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.
This is the original image I am using. 200 x 40 pixels, PNG-24 ARGB premultiplied alpha (All alpha values == 255):
This is the result (screenshot from Simulator), when I do not modify the pixels. Looks good:
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.
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);
Next, I draw the image into that bitmapContext, and obtain the data like this:
void *data = CGBitmapContextGetData(bitmapContext);
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++;
}
}
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];
Question:
What am I doing wrong?
Is this happening because I modify the data while iterating over it? Must I duplicate it?
