Drawing image with CoreGraphics on Retina iPad is slow
core-graphics, ipad, performance, retina-display
Solution
It looks like CoreGraphics is internally doubling the pixels, and then sending that to the GPU,
Pretty much. More accurately (in spirit at least):
- UIKit makes a `CGBitmapContext` the size of your view's bounds, in device pixels
- It makes that context the current context
- You draw your `CGImage` into that context
- ... so CG has to rescale the source image, and touch all of the destination pixels
- After you're done drawing, UIKit makes a `CGImage` from the bitmap context
- and assigns it to the view's layer's contents.
the CGImage that I'm making should be fine for passing to the GPU directly.
If you want that to happen, you need to tell the system to do that, by cutting out some of the steps above.
(There is no link between UIKit, CoreAnimation, and CoreGraphics that provides a "fast path" like you are expecting.)
The easiest way would be to make a `UIImageView`, and set its `image` to a `UIImage` wrapping your `CGImageRef`.
Or, set your `view.layer.contents` to your `CGImageRef`. (And make sure to not override `-drawRect:`, not call `-setNeedsDisplay`, and make sure `contentMode` is not `UIViewContentModeRedraw`. Easier to just use `UIImageView`.)
Problem
In my iPad app, I am rendering to an offscreen bitmap, and then drawing the bitmap to the screen. (This is because I want to re-use existing bitmap rendering code.) On the iPad 2, this works like a charm, but on the new iPad with Retina display, drawing the bitmap is really slow, even though its resolution is still the same. To draw the bitmap, we use the regular Quartz 2D functions: `CGImageCreate` with a data provider created by `CGDataProviderCreateWithData`, 32-bit RGBA format with `kCGImageAlphaNoneSkipLast`. In the `UIView` that displays the bitmap, in `drawRect:`, we use `CGContextDrawImage` to draw it to the context returned by `UIGraphicsGetCurrentContext`. Note that I'm not even trying to draw at double resolution: for now I'm fine with the same resolution as I was using on the iPad 2. It looks like CoreGraphics is internally doubling the pixels, and then sending that to the GPU, even though the `CGImage` that I'm making should be fine for passing to the GPU directly. Any ideas?