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

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

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

DESCRIPTION
    Returns approximate partial derivative of *a* with respect to the
    window-space (vertical) *y* 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 ddy 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
    ddy is not expressible in Cg code.

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

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

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

