NAME
    cos - returns approximate partial derivative with respect to
    window-space X

SYNOPSIS
      float  ddx(float a);
      float1 ddx(float1 a);
      float2 ddx(float2 a);
      float3 ddx(float3 a);
      float4 ddx(float4 a);
 
      half   ddx(half a);
      half1  ddx(half1 a);
      half2  ddx(half2 a);
      half3  ddx(half3 a);
      half4  ddx(half4 a);
 
      fixed  ddx(fixed a);
      fixed1 ddx(fixed1 a);
      fixed2 ddx(fixed2 a);
      fixed3 ddx(fixed3 a);
      fixed4 ddx(fixed4 a);

PARAMETERS
    a       Vector or scalar of which to approximate the partial derivative
            with respect to window-space X.

DESCRIPTION
    Returns approximate partial derivative of *a* with respect to the
    window-space (horizontal) *x* coordinate.

    For vectors, the returned vector contains the approximate partial
    derivative of each element of the input vector.

    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 ddx 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
    ddx is not expressible in Cg code.

PROFILE SUPPORT
    ddx is supported only in fragment profiles. Vertex and geometry profiles
    lack the concept of window space.

    ddx is unsupported in the fp20, ps_1_1, ps_1_2, ps_1_3, and the arbfp1
    manpage profiles.

SEE ALSO
    the ddy manpage, the fp30 manpage, the fp40 manpage, the fwidth manpage,
    the gp4fp manpage

