mapnik-org/mapnik — Issue #62: im.blend treats transparency on/off
Author: artemp
Created: 2011-10-11 07:33:32+00:00
State: open
Labels: Core_Library

(originally reported via email to Artem)

im.blend doesn't take alpha-transparency into account properly when using an image with a transparent background. Each pixel is assumed to be either fully transparent or fully opaque, which leads to jagged outlines.

The attached blend.png shows a transparent layer containing circles blended onto a white background. Notice that the circles are fine when overlapping, but jagged where the layers have been blended.

Patch follows.