Images lose quality after saving as GIF - ios

Question

Im developing an iOS app which allows users to take a sequence of photos - afterwards the photos are put in an animation and exported as MP4 and GIF.
While the MP4 presents the source quality, the GIF color grades are visible.
Here the visual comparison:
GIF:
MP4
The code I use for exporting as GIF:
var dictFile = new NSMutableDictionary();
var gifDictionaryFile = new NSMutableDictionary();
gifDictionaryFile.Add(ImageIO.CGImageProperties.GIFLoopCount, NSNumber.FromFloat(0));
dictFile.Add(ImageIO.CGImageProperties.GIFDictionary, gifDictionaryFile);
var dictFrame = new NSMutableDictionary();
var gifDictionaryFrame = new NSMutableDictionary();
gifDictionaryFrame.Add(ImageIO.CGImageProperties.GIFDelayTime, NSNumber.FromFloat(0f));
dictFrame.Add(ImageIO.CGImageProperties.GIFDictionary, gifDictionaryFrame);
InvokeOnMainThread(() =>
{
var imageDestination = CGImageDestination.Create(fileURL, MobileCoreServices.UTType.GIF, _images.Length);
imageDestination.SetProperties(dictFile);
for (int i = 0; i < this._images.Length; i++)
{
imageDestination.AddImage(this._images[i].CGImage, dictFrame);
}
imageDestination.Close();
});
The code I use for exporting as MP4:
var videoSettings = new NSMutableDictionary();
videoSettings.Add(AVVideo.CodecKey, AVVideo.CodecH264);
videoSettings.Add(AVVideo.WidthKey, NSNumber.FromNFloat(images[0].Size.Width));
videoSettings.Add(AVVideo.HeightKey, NSNumber.FromNFloat(images[0].Size.Height));
var videoWriter = new AVAssetWriter(fileURL, AVFileType.Mpeg4, out nsError);
var writerInput = new AVAssetWriterInput(AVMediaType.Video, new AVVideoSettingsCompressed(videoSettings));
var sourcePixelBufferAttributes = new NSMutableDictionary();
sourcePixelBufferAttributes.Add(CVPixelBuffer.PixelFormatTypeKey, NSNumber.FromInt32((int)CVPixelFormatType.CV32ARGB));
var pixelBufferAdaptor = new AVAssetWriterInputPixelBufferAdaptor(writerInput, sourcePixelBufferAttributes);
videoWriter.AddInput(writerInput);
if (videoWriter.StartWriting())
{
videoWriter.StartSessionAtSourceTime(CMTime.Zero);
for (int i = 0; i < images.Length; i++)
{
while (true)
{
if (writerInput.ReadyForMoreMediaData)
{
var frameTime = new CMTime(1, 10);
var lastTime = new CMTime(1 * i, 10);
var presentTime = CMTime.Add(lastTime, frameTime);
var pixelBufferImage = PixelBufferFromCGImage(images[i].CGImage, pixelBufferAdaptor);
Console.WriteLine(pixelBufferAdaptor.AppendPixelBufferWithPresentationTime(pixelBufferImage, presentTime));
break;
}
}
}
writerInput.MarkAsFinished();
await videoWriter.FinishWritingAsync();
I would appreciate for your help!
Kind regards,
Andre

Answer 1

This is just summarization of mine comments...
I do not code on your platform so I only provide generic answer (and insights from mine own GIF encoder/decoder coding experience).
GIF image format supports up to 8bit per pixel leading to max 256 colors per pixel with naive encoding. Cheap encoders just truncates input image to 256 or less colors usually leading to ugly pixelated results. To increase coloring quality of GIF there are 3 approaches I know of:
Multiple frames covering screen with own palettes
Simply you divide image into overlays each with its own palette. This is slow (in therm of decoding as you need to process more frames per single image which can cause sync errors with some viewers and you need to process all frame related chunks multiple times per single image). The encoding itself is fast as you just either separate the frames based on colors or region/position to multiple frames. Here (region/position based) example:
The sample image is taken from here: Wiki
The GIF supports transparency so the sub frames can overlap ... This approach physically increase the colors per pixel possible to N*256 (or N*255 for transparent frames) where N is the number of frames or palettes used per single image.
Dithering
Dithering is technique that approximate color of area to match colors as closely as possible while using only specified colors (from palette) only. This is fast and easily implementable but the result is kind of noisy. For more info see some related answers of mine:
Converting BMP image to set of instructions for a plotter?
c# image dithering routine that accepts an amount of dithering?
Better color quantization method
Cheap encoders just truncate the colors to predefined palette. Much better results are obtained by clustering the used colors based on histogram. For example see:
Effective gif/image color quantization?
The result is usually much better then dithering but the encoding time is huge in comparison to dithering...
The #1 and #3 can be used together to enhance quality even more ...
If you do not have access to the encoding code or pipeline you still can transform image itself before encoding doing the quantization and palette computation instead and load the result directly to GIF encoder which should be possible (if the GIF encoder you are using is at least a bit sophisticated ...)