Transcode a movie to a different framerate.

gistfile1.txt

// This sketches a script that will sample images from an AVAsset at a
// new framerate. This might be used to transcode a movie or create an
// animated GIF.

import AppKit
import AVFoundation
import CoreMedia

let FPS = 12 // The target framerate for the new movie.
let frameDuration = CMTimeMake(1, Int32(FPS))

let srcURL = NSURL(fileURLWithPath: "./cat.mov", isDirectory: false)
let options: [String : AnyObject] = [AVURLAssetPreferPreciseDurationAndTimingKey: NSNumber(bool: true)]
let asset = AVURLAsset(URL: srcURL, options: options)

// Compile a list of times to sample the movie. By picking times that
// correspond to the new target framerate, we can ensure that the
// transcode movie or GIF will play smoothly.
let duration = asset.duration
var time = kCMTimeZero
var requestedTimes: [NSValue] = []
while CMTimeCompare(time, duration) <= 0 {
    requestedTimes.append(NSValue(CMTime: time))
    time = CMTimeAdd(time, frameDuration)
}

// Configure the image sampler. By default this generates images at
// their native resolution in the source, but smaller sizes can also
// be requested.
let imageGenerator = AVAssetImageGenerator(asset: asset)
imageGenerator.appliesPreferredTrackTransform = true
imageGenerator.requestedTimeToleranceBefore = kCMTimeZero
imageGenerator.requestedTimeToleranceAfter = kCMTimeZero

// Asynchronously generate and process the sampled images. The given
// callback will be called for each image in order. A semaphore is
// used to block further progress of the script until the conversion
// has finished.
//
// This should be an efficient operation, perhaps even more efficient
// than reading the movie directly from an AVAssetReader as intermediate
// frames do not need to be decoded.
var loop = 0
let semaphore = dispatch_semaphore_create(0)
imageGenerator.generateCGImagesAsynchronouslyForTimes(requestedTimes) {
    (imageTime, image, _, result, error) -> Void in

    // Make sure to count every callback request, whether or not it
    // completed successfully, so that the semaphore can be singaled
    // when all requested times have been processed.
    defer {
        loop = loop + 1
        if loop == requestedTimes.count {
            dispatch_semaphore_signal(semaphore)
        }
    }

    guard let image = image else {
        return
    }

    // DO SOMETHING WITH IMAGE
    // * Convert to CVPixelBuffer and append to an AVAssetWriter
    // * Add to CGImageDestination to create animated GIF
}

// Wait for processing to finish.
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER)