How to remove opacity but keep the alpha channel of UIImage? - ios

Question

I have a layer where I want the user to draw a 'mask' for cutting out images. It is semi-opaque so that they can see beneath what they are selecting.
How can I process this so that the drawing data has an alpha of 1.0, but retain the alpha channel (for masking)?
TL:DR - I'd like the black area to be a solid, single colour.
Here is the desired before and after (the white background should be transparent in both):
something like this:
for (pixel in image) {
if (pixel.alpha != 0.0) {
fill solid black
}
}

Answer 1

The following should do what you're after. Majority of the code is from How to set the opacity/alpha of a UIImage? I only added a test for the alpha value, before converting the colour of the pixel to black.
// Create a pixel buffer in an easy to use format
CGImageRef imageRef = [[UIImage imageNamed:#"testImage"] CGImage];
NSUInteger width = CGImageGetWidth(imageRef);
NSUInteger height = CGImageGetHeight(imageRef);
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
UInt8 * m_PixelBuf = malloc(sizeof(UInt8) * height * width * 4);
NSUInteger bytesPerPixel = 4;
NSUInteger bytesPerRow = bytesPerPixel * width;
NSUInteger bitsPerComponent = 8;
CGContextRef context = CGBitmapContextCreate(m_PixelBuf, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
CGContextRelease(context);
//alter the alpha when the alpha of the source != 0
int length = height * width * 4;
for (int i=0; i<length; i+=4) {
if (m_PixelBuf[i+3] != 0) {
m_PixelBuf[i+3] = 255;
}
}
//create a new image
CGContextRef ctx = CGBitmapContextCreate(m_PixelBuf, width, height,
bitsPerComponent, bytesPerRow, colorSpace,
kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
CGImageRef newImgRef = CGBitmapContextCreateImage(ctx);
CGColorSpaceRelease(colorSpace);
CGContextRelease(ctx);
free(m_PixelBuf);
UIImage *finalImage = [UIImage imageWithCGImage:newImgRef];
CGImageRelease(newImgRef);
finalImage will now contain an image where all pixels that don't have an alpha of 0.0 have alpha of 1.

Answer 2

The underlying model for this app should not be images. This is not a question of "how do I create one rendition of the image from the other."
Instead, the underlying object model should be an array of paths. Then, when you want to create the image with translucent paths vs opaque paths, it's just a question of how you render this array of paths. Once you tackle it that way, the problem is not a complex image manipulation question but a simple rendering question.
By the way, I really like this array-of-paths model, because then it becomes quite trivial to do things like "gee, let me provide an undo function, letting the user remove one stroke at a time." It opens you up to all sorts of nice functional enhancements.
In terms of specifics of how to render these paths, it can be implemented in a variety of different ways. You could use custom drawRect function for UIView subclass that renders the paths with the appropriate alpha. Or you can do it with CAShapeLayer objects, too. Or you can do some hybrid (creating new image snapshots as you finish adding each path, saving you from having to re-render all of the paths each time). There are tons of ways of tackling this.
But the key insight is to employ an underlying model of an array of paths, and then the rendering of your two types of images becomes fairly trivial exercise:
The first image is a rendering of a bunch of paths as CAShapeLayer objects with alpha of 0.5. The second is the same rendering, but with an alpha of 1.0. Again, it doesn't matter if you use shape layers or low level Core Graphics calls, but the underlying idea is the same. Either render your paths with translucency or not.