Return the partial derivative of an argument with respect to x
genType dFdx(float x);
p specifies the expression of which to take the partial derivative.
Available only in the fragment shader,
dFdx return the partial derivative of expression
x. Derivatives are calculated using local differencing. Expressions that imply higher order derivatives such as
dFdx(dFdx(n)) have undefined results, as do mixed-order derivatives such as
dFdx(dFdy(n)). It is assumed that the expression
p is continuous and therefore, expressions evaluated via non-uniform control flow may be undefined.