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

The short answer is no. However, you say you have to make a copy anyhow, so why not just get an NSData object and manipulate its bytes.

From the Apple docs on UIImage:

Because image objects are immutable, they also do not provide direct access to their underlying image data. However, you can get an NSData object containing either a PNG or JPEG representation of the image data using the UIImagePNGRepresentation and UIImageJPEGRepresentation functions.

To get the data as a PNG, use:

NSData * UIImagePNGRepresentation (
   UIImage *image
);

for JPEG, use:

NSData * UIImageJPEGRepresentation (
   UIImage *image,
   CGFloat compressionQuality
);

Answer 2

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

Answer 3

This may help you out.

When you're done, you can use CGImageCreate to create a CGImageRef, then use +[UIImage imageWithCGImage:] to create a new UIImage.

Answer 4

For the more obtuse among us (read: future me) here's some working code based on Itay and Dave R's answers. It starts with a UIImage and ends with a modified UIImage:

    // load image
    UIImage *image      = [UIImage imageNamed:@"test.png"];
    CGImageRef imageRef = image.CGImage;
    NSData *data        = (NSData *) CGDataProviderCopyData(CGImageGetDataProvider(imageRef));
    char *pixels        = (char *)[data bytes];

    // manipulate the individual pixels
    for(int i = 0; i < [data length]; i += 4)
    {
        int r = i;
        int g = i+1;
        int b = i+2;
        int a = i+3;

        pixels[r]   = 0;
        pixels[g]   = pixels[g];
        pixels[b]   = pixels[b];
        pixels[a]   = pixels[a];
    }

    // create a new image from the modified pixel data
    size_t width                    = CGImageGetWidth(imageRef);
    size_t height                   = CGImageGetHeight(imageRef);
    size_t bitsPerComponent         = CGImageGetBitsPerComponent(imageRef);
    size_t bitsPerPixel             = CGImageGetBitsPerPixel(imageRef);
    size_t bytesPerRow              = CGImageGetBytesPerRow(imageRef);

    CGColorSpaceRef colorspace      = CGColorSpaceCreateDeviceRGB();
    CGBitmapInfo bitmapInfo         = CGImageGetBitmapInfo(imageRef);
    CGDataProviderRef provider      = CGDataProviderCreateWithData(NULL, pixels, [data length], NULL);

    CGImageRef newImageRef = CGImageCreate (
                              width,
                              height,
                              bitsPerComponent,
                              bitsPerPixel,
                              bytesPerRow,
                              colorspace,
                              bitmapInfo,
                              provider,
                              NULL,
                              false,
                              kCGRenderingIntentDefault
                              );
    // the modified image
    UIImage *newImage   = [UIImage imageWithCGImage:newImageRef];

    // cleanup
    free(pixels);
    CGImageRelease(imageRef);
    CGColorSpaceRelease(colorspace);
    CGDataProviderRelease(provider);
    CGImageRelease(newImageRef);