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float area(vector p, ...)
This is a more accurate and convenient method to get the micropolygon area
than multiplying the length of Du(P)
by the length of Dv(P)
.
This function is typically used to get the shading area in pixels.
Note
This function works because VEX “knows” that the variable P
has derivatives (dPdu
and dPdv
). Passing a literal vector
instead of a special variables such as P
will return 0
since
VEX will not be able to access the derivatives.
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 nonuniformly 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
Return the area of the current micropolygon in camera space:
area(P)
Return the area of the current micropolygon in NDC space:
area(transform("ndc", P))
Returns 0
, since the argument is not a variable VEX knows the derivatives for:
area({0.1, 2.3, 4.5})
See also  
shading 
