Fast lossless encoding of SKBitmap images

Question

I'm trying to store large 4096x3072 SKBitmap images with lossless compression as fast as I can. I've tried storing them as PNG using SKImage.FromBitmap(bitmap).Encode(SKEncodedImageFormat.Png, 100) but this was really slow. Then using information from this question and this example code I made a method to store them as a Tiff image, which was a lot faster but still not fast enough for my purposes. The code has to work on Linux as well. This is my current code:

public static class SKBitmapExtensions
{
    public static void SaveToPng(this SKBitmap bitmap, string filename)
    {
        using (Stream s = File.OpenWrite(filename))
        {
            SKData d = SKImage.FromBitmap(bitmap).Encode(SKEncodedImageFormat.Png, 100);
            d.SaveTo(s);
        }
    }
    public static void SaveToTiff(this SKBitmap img, string filename)
    {
        using (var tifImg = Tiff.Open(filename, "w"))
        {
            // Set the tiff information
            tifImg.SetField(TiffTag.IMAGEWIDTH, img.Width);
            tifImg.SetField(TiffTag.IMAGELENGTH, img.Height);
            tifImg.SetField(TiffTag.COMPRESSION, Compression.LZW);
            tifImg.SetField(TiffTag.PHOTOMETRIC, Photometric.RGB);
            tifImg.SetField(TiffTag.ROWSPERSTRIP, img.Height);
            tifImg.SetField(TiffTag.BITSPERSAMPLE, 8);
            tifImg.SetField(TiffTag.SAMPLESPERPIXEL, 4);
            tifImg.SetField(TiffTag.XRESOLUTION, 1);
            tifImg.SetField(TiffTag.YRESOLUTION, 1);
            tifImg.SetField(TiffTag.PLANARCONFIG, PlanarConfig.CONTIG);
            tifImg.SetField(TiffTag.EXTRASAMPLES, 1, new short[] { (short)ExtraSample.UNASSALPHA });

            // Copy the data
            byte[] bytes = img.Bytes;

            // Swap red and blue
            convertSamples(bytes, img.Width, img.Height);

            // Write the image into the memory buffer
            for (int i = 0; i < img.Height; i++)
                tifImg.WriteScanline(bytes, i * img.RowBytes, i, 0);
        }
    }
    private static void convertSamples(byte[] data, int width, int height)
    {
        int stride = data.Length / height;
        const int samplesPerPixel = 4;

        for (int y = 0; y < height; y++)
        {
            int offset = stride * y;
            int strideEnd = offset + width * samplesPerPixel;

            for (int i = offset; i < strideEnd; i += samplesPerPixel)
            {
                byte temp = data[i + 2];
                data[i + 2] = data[i];
                data[i] = temp;
            }
        }
    }
}

And the test code:

SKBitmap bitmap = SKBitmap.Decode("test.jpg");

Stopwatch stopwatch = new();
stopwatch.Start();

int iterations = 20;
for (int i = 0; i < iterations; i++)
    bitmap.SaveToTiff("encoded.tiff");

stopwatch.Stop();
Console.WriteLine($"Average Tiff encoding time for a {bitmap.Width}x{bitmap.Height} image = {stopwatch.ElapsedMilliseconds / iterations} ms");

stopwatch.Restart();

for (int i = 0; i < iterations; i++)
    bitmap.SaveToPng("encoded.png");

stopwatch.Stop();
Console.WriteLine($"Average PNG encoding time for a {bitmap.Width}x{bitmap.Height} image = {stopwatch.ElapsedMilliseconds / iterations} ms");

As a result I get:

Average Tiff encoding time for a 4096x3072 image = 630 ms
Average PNG encoding time for a 4096x3072 image = 3092 ms

Is there any faster way to store these images? I can imagine that I can avoid copying the data at var bytes = img.Bytes but I'm not sure how. The encoded file size for the PNG is 10.3MB and for the Tiff it is 26MB now.

score 2 · Answer 1 · answered Jul 14 '22 at 15:27

If you are not so interested in making the most optimal png (from a file size point of view) then you can get access to some faster encoding options through:

SKBitmap bitmap = SKBitmap.Decode("test.jpg");
using(var pixmap= bitmap.PeekPixels())
{
    var filters = SKPngEncoderFilterFlags.NoFilters;
    int compress = 0;
    var options = new SKPngEncoderOptions(filters, compress);
    using (var data = pixmap.Encode(options))
    {
        byte[] bytes = data.ToArray();
        // use data - write bytes to file etc
    }
}

In the above example:

compress = 0 will use no zlib compression so the pngs will effectively be similar size to an uncompressed TIFF. You could try a higher value for compress (I think 9 is maximum but slowest).
filters = SKPngEncoderFilterFlags.NoFilters will be fastest under all scenarios. It could make files produced with compress != 0 larger in file size. The filters option is used to try to improve compressibility at the file with SKPngEncoderFilterFlags.AllFilters producing potentially the most compressed file.

Fast lossless encoding of SKBitmap images

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