Add WebP 'exact' option for control over transparent pixels

lib/constructor.js

@@ -350,6 +350,7 @@ const Sharp = function (input, options) {
     webpEffort: 4,
     webpMinSize: false,
     webpMixed: false,
+    webpExact: false,
     gifBitdepth: 8,
     gifEffort: 7,
     gifDither: 1,

lib/index.d.ts

@@ -1394,11 +1394,13 @@ declare namespace sharp {
         /** Level of CPU effort to reduce file size, integer 0-6 (optional, default 4) */
         effort?: number | undefined;
         /** Prevent use of animation key frames to minimise file size (slow) (optional, default false) */
-        minSize?: boolean;
+        minSize?: boolean | undefined;
         /** Allow mixture of lossy and lossless animation frames (slow) (optional, default false) */
-        mixed?: boolean;
+        mixed?: boolean | undefined;
         /** Preset options: one of default, photo, picture, drawing, icon, text (optional, default 'default') */
         preset?: keyof PresetEnum | undefined;
+        /** Preserve the colour data in transparent pixels (optional, default false) */
+        exact?: boolean | undefined;
     }
 
     interface AvifOptions extends OutputOptions {

lib/output.js

@@ -749,6 +749,7 @@ function png (options) {
  * @param {number|number[]} [options.delay] - delay(s) between animation frames (in milliseconds)
  * @param {boolean} [options.minSize=false] - prevent use of animation key frames to minimise file size (slow)
  * @param {boolean} [options.mixed=false] - allow mixture of lossy and lossless animation frames (slow)
+ * @param {boolean} [options.exact=false] - preserve the colour data in transparent pixels
  * @param {boolean} [options.force=true] - force WebP output, otherwise attempt to use input format
  * @returns {Sharp}
  * @throws {Error} Invalid options
@@ -801,6 +802,9 @@ function webp (options) {
     if (is.defined(options.mixed)) {
       this._setBooleanOption('webpMixed', options.mixed);
     }
+    if (is.defined(options.exact)) {
+      this._setBooleanOption('webpExact', options.exact);
+    }
   }
   trySetAnimationOptions(options, this.options);
   return this._updateFormatOut('webp', options);