> ## Name

> fwidth - returns sum of approximate window-space partial derivatives magnitudes

> ## Synopsis

>     float  fwidth(float a);
>     float1 fwidth(float1 a);
>     float2 fwidth(float2 a);
>     float3 fwidth(float3 a);
>     float4 fwidth(float4 a);

>     half   fwidth(half a);
>     half1  fwidth(half1 a);
>     half2  fwidth(half2 a);
>     half3  fwidth(half3 a);
>     half4  fwidth(half4 a);

>     fixed  fwidth(fixed a);
>     fixed1 fwidth(fixed1 a);
>     fixed2 fwidth(fixed2 a);
>     fixed3 fwidth(fixed3 a);
>     fixed4 fwidth(fixed4 a);

> ## Parameters

> a
>   Vector or scalar of which to sum its approximate window-space partial derivative magnitudes. with respect to window-space X and Y.

> ## Description

> Returns sum of the absolute values of each approximate partial derivative of a with respect to both the window-space (horizontal) x and (vertical) I<y) coordinate.

> For vectors, the returned vector contains the sum of partial derivative magnitudes of each element of the input vector.

> This function can be used to approximate the fragment width (hence the name "fwidth") for level-of-detail computations dependent on change in window-space.

> This function is only available in fragment program profiles (but not all of them).

> The specific way the partial derivative is computed is implementation-dependent. Typically fragments are rasterized in 2x2 arrangements of fragments (called quad-fragments) and the partial derivatives of a variable is computed by differencing with the adjacent horizontal fragment in the quad-fragment.

> The partial derivative computation may incorrect when fwidth is used in control flow paths where not all the fragments within a quad-fragment have branched the same way.

> The partial derivative computation may be less exact (wobbly) when the variable is computed based on varying parameters interpolated with centroid interpolation.

> ## Reference Implementation

> fmod for float3 vectors could be implemented this way:

>     float3 fwidth(float3 a)
>     {
>       return abs(ddx(a)) + abs(ddy(a));
>     }

> ## Profile Support

> fwidth is supported only in fragment profiles. Vertex and geometry profiles lack the concept of window space.

> fwidth is unsupported in the fp20, ps_1_1, ps_1_2, ps_1_3, and arbfp1 profiles.

> ## See Also

> ddx, ddy, fp30, fp40, gp4fp