I'm trying to change the color of the pixels (lighten or darken) without changing the value of the alpha channel using CGDataProviderCopyData. I leave every 4th databyte untouched. It work fine of the iphone simulator, however on the real thing the alpha goes white as I increase the values of the other pixels. I've tried changing just the first byte, or the second, or the third. Does anybody have any idea what is going on? The basic code is borrowed from Jorge. I like this simple approach --I'm new to this. But I want to make it work with png images with some transparency.

here is most of the code by Jorge :

CFDataRef CopyImagePixels(CGImageRef inImage){ return CGDataProviderCopyData(CGImageGetDataProvider(inImage)); }

CGImageRef img=originalImage.CGImage; CFDataRef dataref=CopyImagePixels(img); UInt8 *data=(UInt8 *)CFDataGetBytePtr(dataref); int length=CFDataGetLength(dataref); for(int index=0;index255){ data[index+i]=255; }else{ data[index+i]+=value; } } }

}
size_t width=CGImageGetWidth(img);
size_t height=CGImageGetHeight(img);
size_t bitsPerComponent=CGImageGetBitsPerComponent(img);
size_t bitsPerPixel=CGImageGetBitsPerPixel(img);
size_t bytesPerRow=CGImageGetBytesPerRow(img);
CGColorSpaceRef colorspace=CGImageGetColorSpace(img);
CGBitmapInfo bitmapInfo=CGImageGetBitmapInfo(img);
CGImageAlphaInfo alphaInfo = kCGBitmapAlphaInfoMask(img);
NSLog(@"bitmapinfo:  %d",bitmapInfo);
CFDataRef newData=CFDataCreate(NULL,data,length);
CGDataProviderRef provider=CGDataProviderCreateWithCFData(newData);
CGImageRef newImg=CGImageCreate(width,height,bitsPerComponent,bitsPerPixel,bytesPerRow,colorspace,bitmapInfo,provider,NULL,true,kCGRenderingIntentDefault);
[iv setImage:[UIImage imageWithCGImage:newImg]];
CGImageRelease(newImg);
CGDataProviderRelease(provider);