iphone - Can I edit the pixels of the UIImage's property CGImage

Question

UIImage has a read-only property CGImage. I have to read its pixels to a memory block and edit them and then make a new UIImage to replace the old one. I want to know if there is a way bypass the read-only property and edit those pixels directly.

Thanks.

---

Thanks all. I have found a way to do it. Write a class with those method:

-(void)preProcess:(UIImage*)srcImage {
    m_Context = ...// Created by calling CGBitmapContextCreate(...)
    ...
    CGContextDrawImage(m_Context, rect, srcImage.CGImage);
    m_Bits = (unsigned char*)CGBitmapContextGetData (mContext);
}

-(void)postProcess {
    CGContextRelease(m_Context);
    free(m_Bits);
}

-(UIImage*)doProcess:(CGPoint)pt {// just a example 
    unsigned char* ppxl = m_Bits + ...
    // do something...
    CGImageRef imRef = CGBitmapContextCreateImage(mContext);
    return [UIImage imageWithCGImage:imRef];
}

And preProcess and postProcess are called just once.

Answer 1

You cannot get at the original pixels. However, you can get a copy. One option is to do what Matt suggested, and convert it into a PNG/JPG - though remember, the image is now compressed, and you will be manipulating the compressed file and not the pixels directly.

If you want to get at a copy of the raw pixels, you can do something like:

UIImage* image = ...; // An image
NSData* pixelData = (NSData*) CGDataProviderCopyData(CGImageGetDataProvider(image.CGImage));
void* pixelBytes = [pixelData bytes];

// Take away the red pixel, assuming 32-bit RGBA
for(int i = 0; i < [pixelData length]; i += 4) {
    bytes[i] = 0; // red
    bytes[i+1] = bytes[i+1]; // green
    bytes[i+2] = bytes[i+2]; // blue
    bytes[i+3] = bytes[i+3]; // alpha
}

Now, if you wanted to make this into a new UIImage, you can do something like:

NSData* newPixelData = [NSData dataWithBytes:pixelBytes length:[pixelData length]];
UIImage* newImage = [UIImage imageWithData:newPixelData]; // Huzzah