TurboJpeg.Wrapper 3.2.0-pre.2

TurboJpeg.Wrapper

A .NET wrapper around the libjpeg-turbo TurboJPEG API. The managed package TurboJpeg.Wrapper (namespace TurboJpegWrapper) contains no native code; add exactly one native package for the deployment runtime.

• Decodes JPEG to complete 8-, 24-, or 32-bit BMP files: 2- to 16-bit RGB, YCbCr, grayscale, CMYK, and YCCK input; scaling, cropping, EXIF auto-orientation.
• Encodes uncompressed 1-, 4-, 8-, 16-, 24-, and 32-bit BMP files to JPEG: quality, subsampling, colorspace, lossless, progressive, arithmetic coding, restart markers, density.

Installation

Native libraries ship as separate packages for each supported runtime. Add the managed package and the native package for the deployment runtime; the example below uses Windows x64:

dotnet add package TurboJpeg.Wrapper --prerelease
dotnet add package TurboJpeg.Wrapper.Native.win-x64 --prerelease

Available packages:

If the matching native package is missing, the first decode, encode, or native-handle creation throws TurboJpegException with the expected native library paths.

Requirements

• .NET 10 or later.
• Windows x64/ARM64, Linux x64/ARM64, or macOS x64/ARM64.
• Bundled native library: libjpeg-turbo 3.2.0.
• Linux: glibc 2.17 or later. musl/Alpine requires a musl libjpeg-turbo build supplied through TURBOJPEG_NATIVE_LIBRARY_PATH.
• macOS: 10.7 or later on x64, 11 or later on ARM64.

Quick start

using TurboJpegWrapper;

byte[] jpeg = File.ReadAllBytes("photo.jpg");
byte[] bmp = TurboJpeg.DecodeToBmp(jpeg);
File.WriteAllBytes("photo.bmp", bmp);

byte[] reencoded = TurboJpeg.EncodeToJpeg(bmp);
File.WriteAllBytes("photo.jpg", reencoded);

Decoding returns a complete BMP file, not raw pixels. Encoding accepts a complete BMP file, not raw pixel data, and returns a complete JPEG file.

Decoding

BMP samples are always 8 bits per channel. Lossy, lossless, progressive, and arithmetic-coded JPEGs are supported with any chroma subsampling scheme TurboJPEG reports.

Static facade

TurboJpeg is thread-safe.

byte[] bmp = TurboJpeg.DecodeToBmp(jpeg);
byte[] scaled = TurboJpeg.DecodeToBmp(
    jpeg,
    new TurboJpegDecodeOptions { ScalingFactor = new TurboJpegScalingFactor(1, 2) });

TurboJpegImageInfo info = TurboJpeg.GetImageInfo(jpeg);
ReadOnlySpan<TurboJpegScalingFactor> factors = TurboJpeg.GetSupportedScalingFactors();

int size = TurboJpeg.GetBmpSize(jpeg);
byte[] buffer = new byte[size];
int written = TurboJpeg.DecodeToBmp(jpeg, TurboJpegDecodeOptions.Default, buffer);

GetBmpSize performs the same validation as decoding and returns the exact BMP size. The span overload returns the number of bytes written; the destination must not overlap the JPEG source, and overlap throws ArgumentException.

Instance decoder

Use an instance to keep one native handle on a thread. Instances are not thread-safe.

using var decoder = new TurboJpegDecoder();
byte[] bmp = decoder.DecodeToBmp(jpeg);
TurboJpegImageInfo info = decoder.GetImageInfo(jpeg);

int size = decoder.GetBmpSize(jpeg);
byte[] buffer = new byte[size];
int written = decoder.DecodeToBmp(jpeg, TurboJpegDecodeOptions.Default, buffer);

All decode and size methods throw ObjectDisposedException after the decoder is disposed.

Options

TurboJpegDecodeOptions is an immutable record; use with to derive modified options, for example TurboJpegDecodeOptions.Default with { AutoOrient = true }. All limits are disabled by 0.

Cropping

TurboJpegDecodeOptions.Crop decodes a rectangular region instead of the full image. Coordinates refer to the scaled, stored image: scaling is applied first, then the crop, then AutoOrient.

• X must be divisible by the scaled iMCU width: 8 for 4:4:4, 4:4:0, 4:4:1, and grayscale; 16 for 4:2:2, 4:2:0, and 2x4; 32 for 4:1:1 and 4:1:0. Y does not need to be aligned.
• The region must fit inside the scaled image and have positive width and height.
• Only lossy JPEGs with a known chroma subsampling scheme can be cropped.

Orientations 5-8 transpose the crop width and height. GetBmpSize returns the cropped BMP size. Invalid crops throw ArgumentException.

Precision and CMYK

BMP output always uses 8-bit channels. JPEG precision 2-16 is accepted; precisions 2-7, 9-11, and 13-15 are mapped exactly to 8 bits, and 12- and 16-bit output is within 1 LSB of exact rounding.

