the sum of the absolute derivative in x and

y using local differencing for the input argument p ;

i.e.,

result = abs ( dFdx ( p )) + abs ( dFdy ( p )).