Colors transitions are based on a line, where colors are assigned to stop points along the line, and colors for in-between points are interpolated from the stop-point colors. The color of a point on the gradient’s line is propagated to all points in the drawing context that are touched by a line through the point and perpendicular to the gradient’s line.

Examples:

> (define no-pen (make-object pen% "BLACK" 1 'transparent))

> (define brush-grad (new brush% [gradient       (new linear-gradient%            [x0 300] [y0 250] [x1 500] [y1 50]            [stops             (list (list 0   (make-object color% 255 0 0))                   (list 0.5 (make-object color% 0 255 0))                   (list 1   (make-object color% 0 0 255)))])]))

> (define brush-grad2 (new brush% [gradient       (make-object linear-gradient%                    50 150 250 150                    (list (list 0   (make-object color% 255 0 0))                          (list 0.5 (make-object color% 0 255 0))                          (list 1   (make-object color% 0 0 255))))]))

> (dc (lambda (dc x y)       (send dc set-pen no-pen)       (send dc set-brush brush-grad2)       (send dc draw-ellipse 50 50 200 200)            (send dc set-brush brush-grad)       (send dc draw-rectangle 300 50 200 200)) 550 300)