Cg 3.1 Toolkit Documentation Cg / Standard Library / ddy

## Name

ddy - 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 arbfp1 profiles.