I've been doing some Canvas work to implement a simple image editing system, and I noticed that some things are quite hard to do due to a lack of certain blend modes.
1) Given an empty fully transparent Canvas, you can easily draw opaque ovals onto it (using a circular brush) but cannot use `fillOval` to clear an area (back to fully transparent). The only option available here seems to be `clearRect` which obviously is not going to look very nice when working with a circular brush... A blend mode "CLEAR" (which is pretty common) would really help here. 2) Given an empty fully transparent Canvas, drawing partially transparent ovals (or even a RadialGradient where the brush becomes less transparent at the edges) doesn't work nicely when strokes partially overlap, or when you compensate for mouse movements by drawing intermediate ovals -- the intended transparency at the edges of strokes quickly disappears when the stroke overlaps multiple times because of repeated fillOvals to draw the full stroke made by the mouse. Single clicks work as expected. A blend mode "ALPHA_MAX" (uncommon) could allow this; given a source and destination pixel, it would never set alpha to a value that is higher than either source or destination (it just takes the maximum); SRC_OVER will keep increasing alpha until it is fully opaque. This blend mode would be useful when drawing into an overlapping Canvas and merging the results with the underlying Canvas after each stroke completes. -- Now it seems to me that all it would take to add additional blend modes is extending the BlendMode enum, the internal Mode enum, and adding a new shader file per blend mode. The software pipeline could also be extended to support these (but I think that may be optional as this pipeline doesn't need to support all blend modes?) So my question, is this something that would get support if a PR is provided, or is there a reason to limit the BlendModes and/or shader files to some specific set? --John
