Did you finally find a decent way to draw a line over your CGImage (in your scenario and context) ? I'd LOVE to see how. I have similar task, but I'm on the Mac, and my "line"(s) aren't coming from user touches, but rather from some image analysis that finds shapes in the image, and I don't need to ever draw the thing onscreen - just process the image and produce another image. Still - I'd like to be efficient and not copy the whole (huge! in my case) image every time I need to draw a line!!!.
Have you found the optimized way to work with this? BTW -- I think if you have a CALayer with a CGImage, you should be able to continuously draw over the layer itself, and the final "merging" of the layer with its image will only be done when you "draw" (or otherwise "output") an image from that. I don't know how to do any of this (or I wouldn't be writing this) but I think that might work?
In anyway, be kind to add your own answer hopefully with little helpful code sample. Thanks
