PathGradientBrush Class

Triangles. The polygon of PathGradientBrush.Points is cut into triangles from PathGradientBrush.Center: the triangle i has the vertices PathGradientBrush.Center, Points[i] and Points[(i + 1) mod n]. A position takes the color of the first of these triangles that contains it, edges included; a position no triangle contains has no color, and nothing is painted there, whatever the shape that is painted. When the center sees every edge of the polygon (a convex polygon around its center, a star around its middle), the triangles cover the polygon exactly once. A triangle whose three vertices are on one line contains nothing.

Colors. The color of a pixel is the color at the center of that pixel. In the triangle of the center c and of the points a and b, with the cross product u × v = ux vy - uy vx, the position p has the weights s = ((p - c) × (b - c)) / ((a - c) × (b - c)) for a and t = ((a - c) × (p - c)) / ((a - c) × (b - c)) for b, and is in the triangle when s >= 0, t >= 0 and s + t <= 1. The colors are converted to the pixel format of the frame first, then interpolated on alpha-premultiplied values, as the stops of a gradient: a = ac + s (aa - ac) + t (ab - ac) for alpha and the same for each premultiplied sample, divided by a, each rounded as floor(v + 1/2 + 2^-20). A point has exactly its color, and a brush whose colors are all the same paints exactly what a SolidBrush of that color paints inside the triangles.

Cost. Each pixel tests the triangles in order until one contains it (those that cannot, from their bounds, are skipped): the cost of a pixel grows with the number of points. The arithmetic uses + - * / only: the pixels are the same on every platform.