how to add fade in/out effect in avassetwriter (img to video) in iphone

问题

I'm creating a .mov from the array of images with audio using avassetwriter and i want to add the fade in/out effect in the creating video, have anyone used any code? Thanks in advance for any help!

...
for(UIImage * img in imageArray)
    {
        //UIImage * img = frm._imageFrame;
        buffer = [self pixelBufferFromCGImage:[img CGImage]];

    BOOL append_ok = NO;
    int j = 0;
    while (!append_ok && j < 30) {
        if (adaptor.assetWriterInput.readyForMoreMediaData)  {
            //print out status:
            NSLog(@"Processing video frame (%d,%d)",frameCount,[imageArray count]);

            CMTime frameTime = CMTimeMake(frameCount*frameDuration,(int32_t) fps);
            append_ok = [adaptor appendPixelBuffer:buffer withPresentationTime:frameTime];
            if(!append_ok){
                NSError *error = videoWriter.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++;
    }
    if (!append_ok) {
        printf("error appending image %d times %d\n, with error.", frameCount, j);
    }
    frameCount++;
}...

来源：https://stackoverflow.com/questions/13560476/how-to-add-fade-in-out-effect-in-avassetwriter-img-to-video-in-iphone