WebP for .NET

Install

Two packages. Darnelix.Webp is the library; Darnelix.Webp.Cli is the webpx command-line tool, installed as a .NET tool. Both need the .NET 8, .NET 9 or .NET 10 runtime.

dotnet add package Darnelix.Webp

dotnet tool install --global Darnelix.Webp.Cli

What ships

A WebP codec written entirely in C#, implemented from the published WebP container, VP8 and VP8L specifications. It decodes and encodes WebP — lossy, lossless, near-lossless, with alpha, still and animated — as one managed assembly with no native code and zero runtime dependencies.

• Decode. Still images — lossy VP8 and lossless VP8L, with alpha — into RGBA32, BGRA32, ARGB32, RGB24 or BGR24; cropped and scaled decode; incremental decode from bytes as they arrive; header-only feature reads; animated WebP played frame by frame onto a composited canvas.

• Encode. Still images, lossy (quality, effort, target size or PSNR, alpha quality and compression, sharp YUV, loop-filter settings) or lossless (effort, near-lossless preprocessing; the RGB under transparent pixels is always preserved); animated WebP assembled from full-canvas frames with per-frame durations, a loop count and a background colour.

• Deterministic. The same input and options produce the same bytes — with or without the optional worker threads (UseThreads on the decode and encode options, off by default).

• Defined failure. A truncated, corrupt or unsupported file raises a WebpException subtype (WebpFormatException, WebpDecodeException, WebpUnsupportedFeatureException); a bad argument raises an ArgumentException.

• SIMD-accelerated DSP kernels on every framework the package carries, each proven bit-identical to the scalar reference implementation they replace.

Two API styles cover the same functionality. The idiomatic API — WebpDecoder, WebpEncoder, WebpAnimDecoder, WebpAnimEncoder, each with an options type — exposes everything; the static Webp class mirrors the shape of the reference C API's classic entry points for the common cases (Decode, EncodeLossy, EncodeLossless, GetFeatures). Container metadata (ICC, EXIF, XMP) and animation parameters are edited with the command-line tool.

Quick start

Read a header without decoding, decode to RGBA32, encode lossless and lossy, then build the options yourself — the package README's quick start, as written.

using System;
using System.IO;
using Darnelix.Webp;
using Darnelix.Webp.Animation;
using Darnelix.Webp.Decoding;
using Darnelix.Webp.Encoding;

byte[] file = File.ReadAllBytes("input.webp");

// Header only — no pixels decoded; false (never an exception) when the bytes are not a WebP header.
if (WebpDecoder.TryGetInfo(file, out WebpImageInfo info))
{
    Console.WriteLine($"{info.Width}x{info.Height} {info.Format} alpha={info.HasAlpha} animated={info.HasAnimation}");
}

// Decode. The image owns a pooled pixel buffer: dispose it when you are done with the pixels.
using (WebpImage image = WebpDecoder.Decode(file))
{
    Span<byte> rgba = image.Pixels;   // RGBA32, top-down, 4 bytes per pixel, no row padding
    Console.WriteLine($"{image.Width}x{image.Height} {image.Format}, {rgba.Length} bytes");
}

// Encode RGBA32 pixels. The Webp facade applies the recommended defaults (effort 4, loop-filter
// strength 60, lossless alpha at effort 100); EncodeLossy takes the quality.
int width = 64, height = 64;
byte[] pixels = new byte[width * height * 4];   // RGBA32, top-down
byte[] lossless = Webp.EncodeLossless(pixels, width, height);
byte[] lossy = Webp.EncodeLossy(pixels, width, height, 85f);
File.WriteAllBytes("output.webp", lossy);

// For anything else, build the options yourself and set every field you rely on: a new
// WebpEncodeOptions starts at zero throughout, and zero means "off" for the loop filter and the
// alpha effort. Nothing is filled in for you.
var options = new WebpEncodeOptions { Quality = 85, Method = 4, FilterStrength = 60, AlphaQuality = 100, SharpYuv = true };
byte[] tuned = WebpEncoder.Encode(pixels, width, height, PixelFormat.Rgba32, options);

Animated WebP, frame by frame:

using (var animation = new WebpAnimDecoder(file))
{
    Console.WriteLine($"{animation.Info.CanvasWidth}x{animation.Info.CanvasHeight}, {animation.Info.FrameCount} frames");
    while (animation.TryGetNextFrame(out WebpFrame frame))
    {
        // frame.Pixels is the full composited canvas (RGBA32); frame.DurationMs is its display time.
    }
}

The webpx command-line tool

The same codec drives webpx, packaged separately as Darnelix.Webp.Cli: encode, decode, inspect, edit container metadata and animation parameters, and build or extract animations from the shell.

webpx encode photo.ppm photo.webp -q 90

webpx encode

image → .webp — lossy or lossless, alpha, target size or PSNR

webpx decode

.webp → image — pixel layout, scale, crop, flip

webpx info

print the header features

webpx mux

set or strip ICC, EXIF and XMP, the loop count and the background colour

webpx demux

list chunks and frames; extract metadata

webpx anim build

frames → animated .webp

webpx anim extract

animated .webp → frames and durations

webpx version

print the codec version

Licence

The packages are free of charge. They are not open source. In plain words, for the packages as published by Darnelix on nuget.org, you may:

• use them in any application, service or workflow, commercial or not;
• distribute the unmodified Darnelix.Webp assembly inside your application;
• copy and redistribute the unmodified packages, for example through mirrors, caches and private feeds.

You may not:

• modify, decompile or reverse engineer them, except where applicable law expressly permits it;
• sell, sublicense or rent the packages or their contents on their own, or offer them as part of a product whose main purpose is to provide WebP encoding or decoding to others as a library or tool;
• use the library, or any assembly from the packages, in a Unity project, the Unity Editor or a Unity build — that use is licensed only through WebP for Unity on the Unity Asset Store. Running the webpx tool as a separate program on a Unity project's files is allowed.

The full terms are in the LICENSE file packed with each package.

Answers

The problems this tool was built for, answered in full — with the code you would write yourself.

• Convert WebP to PNG or JPEG, and back, in C#

  Decode the WebP to raw pixels with a WebP codec and write them with a PNG or JPEG encoder; to go the other way, read the image to pixels and encode those as WebP.

  Darnelix.Webp 1.1.0 · .NET 8 – 10

• Load a WebP into a System.Drawing Bitmap or a WPF image in C#

  System.Drawing cannot read or write WebP, and WPF reads it only when a Microsoft Store extension is installed, so decode the file to BGRA pixels yourself.

  Darnelix.Webp 1.1.0 · .NET 8 – 10 · Windows 11 25H2

Using Unity? WebP for Unity is the same codec with an importer, components and samples.

Support

Questions and bug reports go to support@darnelix.com . Replies usually within a couple of business days.