feat: Parse GeoTIFF ColorMap tag

packages/deck.gl-geotiff/src/geotiff.ts

@@ -97,3 +97,33 @@ function addAlphaChannel(rgbImage: TypedArrayWithDimensions): ImageData {
     );
   }
 }
+
+/**
+ * Parse the GeoTIFF `ColorMap` tag into an ImageData.
+ *
+ * @param   {Uint16Array}  cmap  The colormap array from the GeoTIFF `ColorMap` tag.
+ *
+ * @return  {ImageData}          The parsed colormap as an ImageData object.
+ */
+export function parseColormap(cmap: Uint16Array): ImageData {
+  // TODO: test colormap handling on a 16-bit image with 2^16 entries?
+  const size = cmap.length / 3;
+  const rgba = new Uint8ClampedArray(size * 4);
+
+  const rOffset = 0;
+  const gOffset = size;
+  const bOffset = size * 2;
+
+  // Note: >> 8 is needed to convert from 16-bit to 8-bit color values
+  // It just divides by 256 and floors to nearest integer
+  for (let i = 0; i < size; i++) {
+    rgba[4 * i + 0] = cmap[rOffset + i]! >> 8;
+    rgba[4 * i + 1] = cmap[gOffset + i]! >> 8;
+    rgba[4 * i + 2] = cmap[bOffset + i]! >> 8;
+
+    // Full opacity
+    rgba[4 * i + 3] = 255;
+  }
+
+  return new ImageData(rgba, size, 1);
+}

packages/deck.gl-geotiff/src/index.ts

@@ -7,7 +7,7 @@ export {
   extractGeotiffReprojectors,
   fromGeoTransform,
 } from "./geotiff-reprojection.js";
-export { loadRgbImage } from "./geotiff.js";
+export { loadRgbImage, parseColormap } from "./geotiff.js";
 
 export * as proj from "./proj.js";
 export * as texture from "./texture.js";