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.rm #[ #] #H #V #F C
.IX Title "ddx Cg"
.TH ddx Cg "" "Cg Toolkit 2.2" "Cg Standard Library"
.UC
.SH "NAME"
\&\fBddx\fR \- returns approximate partial derivative with respect to window-space X
.SH "SYNOPSIS"
.IX Header "SYNOPSIS"
.Vb 5
\&  float  ddx(float a);
\&  float1 ddx(float1 a);
\&  float2 ddx(float2 a);
\&  float3 ddx(float3 a);
\&  float4 ddx(float4 a);
.Ve
.Vb 5
\&  half   ddx(half a);
\&  half1  ddx(half1 a);
\&  half2  ddx(half2 a);
\&  half3  ddx(half3 a);
\&  half4  ddx(half4 a);
.Ve
.Vb 5
\&  fixed  ddx(fixed a);
\&  fixed1 ddx(fixed1 a);
\&  fixed2 ddx(fixed2 a);
\&  fixed3 ddx(fixed3 a);
\&  fixed4 ddx(fixed4 a);
.Ve
.SH "PARAMETERS"
.IX Header "PARAMETERS"
.Ip "a" 8
.IX Item "a"
Vector or scalar of which to approximate the partial derivative with
respect to window-space X.
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
Returns approximate partial derivative of \fIa\fR with respect to the
window-space (horizontal) \fIx\fR coordinate.
.PP
For vectors, the returned vector contains the approximate partial
derivative of each element of the input vector.
.PP
This function is only available in fragment program profiles (but not
all of them).
.PP
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.
.PP
The partial derivative computation may incorrect when \fBddx\fR is used in
control flow paths where not all the fragments within a quad-fragment
have branched the same way.
.PP
The partial derivative computation may be less exact (wobbly) when
the variable is computed based on varying parameters interpolated with
centroid interpolation.
.SH "REFERENCE IMPLEMENTATION"
.IX Header "REFERENCE IMPLEMENTATION"
\&\fBddx\fR is not expressible in Cg code.
.SH "PROFILE SUPPORT"
.IX Header "PROFILE SUPPORT"
\&\fBddx\fR is supported only in fragment profiles.  Vertex and geometry
profiles lack the concept of window space.
.PP
\&\fBddx\fR is unsupported in the fp20, ps_1_1, ps_1_2, ps_1_3, and arbfp1 profiles.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
ddy, fp30, fp40, fwidth, gp4fp
