Available only in the fragment shader, ```dFdx``` return the partial derivative of expression ```p``` in ```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.
