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:SET-TO-PRODUCT mtl

source files: [xc]mtl.[ch]

SYNTAX

(SEND <mtl> :SET-TO-PRODUCT (list <mtl1> <mtl2> ... ))

DESCRIPTION

This call is used to implement hierarchical displaylist structures in which the color of a leaf is a function of all class-material objects on the path from the displaylist root to the leaf: It is given the ordered list <mtl1> ... of objects on the rootpath, and sets <mtl> appropriately.

In general, each <mtl#> may be set to override those <mtl3> above it on some, all or none of the various material properties, by setting the various :*-WEIGHT properties. A weight of 1.0 means to override the inherited value, a weight of 0.0 means to adopt the inherited value unchanged, intermediate values lead to weighted averages.

For example, when done (assuming only <mtl1> and <mtl2>),

mtl.alpha == mtl1.alpha * (1.0-mtl2.alpha_weight) +
           + mtl2.alpha * (    mtl2.alpha_weight)

Which is to say, <mtl> will use the :alpha from <mtl2> if :alpha-weight in <mtl2> is 1.0, :alpha from <mtl1> if :alpha-weight in <mtl2> is 0.0, else a weighted average.

Other fields are computed similarly.

With more than two input <mtl#>, the 'product' proceeds pairwise, left to right.

This call is typically used in the :per-frame-hook field for a material in a thinglist:

;; mtl1 mtl2 mtl3 are presumed pre-existing:
(setq my-material (:new class-material))
(:set my-material :per-frame-hook
  (lambda nil (:set-to-product my-material (list mtl1 mtl2 mtl3)))
)
(setq my-thing (xg.3d-make-thing-of-triangles
    :want-vertex-normals t
    :material my-material
) )

The product list may contain any number of materials, including zero.

Return value is <mtl>.


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