Possible for AVAssetWriter to write files with transparency? - ios

Question

Every file I write with AVAssetWriter has a black background, if the images I include do not fill the entire render area. Is there any way to write with transparency? Here's the method I use to get the pixel buffer:
- (CVPixelBufferRef)pixelBufferFromCGImage:(CGImageRef)image {
CGSize size = self.renderSize;
NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey,
[NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey,
nil];
CVPixelBufferRef pxbuffer = NULL;
CVPixelBufferPoolCreatePixelBuffer(NULL, self.adaptor.pixelBufferPool, &pxbuffer);
CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault,
size.width,
size.height,
kCVPixelFormatType_32ARGB,
(__bridge CFDictionaryRef) options,
&pxbuffer);
if (status != kCVReturnSuccess){
NSLog(#"Failed to create pixel buffer");
}
CVPixelBufferLockBaseAddress(pxbuffer, 0);
void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pxdata, size.width,
size.height, 8, 4*size.width, rgbColorSpace,
(CGBitmapInfo)kCGImageAlphaPremultipliedFirst);
CGContextConcatCTM(context, CGAffineTransformMakeRotation(0));
CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image),
CGImageGetHeight(image)), image);
CGColorSpaceRelease(rgbColorSpace);
CGContextRelease(context);
CVPixelBufferUnlockBaseAddress(pxbuffer, 0);
return pxbuffer;
}
And the AVAssetWriter code:
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
AVVideoCodecH264, AVVideoCodecKey,
[NSNumber numberWithInt:self.renderSize.width], AVVideoWidthKey,
[NSNumber numberWithInt:self.renderSize.height], AVVideoHeightKey,
nil];
AVAssetWriterInput* videoWriterInput = [AVAssetWriterInput
assetWriterInputWithMediaType:AVMediaTypeVideo
outputSettings:videoSettings];
NSDictionary *bufferAttributes = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithInt:kCVPixelFormatType_32ARGB], kCVPixelBufferPixelFormatTypeKey, nil];
self.adaptor = [AVAssetWriterInputPixelBufferAdaptor
assetWriterInputPixelBufferAdaptorWithAssetWriterInput:videoWriterInput
sourcePixelBufferAttributes:bufferAttributes];
buffer = [self pixelBufferFromCGImage:[stickerImage CGImage]];
BOOL append_ok = YES;
int j = 0;
while (append_ok && j < totalFrames) {
if (self.adaptor.assetWriterInput.readyForMoreMediaData) {
CMTime frameTime = CMTimeMake(frameCount,(int32_t) fps);
append_ok = [self.adaptor appendPixelBuffer:buffer withPresentationTime:frameTime];
if(!append_ok){
NSError *error = self.assetWriter.error;
if(error!=nil) {
NSLog(#"Unresolved error %#,%#.", error, [error userInfo]);
}
}
}
else {
printf("adaptor not ready %d, %d\n", frameCount, j);
[NSThread sleepForTimeInterval:0.1];
}
j++;
frameCount++;
}
if (!append_ok) {
printf("error appending image %d times %d\n, with error.", frameCount, j);
}

Answer 1

In short, This is not possible. AVAssetWriter outputs using the h264 video codec. The h264 video codec does not support an alpha channel, so you are unfortunately out of luck.
A more involved solution would involve rendering the alpha channel (transparency) into a grayscale h264 encoded video and compositiong the rub and alpha information back together when viewing.