On Dec 9, 2013, at 7:47 AM, Graham Cox <graham....@bigpond.com> wrote:
> This last step is where it all falls down, because this one call, to
> CGContextDrawImage, takes a whopping 67% of the overall time for drawRect: to
> run, and normal drawing doesn’t need this call (this is testing in a ‘light’
> view, but nevertheless, it makes the view very noticeably laggy).
A few months ago I saw a great presentation given by Apple engineer Andy
Matuschak about the way rendering works on iOS, the whole pipeline all the way
from the app’s API calls to the pixels appearing onscreen. Its purpose was to
teach some intuition about what rendering techniques will perform better than
others. [This specific talk doesn’t seem to be online, but he has a WWDC talk*
that I think covers similar topics.]
The big picture is that the actual rendering pipeline is
[your app] —> [graphics server process] —> [OpenGL] —> [GPU] —> [Screen]
and the earlier the graphics calls get converted into pixels, the less
efficient the drawing will be, because it’s more expensive to push pixmaps
through the pipeline than lists of drawing commands.
So if you can avoid it, you shouldn’t be doing your own rendering into images.
I haven’t been following the details of this thread, but my guess is you’ll get
better performance by drawing the tiles directly to the view, just setting a
clipping rect beforehand so the drawing is limited to the tile your thread is
working on.
—Jens
* “Understanding UIKit Rendering”:
https://developer.apple.com/itunes/?destination=adc.apple.com.8270634034.08270634040.8367260927?i=1122536585
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