Table of Contents

JPEG-LS

JPEG-LS (ITU-T T.87, ISO/IEC 14495-1) codes 2 to 16-bit samples losslessly, or near-losslessly with every sample within a set error, by predicting each sample from its neighbours and coding the error with Golomb codes. It is quick, and it is one of the transfer syntaxes of DICOM, so medical images often carry it. It reads and writes through the Egami.Formats.Extended package: register it with WithJpegLs() or WithExtendedFormats() (Extended formats).
Reads The whole of T.87: lossless and near-lossless coding, 2 to 16 bits, 1 or more components, interleave none, line and sample, one scan or one per component, restart markers, preset coding parameters, mapping tables (palettes), subsampled components, sizes from an LSE segment or a DNL segment, and HP's colour transforms
Writes Gray, gray and alpha, RGB and RGBA at 2 to 16 bits, lossless or near-lossless, in any interleave mode, with restart markers and HP's colour transforms
Natural pixel type Gray8, GrayAlpha8, Rgb8 or Rgba8 for 8 bits and fewer; Gray16, GrayAlpha16, Rgb16 or Rgba16 above; a palette loads as its colours
FormatInfo JpegLsFormat.Info: name JPEG-LS, extension .jls, MIME types image/jls and image/x-jls 
// JPEG-LS (.jls) is lossless or near-lossless; WithExtendedFormats() registers it too.
EgamiContext context = EgamiContext.Default.WithJpegLs();
using Raster duck = Raster.Load("duck.png", context);
duck.Save("duck.jls");

// Load gives the natural pixel type: Gray8 or Gray16, GrayAlpha, Rgb or Rgba, 8 or 16 bits as stored.
using Raster back = Raster.Load("duck.jls", context);
if (back.Metadata.FormatDetails is JpegLsDetails jls)
{
    Console.WriteLine($"NEAR {jls.Near}, {jls.Interleave}, {jls.BitsPerSample} bits, {jls.Components} components, {jls.ColorTransform}");
}

JPEG-LS and JPEG both start with the SOI marker, so WithJpegLs() puts JPEG-LS ahead of the context's formats, where its detection looks for the JPEG-LS frame header; JPEG files still go to the JPEG reader.

Reading

Samples of other depths than 8 and 16 bits are scaled to the pixel's full range, so a 12-bit file loads as Gray16 with 4095 at 65535. JpegLsDetails in RasterMetadata.FormatDetails keeps the stored depth, NEAR, the interleave mode, the colour transform, the number of components and whether a palette gave the colours. When one scan holds every component at full size (sampling 1 × 1), rows decode as RasterDecoder asks for them; files with one scan per component or with subsampled components are decoded whole first, and subsampled components are repeated up to full size. Files of more than four components load their first four, with a warning.

A lenient read of a damaged file keeps what decoded. Inside a file with restart markers, a restart interval that cannot be decoded is left black and decoding goes on at the next marker (JpegLs.IntervalDamaged); without them the rows from the damage on are left transparent black (JpegLs.Damaged). A file that ends early keeps its rows with JpegLs.RowsMissing. JPEG-LS has no check sums, so damage that still decodes gives wrong samples without a warning. A strict read throws CorruptDataException instead.

Writing

JpegLsWriterOptions sets NEAR, the interleave mode, the colour transform, the bits of a sample and the restart interval. Without them a raster read from JPEG-LS is written with its own depth and NEAR again, and others losslessly at 8 or 16 bits.

// Near-lossless: every sample within 2 of the original, for a much smaller file. Restart markers every 64 lines let a
// damaged file be read again from the next one.
using (FileStream near = File.Create("duck-near.jls"))
{
    duck.Save(near, JpegLsFormat.Info, new JpegLsWriterOptions { Near = 2, RestartInterval = 64 });
}

// Lossless with HP's colour transform, which shrinks most photographs further, sample-interleaved.
using (FileStream hp = File.Create("duck-hp1.jls"))
{
    duck.Save(hp, JpegLsFormat.Info, new JpegLsWriterOptions { ColorTransform = JpegLsColorTransform.Hp1, Interleave = JpegLsInterleave.Sample });
}

// 12-bit samples from a 16-bit raster, as medical images often store them.
using Raster<Gray16> scan = duck.As<Gray16>();
using (FileStream twelve = File.Create("scan-12.jls"))
{
    scan.Save(twelve, JpegLsFormat.Info, new JpegLsWriterOptions { BitsPerSample = 12 });
}
  • NEAR 0 is lossless. NEAR 1 to 3 make photographs much smaller, with no sample more than NEAR from its original.
  • Line interleave, the default, writes rows as they come. Sample interleave does too and is a little slower; interleave none writes one scan per component and keeps the image until the end.
  • The HP transforms code RGB as differences, which shrinks most photographs. HP's software, CharLS and Egami read them; other decoders give the coded components as they are. They apply to lossless, interleaved RGB only and are ignored otherwise.

Files Egami writes decode exactly in CharLS, and lossless files are the size CharLS writes.

Not supported

JPEG-LS part 2 (ITU-T T.870: arithmetic coding, prediction and error-mapping extensions) is not read, and DICOM files themselves are not read here.

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