Question

I’m trying to do something very simple:

1) Draw a UIImage into a CG bitmap context

2) Get a pointer to the data of the image

3) iterate over all pixels and just set all R G B components to 0 and alpha to 255. The result should appear pure black.

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This is the original image I am using. 200 x 40 pixels, PNG-24 ARGB premultiplied alpha (All alpha values == 255):

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This is the result (screenshot from Simulator), when I do not modify the pixels. Looks good:

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This is the result, when I do the modifications! It looks like if the modification was incomplete. But the for-loops went over EVERY single pixel. The counter proves it: Console reports modifiedPixels = 8000 which is exactly 200 x 40 pixels. It looks always exactly the same.

Note: The PNG image I use has no alpha < 255. So no transparent pixels.

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This is how I create the context. Nothing special…

int bitmapBytesPerRow = (width * 4);
int bitmapByteCount = (bitmapBytesPerRow * imageHeight);

colorSpace = CGColorSpaceCreateDeviceRGB();

bitmapData = malloc(bitmapByteCount);
bitmapContext = CGBitmapContextCreate(bitmapData,
                width,
                height,
                8, // bits per component
                bitmapBytesPerRow,
                colorSpace,
                CGImageAlphaPremultipliedFirst);

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Next, I draw the image into that bitmapContext, and obtain the data like this:

void *data = CGBitmapContextGetData(bitmapContext);

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This is the code which iterates over the pixels to modify them:

size_t bytesPerRow = CGImageGetBytesPerRow(img);

NSInteger modifiedPixels = 0;
for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
        long int offset = bytesPerRow * y + 4 * x;

        // ARGB
        unsigned char alpha = data[offset];
        unsigned char red = data[offset+1];
        unsigned char green = data[offset+2];
        unsigned char blue = data[offset+3];

        data[offset] = 255;
        data[offset+1] = 0;
        data[offset+2] = 0;
        data[offset+3] = 0;

        modifiedPixels++;
    }
}

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When done, I obtain a new UIImage from the bitmap context and display it in a UIImageView, to see the result:

CGImageRef imageRef = CGBitmapContextCreateImage(bitmapContext);
UIImage *img = [[UIImage alloc] initWithCGImage:imageRef];

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Question:

What am I doing wrong?

Is this happening because I modify the data while iterating over it? Must I duplicate it?

Answer 1

Use CGBitmapContextGetBytesPerRow(bitmapContext) to get bytesPerRow instead getting from image (image has only 3 bytes per pixels if it hasn’t alpha informations)