cube-simplex-bary.sc

#-------------------------------------------------------------------------------
    implicit cube-to-simplex subdivision

fn cube-simplex (pred)
    """"ivec3 ivec3 ivec3 ivec3 <- ivec3
        for three coordinate predicates
        r: y <= z, g: z < x, b: x < y
        return the two defining vertices of the simplex that the coordinate is in
        and the four planes required for computing barycentric coordinates
        the resulting simplex has the corners (0 0 0) v1 v2 (1 1 1)
        the planes are ordered so that each plane is opposite each defining vertex
    e1 := pred.gbr
    e2 := 1 - pred.brg
    p1 := e1 & e2
    p2 := e1 | e2
    plane0 := -p1
    plane1 := p1 * 2 - p2
    plane2 := 2 * p2 - p1 - 1
    plane3 := 1 - p2
    return p1 p2 plane0 plane1 plane2 plane3

fn cube-simplex-bary (p)
    """"vec4 ivec3 ivec3 <- vec3
        for a cube coordinate in the range (0..1 0..1 0..1)
        return the barycentric coordinate within the sub-simplex of the cube
        as well as the two defining vertices of the simplex with
        corners (0 0 0) v1 v2 (1 1 1)
    pred :=
        ivec3
            ? (p.y <= p.z) 1 0
            ? (p.z < p.x) 1 0
            ? (p.x < p.y) 1 0
    let p1 p2 n0 n1 n2 n3 = (cube-simplex pred)
    let w0 w1 w3 =
        (dot p (vec3 n0)) + 1.0
        dot p (vec3 n1)
        dot p (vec3 n3)
    w2 := 1.0 - w0 - w1 - w3
    return (vec4 w0 w1 w2 w3) p1 p2