Magick.NET

Language	C# (.NET 10 and .NET Standard 2.0)
License	Apache 2.0 (ImageMagick license for the native part)
Repository	github.com/dlemstra/Magick.NET
Documentation	docs in the repository
Depends on	native ImageMagick (bundled in the NuGet package)
Read / write	yes, all common variants including RLE and OS/2 / yes, 1-32 bit, BMP2, BMP3 and V4/V5 headers

Magick.NET puts ImageMagick into a NuGet package with the native library included for Windows, Linux and macOS. It reads and writes more than 100 formats, so it’s a good choice when BMP is one of many formats you handle.

ImageMagick has 3 BMP writers: BMP (the default) uses a V4 or V5 header when the image has alpha, gamma or a color profile, BMP3 always writes the 40-byte Windows 3 header that every program reads, and BMP2 writes the old 12-byte header. Images converted from PNG or JPEG often carry gamma information, so use BMP3 when the output must open in old software.

Installation#

Shell

# 8 bits per channel, any CPU
dotnet add package Magick.NET-Q8-AnyCPU

Basic usage: read, resize and save a BMP#

using ImageMagick;

using var image = new MagickImage("in.bmp");
Console.WriteLine($"{image.Width} x {image.Height}, {image.Depth}-bit {image.ColorType}");

image.Resize(image.Width / 2, 0);  // 0 keeps the aspect ratio
image.Write("out.bmp");

How to write a BMP that old programs can read#

MagickFormat.Bmp3 writes a classic BITMAPINFOHEADER file, 24-bit for color images:

using ImageMagick;

using var image = new MagickImage("in.png");
image.Alpha(AlphaAction.Off);           // drop transparency
image.Write("out.bmp", MagickFormat.Bmp3);

How to get BMP width and height without decoding#

using ImageMagick;

var info = new MagickImageInfo("in.bmp");
Console.WriteLine($"{info.Format}: {info.Width} x {info.Height}, compression {info.Compression}");

How to write a 16-bit BMP#

BmpWriteDefines.Subtype selects a 16-bit layout (RGB565, RGB555, ARGB1555, ARGB4444), common for embedded displays:

using ImageMagick;
using ImageMagick.Formats;

using var image = new MagickImage("in.png");
image.Write("out565.bmp", new BmpWriteDefines { Subtype = BmpSubtype.RGB565 });

How to write an 8-bit palette BMP#

using ImageMagick;

using var image = new MagickImage("in.png");
image.Quantize(new QuantizeSettings { Colors = 256 });
image.ColorType = ColorType.Palette;
image.Write("palette.bmp", MagickFormat.Bmp3);

How to convert BMP to PNG#

using ImageMagick;

using var image = new MagickImage("in.bmp");
image.Write("out.png");  // format from the extension

Tips#

BmpReadDefines { IgnoreFileSize = true } reads files whose header has a wrong file size, which some writers produce.
MagickImageCollection reads ICO files, which contain BMP or PNG images in several sizes.
Width, height and sizes are uint in Magick.NET 14 and later.