CGBitmapContextCreate on the iPhone/iPad

Question

I have a method that needs to parse through a bunch of large PNG images pixel by pixel (the PNGs are 600x600 pixels each). It seems to work great on the Simulator, but on th

Answer 1

Goto Product -> Edit Schemes -> Enable Zombie objects. Put a tick mark before Enable Zombie objects. Now build and run it. It can give you better and to the point description for EXC_BAD_ACCES error.

Answer 2

Maybe replace

CGColorSpaceRelease(colorSpace);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);

with

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGColorSpaceRelease(colorSpace);

as the color space ref might still be needed somehow? Just a random guess... And maybe even put it behind the release of the context?

// cleanup
CGContextRelease(context);
CGColorSpaceRelease(colorSpace);

Answer 3

Running 12 times and then crashing sounds like a running out of memory problem. It might be that internally the CGContext is creating some large autoreleased structures. Since you're doing this in a loop, they are not getting freed, so you run out of memory and die.

I'm not sure how Core Foundation deals with temporary objects though. I don't think CF objects have the equivalent of autorelease, and a Core Graphics context is almost certainly dealing with CF objects rather than NSObjects.

To reduce the memory churn in your code, I would suggest refactoring it to create an offscreen CGContext once before you start processing, and use it repeatedly to process each image. Then release it when you are done. That is going to be faster in any case (since you aren't allocating huge data structures on each pass through the loop.)

I'll wager that will eliminate your crash problem, and I bet it also makes your code much, much faster. Memory allocation is very slow compared to other operations, and you're slinging around some pretty big data structures to handle 600x600 pixel RGBA images.

Answer 4

I used a power of two aligned row size in an app that was defining the row size directly, as I was creating images programmatically. But I would also recommend toastie to try CGImageGetBytesPerRow before trying my suggestion as well.

Answer 5

I was getting a similar crash on my iPad (iPhoneOS 3.2 of course) using CGImageCreate(). Seeing your difficulty gave me a hint. I solved the problem by aligning my bytesPerRow to the next largest power of 2.

size_t bytesPerRowPower2 = (size_t) round( pow( 2.0, trunc( log((double) bytesPerRow) / log(2.0) ) + 1.0 ) );

Let us know if providing power of 2 row alignment also solves your problem. You would need to allocate *rawData with the adjusted size and pass bytesPerRowPower2 to CGBitmapContextCreate()... The height does not seem to need alignment.

Answer 6

I solved the problem by making a square context (width=height). I was having a 512x256 texture and it crashing every time I sent the data to OpenGL. Now I allocate a 512x512 buffer BUT STILL render 512x256. Hope this helps.