I'm writing some tests to detect changes to the lossless image formats (starting with PNG) and finding that on Linux and Windows the image loading mechanisms work as expected - but on iOS (haven't tried on macOS) the image data is always being very slightly changed if I load from a PNG file on disk or save to a PNG file on disk using Apples' methods.
If I create a PNG using any number of tools (GIMP/Paint.NET/whatever) and I use my cross platform PNG reading code to examine each pixel of the resulting loaded data - it matches exactly what I did in the tool (or programmatically generated with my cross platform PNG writing code.) Subsequent reloading into the creation tools yields the exactly same RGBA8888 components.
If I load the PNG from disk using Apple's:
NSString* pPathToFile = nsStringFromStdString( sPathToFile );
UIImage* pImageFromDiskPNG = [UIImage imageWithContentsOfFile:pPathToFile];
...then examine the resulting pixels it's similar but not the same. I would expect, like on other platforms, for the data to be identical.
Now, interestingly, if I load the data from the PNG using my code, and creating a UIImage with it (using some code I show below) I can use that UIImage and display it, copy it, whatever, and if I examine the pixel data - it's exactly what I gave it to begin with (which is why I think it's the loading saving part where Apple is modifying the image data.)
When I instruct it to save what I know to be a good UIImage with perfect pixel data, and then load that Apple saved image with my PNG loading code, I can see it's not exactly the same data. I have used several methods by which Apple suggests to save UIImage's to PNG (UIImagePNGRepresentation primarily.)
The only thing I can really think of is that Apple when loading or saving on iOS doesn't truly support RGBA8888 and is doing some sort of premultiply with the alpha channel - I speculate about this because when I first started using the code I posted below I was choosing
kCGImageAlphaLast
...instead of what I ultimately had to use
kCGImageAlphaPremultipliedLast
because the former is not supported on iOS for some reason.
Does anyone have any experience around this issue on iOS?
Cheers!
The code I use to push/pull RGBA8888 data into and out of UIImages is below:
- (unsigned char *) convertUIImageToBitmapRGBA8:(UIImage*)image dataSize:(NSUInteger*)dataSize
{
CGImageRef imageRef = image.CGImage;
// Create a bitmap context to draw the uiimage into
CGContextRef context = [self newBitmapRGBA8ContextFromImage:imageRef];
if(!context) {
return NULL;
}
size_t width = CGImageGetWidth(imageRef);
size_t height = CGImageGetHeight(imageRef);
CGRect rect = CGRectMake(0, 0, width, height);
// Draw image into the context to get the raw image data
CGContextDrawImage(context, rect, imageRef);
// Get a pointer to the data
unsigned char *bitmapData = (unsigned char *)CGBitmapContextGetData(context);
// Copy the data and release the memory (return memory allocated with new)
size_t bytesPerRow = CGBitmapContextGetBytesPerRow(context);
size_t bufferLength = bytesPerRow * height;
unsigned char *newBitmap = NULL;
if(bitmapData) {
*dataSize = sizeof(unsigned char) * bytesPerRow * height;
newBitmap = (unsigned char *)malloc(sizeof(unsigned char) * bytesPerRow * height);
if(newBitmap) { // Copy the data
for(int i = 0; i < bufferLength; ++i) {
newBitmap[i] = bitmapData[i];
}
}
free(bitmapData);
} else {
NSLog(@"Error getting bitmap pixel data\n");
}
CGContextRelease(context);
return newBitmap;
}
- (CGContextRef) newBitmapRGBA8ContextFromImage:(CGImageRef) image
{
CGContextRef context = NULL;
CGColorSpaceRef colorSpace;
uint32_t *bitmapData;
size_t bitsPerPixel = 32;
size_t bitsPerComponent = 8;
size_t bytesPerPixel = bitsPerPixel / bitsPerComponent;
size_t width = CGImageGetWidth(image);
size_t height = CGImageGetHeight(image);
size_t bytesPerRow = width * bytesPerPixel;
size_t bufferLength = bytesPerRow * height;
colorSpace = CGColorSpaceCreateDeviceRGB();
if(!colorSpace) {
NSLog(@"Error allocating color space RGB\n");
return NULL;
}
// Allocate memory for image data
bitmapData = (uint32_t *)malloc(bufferLength);
if(!bitmapData) {
NSLog(@"Error allocating memory for bitmap\n");
CGColorSpaceRelease(colorSpace);
return NULL;
}
//Create bitmap context
context = CGBitmapContextCreate( bitmapData, width, height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast|kCGBitmapByteOrder32Big );
if( !context )
{
free( bitmapData );
NSLog( @"Bitmap context not created" );
}
CGColorSpaceRelease( colorSpace );
return context;
}
- (UIImage*) convertBitmapRGBA8ToUIImage:(unsigned char*) pBuffer withWidth:(int) nWidth withHeight:(int) nHeight
{
// Create the bitmap context
const size_t nColorChannels = 4;
const size_t nBitsPerChannel = 8;
const size_t nBytesPerRow = ((nBitsPerChannel * nWidth) / 8) * nColorChannels;
CGColorSpaceRef oCGColorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGContextRef oCGContextRef = CGBitmapContextCreate( pBuffer, nWidth, nHeight, nBitsPerChannel, nBytesPerRow , oCGColorSpaceRef, kCGImageAlphaPremultipliedLast|kCGBitmapByteOrder32Big );
// create the image:
CGImageRef toCGImage = CGBitmapContextCreateImage(oCGContextRef);
UIImage* pImage = [[UIImage alloc] initWithCGImage:toCGImage];
return pImage;
}
