getderiv

On this page	* Derivatives options * Examples
Context(s)	displace fog light shadow surface

void  getderiv(float attr, string attrName, int isVertexAttr, float s, float t, float &du, float &dv, ...)

void  getderiv(<vector>attr, string attrName, int isVertexAttr, float s, float t, <vector>&du, <vector>&dv, ...)

Note If derivatives are queried for a polygonal mesh it is interally sampled as a Subdivision Surface.

attr

Attribute value.

attrName

Name of attribute to evaluate.

isVertexAttr

Set to 1 to indicate the attribute is a vertex type.

s

Parametric S shading value. This should be passed from the s global variable.

t

Parametric shading value. This should be passed from the t global variable.

du

Derivative of attribute in U direction.

dv

Derivative of attribute in V direction.

Derivatives options

derivatives-options

Functions which compute derivatives take additional arguments to allow tuning of the derivative computation.

“extrapolate”, int =0

Whether derivatives are “smooth” across patch boundaries. In most cases this is true and if extrapolation is turned on, derivative computation should be exact for C2 surfaces. However, when the VEX variables are changing with a high frequency (for example, a high frequency displacement map causing high frequency changes to the P variable), extrapolation of derivative computation may cause exaggeration of discontinuities between patch boundaries.

“smooth”, int =1

Adjust the magnitude of the differentials non-uniformly over patches. This will usually reduce patch discontinuities in displacement/textured shaders. However, in some odd cases you may want to turn this feature off.

N = computenormal(P, "extrapolate", 1, "smooth", 0);

Examples

examples

// Get derivatives of point attribute 'N'
vector dNdu, dNdv;
getderiv(N, "N", 0, s, t, dNdu, dNdv);