[CREATE] OpenRaster specification: updates to support masking

[Andrew Chadwick]

On 17 January 2014 22:08, Andrew Chadwick <a.t.chadwick at gmail.com> wrote:
> (It has to be in a group because MyPaint does non-isolated rendering
> onto its internal background layer. Currently you see black if a layer
> erases the background layer... The layer group implementation you see
> here uses isolated rendering only, however, and the results of that
> are then composited onto the background normally. This is just a
> workaround for now, allowing some masking experiments.)

Oops, correction. It's because MyPaint renders with an assumed bg
alpha of 1 if the background is turned on, for old speedup reasons.
Thus if something like Clear or XOR tries to write (0,0,0,0), the
result is (0,0,0,1). Kinda suboptimal really; I should make it only
attempt that speedup if the layer stack including the backdrop
_logically can never_ create a pixel with alpha<1 when it renders.

-- 
Andrew Chadwick