Add configurable parameter for blend module to chose image

src/modules/Blend/Module.js

@@ -1,63 +1,67 @@
 module.exports = function Dynamic(options, UI, util) {
 
-  options.func = options.func || "function(r1, g1, b1, a1, r2, g2, b2, a2) { return [ r1, g2, b2, a2 ] }";
+    options.func = options.func || "function(r1, g1, b1, a1, r2, g2, b2, a2) { return [ r1, g2, b2, a2 ] }";
+    options.offset = options.offset || -2;
 
-  var output;
+    var output;
 
-  // This function is called on every draw.
-  function draw(input, callback, progressObj) {
+    // This function is called on every draw.
+    function draw(input, callback, progressObj) {
 
-    progressObj.stop(true);
-    progressObj.overrideFlag = true;
+        progressObj.stop(true);
+        progressObj.overrideFlag = true;
 
-    var step = this;
+        var step = this;
 
-    // convert to runnable code:
-    if (typeof options.func === "string") eval('options.func = ' + options.func);
+        // convert to runnable code:
+        if (typeof options.func === "string") eval('options.func = ' + options.func);
 
-    var getPixels = require('get-pixels');
+        var getPixels = require('get-pixels');
 
-    // save first image's pixels
-    var priorStep = this.getStep(-2);
+        // convert offset as string to int
+        if(typeof options.offset === "string") options.offset = parseInt(options.offset);
 
-    getPixels(priorStep.output.src, function(err, pixels) {
-      options.firstImagePixels = pixels;
+        // save first image's pixels
+        var priorStep = this.getStep(options.offset);
 
-      function changePixel(r2, g2, b2, a2, x, y) {
-        // blend!
-        var p = options.firstImagePixels;
-        return options.func(
-          r2, g2, b2, a2,
-          p.get(x, y, 0),
-          p.get(x, y, 1),
-          p.get(x, y, 2),
-          p.get(x, y, 3)
-        )
-      }
+        getPixels(priorStep.output.src, function(err, pixels) {
+            options.firstImagePixels = pixels;
 
-      function output(image, datauri, mimetype) {
+            function changePixel(r2, g2, b2, a2, x, y) {
+                // blend!
+                var p = options.firstImagePixels;
+                return options.func(
+                    r2, g2, b2, a2,
+                    p.get(x, y, 0),
+                    p.get(x, y, 1),
+                    p.get(x, y, 2),
+                    p.get(x, y, 3)
+                )
+            }
 
-        // This output is accessible by Image Sequencer
-        step.output = { src: datauri, format: mimetype };
+            function output(image, datauri, mimetype) {
 
-      }
+                // This output is accessible by Image Sequencer
+                step.output = { src: datauri, format: mimetype };
 
-      // run PixelManipulatin on second image's pixels
-      return require('../_nomodule/PixelManipulation.js')(input, {
+            }
+
+            // run PixelManipulatin on second image's pixels
+            return require('../_nomodule/PixelManipulation.js')(input, {
+                output: output,
+                changePixel: changePixel,
+                format: input.format,
+                image: options.image,
+                inBrowser: options.inBrowser,
+                callback: callback
+            });
+        });
+    }
+
+    return {
+        options: options,
+        draw: draw,
         output: output,
-        changePixel: changePixel,
-        format: input.format,
-        image: options.image,
-        inBrowser: options.inBrowser,
-        callback: callback
-      });
-    });
-  }
-
-  return {
-    options: options,
-    draw: draw,
-    output: output,
-    UI: UI
-  }
+        UI: UI
+    }
 }

src/modules/Blend/info.json

@@ -1,7 +1,12 @@
 {
   "name": "Blend",
-  "description": "Blend the past two image steps with the given function. Defaults to using the red channel from image 1 and the green and blue and alpha channels of image 2. Easier to use interfaces coming soon!",
+  "description": "Blend two chosen image steps with the given function. Defaults to using the red channel from image 1 and the green and blue and alpha channels of image 2. Easier to use interfaces coming soon!",
   "inputs": {
+    "offset": {
+      "type": "integer",
+      "desc": "Choose which image to blend the current image with. Two steps back is -2, three steps back is -3 etc.",
+      "default": -2
+    },
     "blend": {
       "type": "input",
       "desc": "Function to use to blend the two images.",

test/modules/image-sequencer.js

@@ -184,10 +184,23 @@ test('getStep(offset) returns the step at offset distance relative to current st
 });
 
 test('toCliString() returns the CLI command for the sequence', function(t) {
-  t.deepEqual(sequencer.toCliString(), `sequencer -i [PATH] -s "channel channel channel channel channel invert blend" -d '{"channel":"green"}'`, "works correctly");
+  t.deepEqual(sequencer.toCliString(), `sequencer -i [PATH] -s "channel channel channel channel channel invert blend" -d '{"channel":"green","offset":-2}'`, "works correctly");
   t.end();
 });
 
+test('blend returns different output depending on the set offset', function(t) {
+    var blend_2;
+    sequencer.addSteps('test', 'invert', {});
+    sequencer.addSteps('test', 'invert', {});
+    sequencer.addSteps('test', 'blend', {});
+    // because we've added blend before, so instead of -3 we set it to -4
+    sequencer.addSteps('test', 'blend', {'offset': -4});
+    sequencer.run({ mode: 'test' }, function(out) {
+        t.notStrictEqual(out, sequencer.images.test.steps[sequencer.images.test.steps.length - 2].output.src, 'different offsets give different output');
+        t.end();
+    });
+});
+
 test('replaceImage returns false in NodeJS', function(t) {
   var returnvalue = (sequencer.options.inBrowser) ? false : sequencer.replaceImage("#selector", "test");
   t.equal(returnvalue, false, "It does.");