By default, mask picture is newly created, and each time we need to
clear the whole mask picture, and then composite glyphs to the mask
picture and then composite the mask picture to destination.
Testing results shows that the filling of the mask picture takes a
big portion of the rendering time. As we don't really need to clear
the whole region, we just need to clear the real overlapped region.
This commit is to solve this issue. We split a large glyphs list to
serval lists and each list is non-overlapped or overlapped.
we can reduce the length of overlapped glyphs to do the glyphs_via_mask
to 2 or 3 glyphs one time for most cases. Thus it give us a case to allocate a
small portion of the corresponding cache directly as the mask picture.
Then we can rendering the glyphs to this mask picture, and latter we
can accumulate the second steps, composite the mask to the dest with
the other non-overlapped glyphs's rendering process.
It also make us implement a batch mask cache blocks clearing
algorithm to avoid too frequently small region clearing.
If there is no any overlapping, this method will not get performance gain.
If there is some overlapping, then this algorithm can get about 15% performance
gain.
Signed-off-by: Zhigang Gong <zhigang.gong@linux.intel.com>
0706423bcf