Add support for WebP decompression via the browser

Author: constantinius

src/compression/index.js

@@ -3,6 +3,7 @@ import LZWDecoder from './lzw';
 import JpegDecoder from './jpeg';
 import DeflateDecoder from './deflate';
 import PackbitsDecoder from './packbits';
+import WebImageDecoder from './webimage';
 
 export function getDecoder(fileDirectory) {
   switch (fileDirectory.Compression) {
@@ -20,6 +21,8 @@ export function getDecoder(fileDirectory) {
       return new DeflateDecoder();
     case 32773: // packbits
       return new PackbitsDecoder();
+    case 50001:
+      return new WebImageDecoder();
     default:
       throw new Error(`Unknown compression method identifier: ${fileDirectory.Compression}`);
   }

src/compression/webimage.js

@@ -0,0 +1,40 @@
+import BaseDecoder from './basedecoder';
+
+/**
+ * class WebImageDecoder
+ *
+ * This decoder uses the browsers image decoding facilities to read image
+ * formats like WebP when supported.
+ */
+export default class WebImageDecoder extends BaseDecoder {
+  constructor() {
+    super();
+    if (typeof createImageBitmap === 'undefined') {
+      throw new Error('Cannot decode WebImage as `createImageBitmap` is not available');
+    } else if (typeof document === 'undefined' && typeof OffscreenCanvas === 'undefined') {
+      throw new Error('Cannot decode WebImage as neither `document` nor `OffscreenCanvas` is not available');
+    }
+  }
+
+  async decode(fileDirectory, buffer) {
+    const blob = new Blob([buffer]);
+    const imageBitmap = await createImageBitmap(blob);
+
+    let canvas;
+    if (typeof document !== 'undefined') {
+      canvas = document.createElement('canvas');
+      canvas.width = imageBitmap.width;
+      canvas.height = imageBitmap.height;
+    } else {
+      canvas = new OffscreenCanvas(imageBitmap.width, imageBitmap.height);
+    }
+
+    const ctx = canvas.getContext('2d');
+    ctx.drawImage(imageBitmap, 0, 0);
+
+    // TODO: check how many samples per pixel we have, and return RGB/RGBA accordingly
+    // it seems like GDAL always encodes via RGBA which does not require a translation
+
+    return ctx.getImageData(0, 0, imageBitmap.width, imageBitmap.height).data.buffer;
+  }
+}