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