Problems with CGbitmapcontext and alpha - ios

Question

I'm trying to develop an draw app using Core Graphics. I want to have the background to be in alpha but instead it is in black. I tried using all the different types of bitmapinfo types with no success. kCGImageAlphaPremultipliedLast will not work either. Anyone knows how to fix this?
- (BOOL) initContext:(CGSize)size {
int bitmapByteCount;
int bitmapBytesPerRow;
// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow = (size.width * 4);
bitmapByteCount = (bitmapBytesPerRow * size.height);
// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
cacheBitmap = malloc( bitmapByteCount );
if (cacheBitmap == NULL){
return NO;
}
CGBitmapInfo bitmapInfo = kCGImageAlphaNoneSkipFirst;
cacheContext = CGBitmapContextCreate (cacheBitmap, size.width, size.height, 8, bitmapBytesPerRow, CGColorSpaceCreateDeviceRGB(), bitmapInfo);
CGContextSetRGBFillColor(cacheContext, 0, 0, 0, 0);
CGContextFillRect(cacheContext, (CGRect){CGPointZero, size});
return YES;
}

Answer 1

I'm using this code to do pretty much exactly what you're asking. I've set the color to red with a 50% alpha instead of 0% so you can actually see that the alpha channel is there.
#implementation ViewController
{
CGContextRef cacheContext;
void* cacheBitmap;
__weak IBOutlet UIImageView* _imageView;
}
- (void)viewDidLoad
{
[super viewDidLoad];
}
-(void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
[self setupContext:self.view.bounds.size];
CGImageRef cgImage = CGBitmapContextCreateImage(cacheContext);
_imageView.image = [UIImage imageWithCGImage:cgImage];
CGImageRelease(cgImage);
}
// Name changed to avoid using the magic word "init"
- (BOOL) setupContext:(CGSize)size
{
int bitmapByteCount;
int bitmapBytesPerRow;
// Declare the number of bytes per row. Each pixel in the bitmap in this
// example is represented by 4 bytes; 8 bits each of red, green, blue, and
// alpha.
bitmapBytesPerRow = (size.width * 4);
bitmapByteCount = (bitmapBytesPerRow * size.height);
// Allocate memory for image data. This is the destination in memory
// where any drawing to the bitmap context will be rendered.
cacheBitmap = malloc( bitmapByteCount );
if (cacheBitmap == NULL){
return NO;
}
CGBitmapInfo bitmapInfo = kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrderDefault;
// Create and define the color space
CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
cacheContext = CGBitmapContextCreate (cacheBitmap, size.width, size.height, 8, bitmapBytesPerRow, colorSpace, bitmapInfo);
CGContextSetRGBFillColor(cacheContext, 1., 0, 0, 0.5);
CGContextFillRect(cacheContext, (CGRect){CGPointZero, size});
// Release the color space so memory doesn't leak
CGColorSpaceRelease(colorSpace);
return YES;
}
#end