iOS AVAssetWriter Video Height "Squished" when exported to from UIImages

Question

We are exporting video using AVAssetWriter from UIImages which are first converted to CVPixelBufferRef's - but the images are "squished" in the video by a 160 pixel high black "bar" of empty pixels across the bottom.

Given that the images are 640x480, the fact that 160 pixels of black bar are seen - it is as if the image was rotated and the bottom was lost - BUT - when we look at the video images - the same height of the image is captured - it is just "squished" to fit the black bar.

Below is our UIImage to CVBufferPixelRef code.

Any ideas what may be causing this behavior?

- (CVPixelBufferRef) pixelBufferFromCGImage: (CGImageRef) image
{
    ATHSingleton *singleton = [ATHSingleton singletons];    
    int height = singleton.screenHeight;
    int width = singleton.screenWidth;

    NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
                             [NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
                             [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
                             nil];
    CVPixelBufferRef pxbuffer = NULL;

    CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, 480,
                                          640, kCVPixelFormatType_32ARGB, (__bridge CFDictionaryRef) options,
                                          &pxbuffer);


    NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);

    CVPixelBufferLockBaseAddress(pxbuffer, 0);
    void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
    NSParameterAssert(pxdata != NULL);

    CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
    CGContextRef context = CGBitmapContextCreate(pxdata, width,
                                                 height, 8, 4*width, rgbColorSpace,
                                                 kCGImageAlphaNoneSkipFirst);
    NSParameterAssert(context);
    CGContextConcatCTM(context, CGAffineTransformMakeRotation(0));

    CGContextDrawImage(context, CGRectMake(0, -80, 480, 640), image);

    CGColorSpaceRelease(rgbColorSpace);
    CGContextRelease(context);

    CVPixelBufferUnlockBaseAddress(pxbuffer, 0);

    return pxbuffer;
}

Answer 1

We discovered the cause of this was code we were running which was adding a UIImage logo to each existing UIImage for the video. The code adding the logo was actually causing the bottom of the original image to be cut off.

It was unclear that the logo writing was the source of the problem because the result of the logo operation was an image with a white bar across the bottom. The white bar blended with the background and we didn't see that the image was getting clipped.

Once we deleted the UIImage combining code - we discovered that our video was being produced properly and without the black bar across the bottom.

We fixed the code that combined the two UIImages and wrote the logo properly and now the videos are being produced without the black bar.