Lossless RGB to Bpp8 is converted to luma. Lossless GRAY to Bpp24 or Bpp32 is expanded to B = G = R with alpha 255. Both work at every precision and with AutoOrient.

CMYK and YCCK are decoded as four channels and converted best-effort to BGR, BGRA, or luma. Stored samples are treated as inverted Adobe CMYK regardless of the Adobe APP14 marker, so a producer that stores non-inverted CMYK renders with inverted colors. Bpp8 output is the luma of the converted BGR values.

The wrapper does not perform ICC color management.

Warnings

Truncated entropy data and extraneous bytes can produce a native warning instead of a fatal error.

• With StopOnWarning = true (default), a native warning throws a non-fatal TurboJpegException (IsFatal = false) and no BMP is returned.
• With StopOnWarning = false, warnings are ignored and a best-effort BMP is returned.
• GetImageInfo always treats warnings as errors.

BMP output

• Complete BMP file with a 54-byte BITMAPINFOHEADER; 8-bit output adds a 1024-byte palette.
• Bpp8: grayscale with a 256-entry palette; color is discarded. Bpp24: BGR. Bpp32: BGRA with alpha 255.
• Bottom-up rows (biHeight is positive), BI_RGB compression, rows padded to 4 bytes with zeros.
• Pixel data starts at 54 bytes (24/32 bpp) or 1078 bytes (8 bpp); biSizeImage is stride * height.
• biXPelsPerMeter and biYPelsPerMeter are derived from the JFIF density when present, otherwise 2835 (72 DPI). TurboJpegImageInfo exposes DensityX, DensityY, and DensityUnits.
• With AutoOrient or Crop, the header describes the resulting image; with AutoOrient, orientations 5-8 transpose width and height.

Supported inputs

• JPEG precision 2-16, including 12-bit lossy extended sequential.
• RGB, YCbCr, grayscale, CMYK, and YCCK.
• Baseline, extended sequential, progressive, arithmetic, and lossless entropy coding.
• Any chroma subsampling scheme TurboJPEG reports; cropping requires a known scheme.
• Lossless JPEG at 1/1 scaling.

A precision outside 2-16 throws TurboJpegUnsupportedException.

Without AutoOrient, decoded dimensions are the stored JPEG dimensions; with Crop, the crop dimensions. With AutoOrient, the orientation tag (0x0112) from the first EXIF APP1 segment is applied; all eight values are supported, so the result may be rotated or mirrored. Missing, malformed, or out-of-range orientation is ignored, and malformed metadata never fails decoding.

Encoding

The encoder turns uncompressed BMP files into JPEG.

Static facade

TurboJpeg is thread-safe.

byte[] jpeg = TurboJpeg.EncodeToJpeg(bmp);
byte[] tuned = TurboJpeg.EncodeToJpeg(
    bmp,
    new TurboJpegEncodeOptions { Quality = 95, Subsampling = TurboJpegSubsampling.Yuv444 });

int maxSize = TurboJpeg.GetMaxJpegSize(bmp);
byte[] buffer = new byte[maxSize];
int written = TurboJpeg.EncodeToJpeg(bmp, TurboJpegEncodeOptions.Default, buffer);
File.WriteAllBytes("photo.jpg", buffer[..written]);

GetMaxJpegSize validates the BMP and returns the worst-case JPEG size for the image and effective subsampling; the actual JPEG is usually smaller. The span overload requires a destination of at least that size, writes the JPEG into it, and returns the number of bytes written. The destination must not overlap the BMP source; overlap throws ArgumentException.

Instance encoder

Use an instance to keep one native handle on a thread. Optional default options apply to the parameterless overloads; explicit options replace them for that call. Instances are not thread-safe.

using var encoder = new TurboJpegEncoder();
byte[] jpeg = encoder.EncodeToJpeg(bmp);

using var tuned = new TurboJpegEncoder(
    new TurboJpegEncodeOptions { Quality = 85, Progressive = true });
byte[] progressive = tuned.EncodeToJpeg(bmp);
byte[] oneOff = tuned.EncodeToJpeg(
    bmp,
    new TurboJpegEncodeOptions { Quality = 70 });

int maxSize = encoder.GetMaxJpegSize(bmp);
byte[] buffer = new byte[maxSize];
int written = encoder.EncodeToJpeg(bmp, TurboJpegEncodeOptions.Default, buffer);

All methods throw ObjectDisposedException after Dispose.

Options

TurboJpegEncodeOptions is an immutable record. Use with to derive modified options, for example TurboJpegEncodeOptions.Default with { Quality = 75 }.

Range and enum validation runs when a value is assigned, including values that Lossless later ignores. Cross-property validation runs when encoding or sizing starts: RestartBlocks and RestartRows cannot both be non-zero, RestartBlocks must be zero with Lossless = true, and Density requires lossy YCbCr or grayscale output. Violations throw ArgumentException with parameter name options.

