Canvas performance on Mac OS

Jim Graham james.graham at
Mon Mar 30 19:26:00 UTC 2015

On 3/30/15 12:04 PM, Jim Graham wrote:
> drawPolygon() is a very complex operation that involves things like:
> - dealing with only rendering common points of intersection once

An example of the distinction here - try a test case where you execute 
the exact same diagonal line primitive 1,000 times on top of itself 
(identical coordinates for all of them).

Then change the example to use a Polygon that goes from point A to point 
B and back, over itself 1,000 times.

The result of all of those lines will have jagged edges even though the 
lines themselves are antialiased because the partially filled pixels 
along the edges slowly accumulate opacity until their carefully blended 
edges get lost in the accumulated error.

The result of the polygon will be identical to just drawing an 
antialiased line from point A to point B because it is turned into a 
single coverage result by the software rasterizer.

Another similar example - set an opacity of 0.1 on all of those 
rendering calls.  The (multi-)drawLine example will look like an opaque 
line of 1.0 opacity, but the polygon will still look like it has an 
opacity of 0.1 because the coverages are accumulated across the entire 
polygon before any rendering occurs and so each pixel is only blended 


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