Implement alpha premultiplication (#507)

* Implement alpha premultiplication

* Add benchmark to resize

* Only display "Premultiply alpha" if it's one of the rust resize types.

* Add comment about division by zero

codecs/resize/benchmark.js

@@ -0,0 +1,41 @@
+// THIS IS NOT A NODE SCRIPT
+// This is a d8 script. Please install jsvu[1] and install v8.
+// Then run `npm run --silent benchmark`.
+// [1]: https://github.com/GoogleChromeLabs/jsvu
+
+self = global = this;
+load('./pkg/resize.js');
+
+async function init() {
+  //  Adjustable constants.
+  const inputDimensions = 2000;
+  const outputDimensions = 1500;
+  const algorithm = 3; // Lanczos
+  const iterations = new Array(100);
+
+  // Constants. Don’t change.
+  const imageByteSize = inputDimensions * inputDimensions * 4;
+  const imageBuffer = new Uint8ClampedArray(imageByteSize);
+
+  const module = await WebAssembly.compile(readbuffer("./pkg/resize_bg.wasm"));
+  await wasm_bindgen(module);
+  [false, true].forEach(premulti => {
+    print(`\npremultiplication: ${premulti}`);
+    print(`==============================`);
+    for (let i = 0; i < 100; i++) {
+      const start = Date.now();
+      wasm_bindgen.resize(imageBuffer, inputDimensions, inputDimensions, outputDimensions, outputDimensions, algorithm, premulti);
+      iterations[i] = Date.now() - start;
+    }
+    const average = iterations.reduce((sum, c) => sum + c) / iterations.length;
+    const stddev = Math.sqrt(
+      iterations
+        .map(i => Math.pow(i - average, 2))
+        .reduce((sum, c) => sum + c) / iterations.length
+    );
+    print(`n = ${iterations.length}`);
+    print(`Average: ${average}`);
+    print(`StdDev: ${stddev}`);
+  });
+}
+init().catch(e => console.error(e, e.stack));

codecs/resize/package.json

@@ -2,6 +2,7 @@
   "name": "resize",
   "scripts": {
     "build:image": "docker build -t squoosh-resize .",
-    "build": "docker run --rm -v $(pwd):/src squoosh-resize ./build.sh"
+    "build": "docker run --rm -v $(pwd):/src squoosh-resize ./build.sh",
+    "benchmark": "v8 --no-liftoff --no-wasm-tier-up ./benchmark.js"
   }
 }

codecs/resize/pkg/resize.d.ts

@@ -6,6 +6,7 @@
 * @param {number} arg3 
 * @param {number} arg4 
 * @param {number} arg5 
+* @param {boolean} arg6 
 * @returns {Uint8Array} 
 */
-export function resize(arg0: Uint8Array, arg1: number, arg2: number, arg3: number, arg4: number, arg5: number): Uint8Array;
+export function resize(arg0: Uint8Array, arg1: number, arg2: number, arg3: number, arg4: number, arg5: number, arg6: boolean): Uint8Array;

codecs/resize/pkg/resize.js

@@ -46,13 +46,14 @@
     * @param {number} arg3
     * @param {number} arg4
     * @param {number} arg5
+    * @param {boolean} arg6
     * @returns {Uint8Array}
     */
-    __exports.resize = function(arg0, arg1, arg2, arg3, arg4, arg5) {
+    __exports.resize = function(arg0, arg1, arg2, arg3, arg4, arg5, arg6) {
         const ptr0 = passArray8ToWasm(arg0);
         const len0 = WASM_VECTOR_LEN;
         const retptr = globalArgumentPtr();
-        wasm.resize(retptr, ptr0, len0, arg1, arg2, arg3, arg4, arg5);
+        wasm.resize(retptr, ptr0, len0, arg1, arg2, arg3, arg4, arg5, arg6);
         const mem = getUint32Memory();
         const rustptr = mem[retptr / 4];
         const rustlen = mem[retptr / 4 + 1];

codecs/resize/pkg/resize_bg.d.ts

@@ -3,4 +3,4 @@ export const memory: WebAssembly.Memory;
 export function __wbindgen_global_argument_ptr(): number;
 export function __wbindgen_malloc(a: number): number;
 export function __wbindgen_free(a: number, b: number): void;
-export function resize(a: number, b: number, c: number, d: number, e: number, f: number, g: number, h: number): void;
+export function resize(a: number, b: number, c: number, d: number, e: number, f: number, g: number, h: number, i: number): void;

codecs/resize/src/lib.rs

@@ -22,12 +22,13 @@ cfg_if! {
 #[wasm_bindgen]
 #[no_mangle]
 pub fn resize(
-    input_image: Vec<u8>,
+    mut input_image: Vec<u8>,
     input_width: usize,
     input_height: usize,
     output_width: usize,
     output_height: usize,
     typ_idx: usize,
+    premultiply: bool,
 ) -> Vec<u8> {
     let typ = match typ_idx {
         0 => Type::Triangle,
@@ -36,7 +37,19 @@ pub fn resize(
         3 => Type::Lanczos3,
         _ => panic!("Nope"),
     };
+    let num_input_pixels = input_width * input_height;
     let num_output_pixels = output_width * output_height;
+
+    if premultiply {
+        for i in 0..num_input_pixels {
+            for j in 0..3 {
+                input_image[4 * i + j] = ((input_image[4 * i + j] as f32)
+                    * (input_image[4 * i + 3] as f32)
+                    / 255.0) as u8;
+            }
+        }
+    }
+
     let mut resizer = resize::new(
         input_width,
         input_height,
@@ -48,5 +61,19 @@ pub fn resize(
     let mut output_image = Vec::<u8>::with_capacity(num_output_pixels * 4);
     output_image.resize(num_output_pixels * 4, 0);
     resizer.resize(input_image.as_slice(), output_image.as_mut_slice());
+
+    if premultiply {
+        for i in 0..num_output_pixels {
+            for j in 0..3 {
+                // We don’t need to worry about division by zero, as division by zero
+                // is well-defined on floats to return `±Inf`. ±Inf is converted to 0
+                // when casting to integers.
+                output_image[4 * i + j] = ((output_image[4 * i + j] as f32) * 255.0
+                    / (output_image[4 * i + 3] as f32))
+                    as u8;
+            }
+        }
+    }
+
     return output_image;
 }