Alpha masking module (#1546)

* Mask module added

* add mask module

* add test for alpha masking

* update Readme

Co-authored-by: h <f>
Co-authored-by: Harsh Khandeparkar <34770591+HarshKhandeparkar@users.noreply.github.com>
Co-authored-by: Jeffrey Warren <jeff@unterbahn.com>

src/Modules.js

@@ -31,6 +31,7 @@ module.exports = {
   'gradient': require('./modules/Gradient'),
   'grid-overlay': require('./modules/GridOverlay'),
   'import-image': require('./modules/ImportImage'),
+  'mask': require('./modules/Mask'),
   'minify-image': require('./modules/MinifyImage'),
   // 'invert': require('image-sequencer-invert'),
   'invert': require('./modules/Invert'),

src/modules/Mask/Module.js

@@ -0,0 +1,82 @@
+module.exports = function Mask(options, UI, util) {
+  var defaults = require('./../../util/getDefaults.js')(require('./info.json'));
+  options.offset = options.offset || defaults.offset;
+  options.resize = options.resize || defaults.resize;
+
+  var output;
+
+  // This function is called on every draw.
+  function draw(input, callback, progressObj) {
+    progressObj.stop(true);
+    progressObj.overrideFlag = true;
+
+    var step = this;
+
+    var getPixels = require('get-pixels');
+
+    // convert offset as string to int
+    if (typeof options.offset === 'string')
+      options.offset = parseInt(options.offset);
+
+    // save first image's pixels
+    var priorStep = this.getStep(options.offset);
+
+    if (priorStep.output === undefined) {
+      this.output = input;
+      UI.notify('Offset Unavailable', 'offset-notification');
+      callback();
+    }
+
+    const alpha_masking = function(c, a) {
+      return (a * c + (255 - a) * c) / 255;
+    };
+    const internalSequencer = ImageSequencer({ inBrowser: false, ui: false });
+    internalSequencer.loadImage(priorStep.output.src, function() {
+      internalSequencer.importJSON([{ 'name': 'resize', 'options': { resize: options.resize } }]);
+      internalSequencer.run(function onCallback(internalOutput) {
+
+        getPixels(internalOutput, function(err, pixels) {
+          options.firstImagePixels = pixels;
+
+          function changePixel(r2, g2, b2, a2, x, y) {
+            let p = options.firstImagePixels;
+            let r1 = p.get(x, y, 0),
+              g1 = p.get(x, y, 1),
+              b1 = p.get(x, y, 2),
+              a1 = p.get(x, y, 3);
+
+            return [alpha_masking(r1, a2), alpha_masking(g1, a2), alpha_masking(b1, a2)];
+          }
+
+          function output(image, datauri, mimetype, wasmSuccess) {
+            step.output = {
+              src: datauri,
+              format: mimetype,
+              wasmSuccess,
+              useWasm: options.useWasm
+            };
+          }
+
+          // run PixelManipulatin on second image's pixels
+          return require('../_nomodule/PixelManipulation.js')(input, {
+            output: output,
+            ui: options.step.ui,
+            changePixel: changePixel,
+            format: input.format,
+            image: options.image,
+            inBrowser: options.inBrowser,
+            callback: callback,
+            useWasm: options.useWasm
+          });
+        });
+      });
+    });
+  }
+
+  return {
+    options: options,
+    draw: draw,
+    output: output,
+    UI: UI
+  };
+};

src/modules/Mask/index.js

@@ -0,0 +1,4 @@
+module.exports = [
+  require('./Module'),
+  require('./info.json')
+];

src/modules/Mask/info.json

@@ -0,0 +1,18 @@
+{
+    "name": "mask",
+    "description": "Masks two images according to their Alpha values",
+    "inputs": {
+      "offset": {
+        "type": "integer",
+        "desc": "Choose which image to mask the current image with. Two steps back is -2, three steps back is -3 etc.",
+        "default": -2
+      },
+      "resize": {
+        "type": "string",
+        "desc": "Percentage value by which first image is to be resized",
+        "default": "125%"
+      }
+    },
+    "docs-link":"https://github.com/publiclab/image-sequencer/blob/main/docs/MODULES.md"
+  }
+