Why doesn't CGImage data survive a round trip?

Question

I have a freehand drawing view (users can draw lines with their finger). I only use a few colors, so I have a compression algorithm that I wrote (want to send it over a local network to another iPad) but I can't seem to get the data out of the graphics context accurately, even with this simple test:

//Get the data
UIGraphicsBeginImageContextWithOptions(self.bounds.size, NO, 0.0f);
CGContextRef c = UIGraphicsGetCurrentContext();
[self.layer renderInContext:c];
baseImageView.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

CGImageRef imageRef = baseImageView.image.CGImage;
NSData *dataToUse = (NSData *)CGDataProviderCopyData(CGImageGetDataProvider(imageRef));

//Reuse the data
CGDataProviderRef provider = CGDataProviderCreateWithCFData((CFDataRef)dataToUse);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGImageRef test = CGImageCreate(width,height,8,32,4*width,colorSpace,
kCGBitmapByteOrder32Big|kCGImageAlphaPremultipliedLast,provider,NULL,false,
kCGRenderingIntentDefault); //I get width and height from another part of the program

imageView.image = [UIImage imageWithCGImage:test];

I simply copied out the data from one CGImage and tried to insert it into another. However, the result is garbage and not only that, for some reason it comes out as BGRA when I copy the data, but CGImageCreate wants RGBA. Where am I going wrong with this round-trip test?

borrrden borrrden · Accepted Answer · 2012-05-30T07:43:18

Looks like the answer is that it's not enough to just get the data provider. You need to actually render the image into a bitmap context and take the data from there. Revised way:

//Get the data
CGImageRef imageRef = baseImageView.image.CGImage;
size_t height = CGImageGetHeight(imageRef);
size_t width = CGImageGetWidth(imageRef);
size_t bufferLength = width * height * 4;
unsigned char *rawData = (unsigned char *)malloc(bufferLength);

CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(bitmapData, width, height, 8,
4*width, colorSpace, kCGImageAlphaPremultipliedLast);
CGColorSpaceRelease(colorSpace);

CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);

NSData *dataToUse = [NSData dataWithBytes:rawData length:bufferLength];

//Later free(rawData);

Using the data is still the same.

Why doesn't CGImage data survive a round trip?

1 Answers