JNI wrapper for libimagequant
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README.md

libimagequant-java

libimagequant-java is a JNI wrapper around the C API for libimagequant, enabling you to quantize 32-bit RGBA images.

libimagequant-java is kept intentionally simple, only operating on byte[] arrays and direct ByteBuffer instances.

libimagequant-java the required native shared library and automatically loads the library compatible with your operating system. No messing around with -Djava.library.path. libimagequant-java supports Windows 32- & 64-bit, Linux 32- & 64-bit as well as macOS out of the box.

Installation

libimagequant-java is published to Maven Central. You can include it in your pom.xml as a dependency as follows:

<dependency>
	<groupId>com.badlogicgames</groupId>
	<artifactId>libimagequant-java</artifactId>
	<version>1.0</version>
</dependency>

To include it in your Gradle project, ensure your build.gradle file adds the mavenCentral() repository. Then add the dependency:

compile "com.badlogicgames:libimagequant-java:1.0"

libimagequant-java is also build on every new commit by Jenkins and published as a SNAPSHOT release to SonaType.

Usage

The below code loads a 32-bit RGBA PNG file, quantizes it to 8-bit and then writes it back out to a new file.

new SharedLibraryLoader().load("imagequant-java");

// Read the input image.
BufferedImage input = ImageIO.read(LibImageQuantTest.class.getResourceAsStream("/input.png"));
byte[] pixels = ((DataBufferByte)input.getRaster().getDataBuffer()).getData();

// ABGR -> RGBA.
for (int i = 0; i < pixels.length; i += 4) {
	byte a = pixels[i];
	byte b = pixels[i + 1];
	byte g = pixels[i + 2];
	byte r = pixels[i + 3];
	pixels[i] = r;
	pixels[i + 1] = g;
	pixels[i + 2] = b;
	pixels[i + 3] = a;
}

// Setup libimagequant and quantize the image.
LiqAttribute attribute = new LiqAttribute();
LiqImage image = new LiqImage(attribute, pixels, input.getWidth(), input.getHeight(), 0);
LiqResult result = image.quantize();

// Based on the quantization result, generate an 8-bit indexed image and retrieve its palette.
byte[] quantizedPixels = new byte[input.getWidth() * input.getHeight()];
image.remap(result, quantizedPixels);
LiqPalette palette = result.getPalette();

// The resulting 8-bit indexed image and palette could be written out to an indexed PNG or GIF, but instead we convert it
// back to 32-bit RGBA.
BufferedImage convertedImage = new BufferedImage(input.getWidth(), input.getHeight(), BufferedImage.TYPE_4BYTE_ABGR);
byte[] convertedPixels = ((DataBufferByte)convertedImage.getRaster().getDataBuffer()).getData();
int size = input.getWidth() * input.getHeight();
for (int i = 0, j = 0; i < size; i++, j += 4) {
	int index = quantizedPixels[i] & 0xff; // Java's byte is signed
	int color = palette.getColor(index);
	convertedPixels[j] = LiqPalette.getA(color);
	convertedPixels[j + 1] = LiqPalette.getB(color);
	convertedPixels[j + 2] = LiqPalette.getG(color);
	convertedPixels[j + 3] = LiqPalette.getR(color);
}

ImageIO.write(convertedImage, "png", new File("output.png"));

// Good practice to immediately destroy the native resources but not necessary. If the GC cleans up the Java side object the
// native side will be destroyed as well.
result.destroy();
image.destroy();
attribute.destroy();

Refer to the libimagequant documentation for more information.

Working from source

libimagequant-java includes the upstream libimagequant repository as a submodule in jni/libimagequant. Make sure to clone this repository recursively to also check out the submodule:

git clone --recursive https://github.com/badlogic/libimagequant-java

libimagequant-java is a plain old Maven project and can be imported into any Maven aware IDE. Before importing, run

mvn clean compile

from a terminal. This will download the latest native shared libraries from Jenkins so you do not have to build them yourself. After the above Maven invocation, the native shared libraries will be located in src/main/resources.

Building the native libraries

To build the native shared libraries yourself, you can use the included jni/build-docker.sh script for Windows 32-bit, Windows 64-bit, Linux 32-bit and Linux 64-bit. Install Docker CE, then in your terminal:

cd jni/
./build-docker.sh

This will build a Docker image with all required toolchains for Windows and Linux, compile the libimagequant-java shared libraries and put them in src/main/resources.

Finally, if you want to build a shared library for your specific host, use the jni/build.sh script:

/build.sh --target=<target-os-arch> --build=<release|debug>

Valid values for the --target parameter to pass to build.sh are macosx, windows-x86, windows-x86_64, linux-64 and linux-x86_64.

Valid values for the --build parameter are release and debug.

To build locally, all required toolchains must be installed; Xcode with command line tools on macOS, Mingw64 on Windows (or Linux for cross-compilation) and GCC on Linux.

Modifying the C++ JNI code

libimagequant-java uses gdx-jnigen to generate C++ code embedded in the Java files. When you modify the embedded C++ code, you have to re-generate the C++ files in jni/src/ by running the CodeGenerator class.

License

AGPL 3, See LICENSE. For alternative licensing options contact the libimagequant author.