Save pixels dynamically created and retrieve it back successfully - iphone

Question

I am trying to insert pixels dynamically to a PNG image file and try to retrieve it back without any alteration to pixels I saved using the code below. But I'm not successful in doing so. Can someone help me point where the problem is? Thanks.
// Original pixels for debugging only
NSString *sPixels = #"12345678";
const char *cpPixels8 = sPixels.UTF8String;
char *cpPixelsStore = calloc(sPixels.length + 1, 1);
strncpy(cpPixelsStore, cpPixels8, sPixels.length);
unsigned int r,g,b,a;
for(int j = 0; j < sPixels.length; j += 4)
{
r = cpPixelsStore[j+0];
g = cpPixelsStore[j+1];
b = cpPixelsStore[j+2];
a = cpPixelsStore[j+3];
printf("r:0x%X g:0x%X b:0x%X a:0x%X\n", r, g, b, a);
}
int width = 2;
int height = 1;
int bytesPerRow = 4 * width;
int bitsPerComponent = 8;
int bitsPerPixel = 32;
CGColorSpaceRef colorSpaceRef = CGColorSpaceCreateDeviceRGB();
CGBitmapInfo bitmapInfo = kCGBitmapByteOrder32Host | kCGImageAlphaLast;
CGColorRenderingIntent renderingIntent = kCGRenderingIntentDefault;
CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, cpPixelsStore, (bytesPerRow * height), NULL);
CGImageRef imRef = CGImageCreate(width, height, bitsPerComponent, bitsPerPixel, bytesPerRow, colorSpaceRef, bitmapInfo, provider, NULL, NO, renderingIntent);
CGDataProviderRelease(provider);
UIImage *imNewTemp = [UIImage imageWithCGImage:imRef];
NSData *datPNG = UIImagePNGRepresentation(imNewTemp);
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *sFile = [documentsDirectory stringByAppendingPathComponent:#"Pic.png"];
[datPNG writeToFile:sFile atomically:YES];
CGImageRelease(imRef);
// Cross verify save
UIImage *imTemp = [UIImage imageWithContentsOfFile:sFile];
NSData *datImagePixels = (__bridge NSData *)CGDataProviderCopyData(CGImageGetDataProvider(imTemp.CGImage));
unsigned char *ucpPixelBytes = (unsigned char *)[datImagePixels bytes];
for(int j = 0; j < datImagePixels.length; j += 4)
{
r = ucpPixelBytes[j+0];
g = ucpPixelBytes[j+1];
b = ucpPixelBytes[j+2];
a = ucpPixelBytes[j+3];
printf("r:0x%X g:0x%X b:0x%X a:0x%X\n", r, g, b, a);
}
Initial printf returns this during creation:
r:0x31 g:0x32 b:0x33 a:0x34
r:0x35 g:0x36 b:0x37 a:0x38
printf after saving and retrieving the file gives this output:
r:0xA g:0xA b:0xA a:0x31
r:0xC g:0xB b:0xB a:0x35
I'm lost in translation. Please help.

Answer 1

NSData is immutable - you need a NSMutableData object, then you use 'mutableBytes' to get the pointer. NSMutableData *nData = [NSMutableData dataWithData:datImagePixels];
Now you have the data to create a NEW image with your modifications. You can use the Quartz method CGImageCreate() to get a CGImageRef, and from that a UIImage.