feat(sharp): add kernel option to image service config

.changeset/four-pots-see.md

@@ -0,0 +1,23 @@
+---
+'astro': minor
+---
+
+Adds a new, optional `kernel` configuration option to select a resize algorithm in the Sharp image service
+
+By default, Sharp resizes images with the `lanczos3` kernel. This new config option allows you to set the default resizing algorithm to any resizing option supported by [Sharp](https://sharp.pixelplumbing.com/api-resize/#resize) (e.g. `linear`, `mks2021`).
+
+Kernel selection can produce quite noticeable differences depending on various characteristics of the source image - especially drawn art - so changing the kernel gives you more control over the appearance of images on your site:
+
+```js
+export default defineConfig({
+  image: {
+    service: {
+      entrypoint: 'astro/assets/services/sharp',
+      config: {
+        kernel: "mks2021"
+      }
+  }
+})
+```
+
+This selection will apply to all images on your site, and is not yet configurable on a per-image basis. For more information, see [Sharps documentation on resizing images](https://sharp.pixelplumbing.com/api-resize/#resize).

packages/astro/src/assets/services/sharp.ts

@@ -1,4 +1,4 @@
-import type { FitEnum, FormatEnum, SharpOptions } from 'sharp';
+import type { FitEnum, FormatEnum, ResizeOptions, SharpOptions } from 'sharp';
 import { AstroError, AstroErrorData } from '../../core/errors/index.js';
 import type { ImageFit, ImageOutputFormat, ImageQualityPreset } from '../types.js';
 import {
@@ -13,6 +13,11 @@ export interface SharpImageServiceConfig {
 	 * The `limitInputPixels` option passed to Sharp. See https://sharp.pixelplumbing.com/api-constructor for more information
 	 */
 	limitInputPixels?: SharpOptions['limitInputPixels'];
+
+	/**
+	 * The `kernel` option is passed to resize calls. See https://sharp.pixelplumbing.com/api-resize/ for more information
+	 */
+	kernel?: ResizeOptions['kernel'];
 }
 
 let sharp: typeof import('sharp');
@@ -57,6 +62,7 @@ const sharpService: LocalImageService<SharpImageServiceConfig> = {
 	async transform(inputBuffer, transformOptions, config) {
 		if (!sharp) sharp = await loadSharp();
 		const transform: BaseServiceTransform = transformOptions as BaseServiceTransform;
+		const kernel = config.service.config.kernel;
 
 		// Return SVGs as-is
 		// TODO: Sharp has some support for SVGs, we could probably support this once Sharp is the default and only service.
@@ -81,18 +87,21 @@ const sharpService: LocalImageService<SharpImageServiceConfig> = {
 			result.resize({
 				width: Math.round(transform.width),
 				height: Math.round(transform.height),
+				kernel: kernel,
 				fit,
 				position: transform.position,
 				withoutEnlargement,
 			});
 		} else if (transform.height && !transform.width) {
 			result.resize({
 				height: Math.round(transform.height),
+				kernel: kernel,
 				withoutEnlargement,
 			});
 		} else if (transform.width) {
 			result.resize({
 				width: Math.round(transform.width),
+				kernel: kernel,
 				withoutEnlargement,
 			});
 		}

packages/astro/src/types/public/config.ts

@@ -1313,6 +1313,20 @@ export interface AstroUserConfig<
 		 * Set `false` to bypass the default image size limit for the Sharp image service and process large images.
 		 */
 
+		/**
+		 * @docs
+		 * @name image.service.config.kernel
+		 * @kind h4
+		 * @type {string | undefined}
+		 * @default `undefined`
+		 * @version 5.17.0
+		 * @description
+		 *
+		 * The default [kernel used for resizing images](https://sharp.pixelplumbing.com/api-resize/#resize) in the Sharp image service.
+		 *
+		 * By default this is `undefined`, which maps to Sharp's default kernel of `lanczos3`.
+		 */
+
 		/**
 		 * @docs
 		 * @name image.domains