Add function to apply mask to RawImage.

src/utils/image.js

@@ -1,10 +1,13 @@
+/**
+ * @typedef {import('./tensor.js').DataArray} DataArray
+ */
 
 /**
- * @file Helper module for image processing. 
- * 
- * These functions and classes are only used internally, 
+ * @file Helper module for image processing.
+ *
+ * These functions and classes are only used internally,
  * meaning an end-user shouldn't need to access anything here.
- * 
+ *
  * @module utils/image
  */
 
@@ -91,7 +94,7 @@ export class RawImage {
         this.channels = channels;
     }
 
-    /** 
+    /**
      * Returns the size of the image (width, height).
      * @returns {[number, number]} The size of the image (width, height).
      */
@@ -101,9 +104,9 @@ export class RawImage {
 
     /**
      * Helper method for reading an image from a variety of input types.
-     * @param {RawImage|string|URL} input 
+     * @param {RawImage|string|URL} input
      * @returns The image object.
-     * 
+     *
      * **Example:** Read image from a URL.
      * ```javascript
      * let image = await RawImage.read('https://huggingface.co/datasets/Xenova/transformers.js-docs/resolve/main/football-match.jpg');
@@ -181,7 +184,7 @@ export class RawImage {
 
     /**
      * Helper method to create a new Image from a tensor
-     * @param {Tensor} tensor 
+     * @param {Tensor} tensor
      */
     static fromTensor(tensor, channel_format = 'CHW') {
         if (tensor.dims.length !== 3) {
@@ -304,6 +307,46 @@ export class RawImage {
         return this._update(newData, this.width, this.height, 4);
     }
 
+    /**
+     * Apply an alpha mask to the image.
+     * @param {RawImage} mask The mask to apply. Values should be between 0 and 255, and be a single channel.
+     * @returns {Promise<RawImage>} The masked image.
+     * @throws {Error} If the mask is not the same size as the image.
+     * @throws {Error} If the image does not have 4 channels.
+     * @throws {Error} If the mask is not a single channel.
+     */
+    async putAlpha(mask) {
+        if (mask.width !== this.width || mask.height !== this.height) {
+            throw new Error('Mask must be the same size as the image');
+        }
+
+        // We want the current image to have an alpha channel, but the mask will
+        // just be a single channel.
+        if (this.channels !== 4) {
+            throw new Error('Image must have 4 channels');
+        }
+        if (mask.channels !== 1) {
+            throw new Error('Mask must have 1 channel');
+        }
+
+        const numPixels = this.width * this.height;
+        for (let i = 0; i < numPixels; i++) {
+            const maskPixel = mask.data[i];
+            if (typeof maskPixel === 'undefined') {
+                throw new Error('Invalid mask');
+            }
+
+            // Ensure that the alpha is a range from 0 to 255, and not a range
+            // from 0 to 1.
+            const alpha = maskPixel < 1 ? maskPixel * 255 : maskPixel;
+            // Calculate the offset, multiplying by 4 because of the number of
+            // channels, then offset by 3 to get the alpha channel.
+            this.data[(i * 4) + 3] = alpha;
+        }
+
+        return this;
+    }
+
     /**
      * Resize the image to the given dimensions. This method uses the canvas API to perform the resizing.
      * @param {number} width The width of the new image.
@@ -355,7 +398,7 @@ export class RawImage {
                 case 'nearest':
                 case 'bilinear':
                 case 'bicubic':
-                    // Perform resizing using affine transform. 
+                    // Perform resizing using affine transform.
                     // This matches how the python Pillow library does it.
                     img = img.affine([width / this.width, 0, 0, height / this.height], {
                         interpolator: resampleMethod
@@ -368,7 +411,7 @@ export class RawImage {
                     img = img.resize({
                         width, height,
                         fit: 'fill',
-                        kernel: 'lanczos3', // PIL Lanczos uses a kernel size of 3 
+                        kernel: 'lanczos3', // PIL Lanczos uses a kernel size of 3
                     });
                     break;
 
@@ -447,7 +490,7 @@ export class RawImage {
             // Create canvas object for this image
             const canvas = this.toCanvas();
 
-            // Create a new canvas of the desired size. This is needed since if the 
+            // Create a new canvas of the desired size. This is needed since if the
             // image is too small, we need to pad it with black pixels.
             const ctx = createCanvasFunction(crop_width, crop_height).getContext('2d');
 
@@ -495,7 +538,7 @@ export class RawImage {
             // Create canvas object for this image
             const canvas = this.toCanvas();
 
-            // Create a new canvas of the desired size. This is needed since if the 
+            // Create a new canvas of the desired size. This is needed since if the
             // image is too small, we need to pad it with black pixels.
             const ctx = createCanvasFunction(crop_width, crop_height).getContext('2d');
 
@@ -742,4 +785,4 @@ export class RawImage {
             }
         });
     }
-}
+}