CODEWITHSUNDEEP
iosdrawingalpha
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Reputation: 11

iOS: Overlay two images with Alpha offscreen

sorry, for this question, I know there is a similar question, but I can not get the answer to work. Probably some dumb error on my side ;-)

I want to overlay two images with Alpha on iOS. The images taken from two videos, read by an AssetReader and stored in two CVPixelBuffer. I know that the Alpha channel is not stored in the video, so I get it from a third file. All data looks fine. The Problem is the overlay, if I do it onscreen with [CIContext drawImage] everything is fine ! But if I do it offscreen because the format of the video is not identical to the screen format, I can not get it to work: 1. drawImage does work, but only on-screen 2. render:toCVPixelBuffer works, but ignores Alpha 3. CGContextDrawImage seems to do nothing at all (not even an error message)

So can somebody give me an idea what is wrong:

Init: ...(a lot of code before) Setup color space and bitmap context

                   if(outputContext)
                    {
                        CGContextRelease(outputContext);
                        CGColorSpaceRelease(outputColorSpace);
                    }
                    outputColorSpace = CGColorSpaceCreateDeviceRGB();
                    outputContext =   CGBitmapContextCreate(CVPixelBufferGetBaseAddress(pixelBuffer), videoFormatSize.width, videoFormatSize.height, 8, CVPixelBufferGetBytesPerRow(pixelBuffer), outputColorSpace,(CGBitmapInfo) kCGBitmapByteOrderDefault |kCGImageAlphaPremultipliedFirst);

... (a lot code after)

Drawing:

CIImage *backImageFromSample;
CGImageRef frontImageFromSample;
CVImageBufferRef nextImageBuffer = myPixelBufferArray[0];
CMSampleBufferRef sampleBuffer = NULL;
CMSampleTimingInfo timingInfo;

//draw the frame
CGRect toRect;
toRect.origin.x = 0;
toRect.origin.y = 0;
toRect.size = videoFormatSize;

//background image always full size, this part seems to work
if(drawBack)
{
    CVPixelBufferLockBaseAddress( backImageBuffer,  kCVPixelBufferLock_ReadOnly );
    backImageFromSample = [CIImage imageWithCVPixelBuffer:backImageBuffer];
    [coreImageContext render:backImageFromSample toCVPixelBuffer:nextImageBuffer bounds:toRect colorSpace:rgbSpace];
    CVPixelBufferUnlockBaseAddress( backImageBuffer,  kCVPixelBufferLock_ReadOnly );
}
else
    [self clearBuffer:nextImageBuffer];
//Front image doesn't seem to do anything
if(drawFront)
{
    unsigned long int numBytes = CVPixelBufferGetBytesPerRow(frontImageBuffer)*CVPixelBufferGetHeight(frontImageBuffer);
    CVPixelBufferLockBaseAddress( frontImageBuffer,  kCVPixelBufferLock_ReadOnly );

    CGDataProviderRef provider = CGDataProviderCreateWithData(NULL, CVPixelBufferGetBaseAddress(frontImageBuffer), numBytes, NULL);
    frontImageFromSample = CGImageCreate (CVPixelBufferGetWidth(frontImageBuffer) , CVPixelBufferGetHeight(frontImageBuffer), 8, 32, CVPixelBufferGetBytesPerRow(frontImageBuffer), outputColorSpace, (CGBitmapInfo) kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst, provider, NULL, NO, kCGRenderingIntentDefault);
    CGContextDrawImage ( outputContext, inrect, frontImageFromSample);
    CVPixelBufferUnlockBaseAddress( frontImageBuffer, kCVPixelBufferLock_ReadOnly );
    CGImageRelease(frontImageFromSample);
}

Any ideas anyone ?

Upvotes: 0

Views: 299

Answers (1)

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Reputation: 11

So obviously I should stop to ask questions on stackflow. Every time I do that after hours of debugging I find the answer myself shortly afterwards. Sorry for that. The problem is in the initialisation, you can't do CVPixelBufferGetBaseAddress without locking the adresss first O_o. The adress gets NULL and this seems to be allowed, with the action then beeing not to do anything. So the correct code is:

                if(outputContext)
                {
                    CGContextRelease(outputContext);
                    CGColorSpaceRelease(outputColorSpace);
                }
                CVPixelBufferLockBaseAddress(pixelBuffer);
                outputColorSpace = CGColorSpaceCreateDeviceRGB();
                outputContext =   CGBitmapContextCreate(CVPixelBufferGetBaseAddress(pixelBuffer), videoFormatSize.width, videoFormatSize.height, 8, CVPixelBufferGetBytesPerRow(pixelBuffer), outputColorSpace,(CGBitmapInfo) kCGBitmapByteOrderDefault |kCGImageAlphaPremultipliedFirst);
                CVPixelBufferUnlockBaseAddress(pixelBuffer);

Upvotes: 1

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