ddx - returns approximate partial derivative with respect to window-space X
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);
Vector or scalar of which to approximate the partial derivative with
respect to window-space X.
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
ddx is not expressible in Cg code.
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 arbfp1 profiles.
ddy, fp30, fp40, fwidth, gp4fp