Cada archivo que escribo con AVAssetWriter tiene un fondo negro, si las imágenes que incluyo no llenan el área de renderización completa. ¿Hay alguna forma de escribir con transparencia? Aquí está el método que utilizo para obtener el buffer de píxeles:

- (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;
}

Y el código AVAssetWriter:

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);
    }