Colorspace = Rgb forces 4:4:4 and ignores Subsampling. Other invalid combinations are rejected by libjpeg-turbo and surface as TurboJpegException.

BMP input support

The encoder accepts complete BMP files with DIB header sizes 40, 52, 56, 108, and 124, bottom-up (positive height) or top-down (negative height), with 1, 4, 8, 16, 24, or 32 bits per pixel.

"Grayscale output requested" means Colorspace = Grayscale, or Subsampling = Grayscale when Colorspace is not explicitly Rgb. Explicit Rgb wins and forces 4:4:4. It applies only to lossy encoding; lossless mode does not convert color to grayscale.

Palette images use clrUsed entries when non-zero, otherwise 1 << bitCount entries. Indices beyond the declared entries are treated as black. Alpha and color management are not applied.

The following BMP variants are rejected:

• OS/2 core bitmaps (DIB header size 12) and any DIB header size other than 40, 52, 56, 108, or 124: TurboJpegUnsupportedException.
• RLE4/RLE8, BI_JPEG, BI_PNG, and compression other than BI_RGB/BI_BITFIELDS: TurboJpegUnsupportedException.
• BI_BITFIELDS on 24-bit bitmaps, and empty, non-contiguous, or overlapping masks: TurboJpegUnsupportedException.
• Malformed or truncated files, including a missing BM signature, non-positive width, zero or int.MinValue height, invalid planes or bit counts, offsets outside the buffer, and truncated pixel data: ArgumentException.

Density

Density is a TurboJpegDensity with X and Y from 1 to 65535 and PixelsPerInch, PixelsPerCentimeter, or Unknown units. The default struct (0 x 0) is invalid.

Density is written only for lossy YCbCr and grayscale output. Lossless and RGB output reject an explicit density and ignore the BMP density, writing 1 x 1 with Unknown units.

With Density = null and lossy YCbCr or grayscale output, the encoder derives density from the BMP biXPelsPerMeter and biYPelsPerMeter:

• A non-zero value is converted to DPI and clamped to 1-65535.
• If only one axis is non-zero, its value is used for both axes.
• With no BMP density, the encoder writes 1 x 1 with Unknown units.

An explicit Density overrides the BMP values.

Errors

• ArgumentException: empty input; invalid options (unsupported scaling factor, invalid crop, invalid restart or density combinations, invalid colorspace for encoding); malformed or truncated BMP; destination too small; destination overlapping the source.
• ArgumentNullException: a null options object.
• ArgumentOutOfRangeException: negative crop offset, non-positive crop dimension, invalid BMP bit depth, or an option or density value outside its allowed range or not supported for the requested direction.
• TurboJpegUnsupportedException: image precision outside 2-16 bits, or a BMP encoding the library does not support.
• TurboJpegImageTooLargeException: MaxPixels is exceeded, or the BMP, worst-case JPEG, or a conversion buffer would exceed Array.MaxLength.
• TurboJpegException: native error, native-library load failure, or a native warning with the direction's StopOnWarning enabled.
• ObjectDisposedException: an instance method was called after Dispose.
• PlatformNotSupportedException: no known runtime identifier for the current OS/architecture and no explicit native path.

Native library loading

Set TURBOJPEG_NATIVE_LIBRARY_PATH to the full path of a native library file to override discovery. A non-blank value is the only location used; an invalid path fails immediately.

Otherwise the resolver tries, in order:

1. runtimes/{rid}/native/{library} under AppContext.BaseDirectory.
2. On macOS only, runtimes/osx/native/{library} under AppContext.BaseDirectory.
3. {library} in AppContext.BaseDirectory.
4. The operating system's normal library search.

Threading

• TurboJpeg is safe for concurrent use.
• TurboJpegDecoder and TurboJpegEncoder are not thread-safe. Use one instance per thread or synchronize access.
• TurboJpegDecodeOptions and TurboJpegEncodeOptions are immutable and can be shared across threads and calls.

Benchmarks

Ratio and Alloc Ratio are relative to the TurboJpeg row.

Encoding

TurboJpeg.EncodeToJpeg with default options (quality 90, 4:2:0), GDI+ JPEG quality 90, and ImageSharp JpegEncoder { Quality = 90 }.

Decoding

TurboJpeg.DecodeToBmp with AutoOrient = true and Bpp24; GDI+ and ImageSharp auto-orient from EXIF and produce a 24-bit BMP as well.

GDI+ and ImageSharp do not expose the same chroma-subsampling and TurboJPEG-specific controls, so these results are throughput and allocation indicators, not a byte-identical output comparison.

The harnesses are in bench/TurboJpeg.Wrapper.Encoder.Benchmark and bench/TurboJpeg.Wrapper.Decoder.Benchmark.

License and third-party notices

The source code is licensed under the MIT License. See LICENSE.

The bundled native libraries are libjpeg-turbo 3.2.0, distributed under the BSD-3-Clause license with the Independent JPEG Group and zlib notices. See native/THIRD-PARTY-NOTICES.md.