Length of data returned from CGImageGetDataProvider is larger than expected

Question

I'm loading a grayscale png image and I want to access the underlying pixel data. However after I load get the pixel data via CGImageGetDataProvider, the length of the data returned is longer than expected.

CCGDataProviderRef provider = CGDataProviderCreateWithFilename(cStr);

CGImageRef image = CGImageCreateWithPNGDataProvider(provider, NULL, FALSE, kCGRenderingIntentDefault);

mapWidth = CGImageGetWidth(image); mapHeight = CGImageGetHeight(image);

lookupMap = CGDataProviderCopyData(CGImageGetDataProvider(image));

mapWidth comes out to 1804 and mapHeight comes out to 1005. The product of which is 1813020

When I call

CFDataGetLength(lookupMap)

the response is 1833120.

Where are these extra 20100 bytes coming from? Any help here is much appreciated. Am I missing something about the underlying format of the image?

Upon further examination of the CFDataRef I found that if I loop through the buffer, for each row bytes: 0 to 1803 will be correct from my image, and then the next 20 bytes are all zero. So this means that my returned image is actually coming back as a 1824 by 1005 image instead of 1804 by 1005. Still no explanation as to why.

Answer 1

There's a buffer being added to the end of each one of my rows. I started using

CGImageGetBytesPerRow

and solved the mystery.