Working with Images in Kotlin: Libraries and Code Examples…

Working with Images in Kotlin: Library Overview and Practical Examples

Kotlin is a modern programming language that runs on top of the JVM. Thanks to full compatibility with Java, all standard Java libraries such as javax.imageio, java.awt, and javax.swing can be used for image processing. In this article, we will look at the main approaches to working with images in Kotlin: reading, writing, resizing, applying filters, and format conversion.

Required Libraries

To work with images in Kotlin (JVM), no additional dependencies are required — the standard JDK library is sufficient. However, for more complex tasks (e.g., processing large images or working with metadata), third-party libraries may be needed:

java.awt.image.BufferedImage — the main class for representing an image in memory.
javax.imageio.ImageIO — reading and writing images in various formats (PNG, JPEG, BMP, GIF).
java.awt.Graphics2D — drawing and transformations.
Thumbnailator (optional) — simplifies resizing and creating thumbnails.
BoofCV (optional) — a computer vision library with Kotlin support.

Reading and Writing Images

The simplest way to load an image is to use ImageIO.read().

import java.awt.image.BufferedImageimport java.io.Fileimport javax.imageio.ImageIO

fun main() {    // Reading an image    val inputFile = File("input.jpg")    val image: BufferedImage = ImageIO.read(inputFile) ?: throw Exception("Failed to load image")

    // Writing in a different format    val outputFile = File("output.png")    ImageIO.write(image, "png", outputFile)

    println("Image saved: ${outputFile.absolutePath}")}

Resizing an Image

To resize, we use Graphics2D with smoothing parameters.

import java.awt.Graphics2Dimport java.awt.RenderingHintsimport java.awt.image.BufferedImageimport javax.imageio.ImageIOimport java.io.File

fun resizeImage(inputPath: String, outputPath: String, targetWidth: Int, targetHeight: Int) {    val original: BufferedImage = ImageIO.read(File(inputPath))    val resized = BufferedImage(targetWidth, targetHeight, BufferedImage.TYPE_INT_RGB)    val g2d: Graphics2D = resized.createGraphics()

    // Enable smoothing for high-quality scaling    g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BILINEAR)    g2d.drawImage(original, 0, 0, targetWidth, targetHeight, null)    g2d.dispose()

    ImageIO.write(resized, "jpg", File(outputPath))    println("Image resized: $outputPath")}

fun main() {    resizeImage("input.jpg", "resized.jpg", 800, 600)}

Applying Filters (Blur, Contrast)

For simple effects, you can use BufferedImageOp (e.g., ConvolveOp for blur).

import java.awt.image.BufferedImageimport java.awt.image.ConvolveOpimport java.awt.image.Kernelimport javax.imageio.ImageIOimport java.io.File

fun applyBlur(inputPath: String, outputPath: String) {    val image: BufferedImage = ImageIO.read(File(inputPath))

    // 5x5 blur kernel    val size = 5    val weight = 1.0f / (size * size)    val kernelData = FloatArray(size * size) { weight }    val kernel = Kernel(size, size, kernelData)    val blurOp = ConvolveOp(kernel)

    val blurred = blurOp.filter(image, null)    ImageIO.write(blurred, "jpg", File(outputPath))}

fun main() {    applyBlur("input.jpg", "blurred.jpg")}