#819 Toogle alpha channel reduction

src/features/processors/quantize/client/index.tsx

@@ -2,13 +2,15 @@ import { h, Component } from 'preact';
 import { Options as QuantizeOptions } from '../shared/meta';
 import * as style from 'client/lazy-app/Compress/Options/style.css';
 import {
+  inputFieldChecked,
   inputFieldValueAsNumber,
   konami,
   preventDefault,
 } from 'client/lazy-app/util';
 import Expander from 'client/lazy-app/Compress/Options/Expander';
 import Select from 'client/lazy-app/Compress/Options/Select';
 import Range from 'client/lazy-app/Compress/Options/Range';
+import Checkbox from 'client/lazy-app/Compress/Options/Checkbox';
 
 const konamiPromise = konami();
 
@@ -43,6 +45,7 @@ export class Options extends Component<Props, State> {
         options.maxNumColors,
       ),
       dither: inputFieldValueAsNumber(form.dither),
+      alphaOnly: inputFieldChecked(form.alphaOnly),
     };
     this.props.onChange(newOptions);
   };
@@ -92,6 +95,14 @@ export class Options extends Component<Props, State> {
             Dithering:
           </Range>
         </div>
+        <label class={style.optionToggle}>
+          Alpha only
+          <Checkbox
+            name="alphaOnly"
+            checked={options.alphaOnly}
+            onChange={this.onChange}
+          />
+        </label>
       </form>
     );
   }

src/features/processors/quantize/shared/meta.ts

@@ -14,10 +14,12 @@ export interface Options {
   zx: number;
   maxNumColors: number;
   dither: number;
+  alphaOnly: boolean;
 }
 
 export const defaultOptions: Options = {
   zx: 0,
   maxNumColors: 256,
   dither: 1.0,
+  alphaOnly: false,
 };

src/features/processors/quantize/worker/quantize.ts

@@ -25,16 +25,54 @@ export default async function process(
   }
 
   const module = await emscriptenModule;
+  let alpha_data: Uint8ClampedArray;
+
+  if (opts.alphaOnly) {
+    alpha_data = getAlphaData(data.data);
+  }
 
   const result = opts.zx
-    ? module.zx_quantize(data.data, data.width, data.height, opts.dither)
+    ? module.zx_quantize(
+        opts.alphaOnly ? alpha_data! : data.data,
+        data.width,
+        data.height,
+        opts.dither,
+      )
     : module.quantize(
-        data.data,
+        opts.alphaOnly ? alpha_data! : data.data,
         data.width,
         data.height,
         opts.maxNumColors,
         opts.dither,
       );
 
-  return new ImageData(result, data.width, data.height);
+  //Apply the result back to the original image if only alpha is checked
+  if (opts.alphaOnly) {
+    for (let i = 0; i < result.length; i += 4) {
+      const alpha = alpha_data![i + 3];
+      data.data[i + 3] = alpha;
+    }
+  }
+
+  return new ImageData(
+    opts.alphaOnly ? data.data : result,
+    data.width,
+    data.height,
+  );
+}
+
+/**
+ * Return a single color version of the image with alpha channel preserved
+ * @param data The original image data
+ * @returns A single color image with alpha channel preseved
+ */
+function getAlphaData(data: Uint8ClampedArray): Uint8ClampedArray {
+  let copy = new Uint8ClampedArray(data);
+  for (let i = 0; i < copy.length; i += 4) {
+    copy[i] = 0;
+    copy[i + 1] = 0;
+    copy[i + 2] = 0;
+  }
+
+  return copy;
 }