Add configurable parameter for blend module to chose image (#424)

* Add configurable parameter for blend module to chose image

* Made minor changes

* Edit test to work with new parameter

* blend offset test added

* blend offset test added

* fixed blend offset test

src/modules/Blend/Module.js

@@ -1,6 +1,7 @@
 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.offset = options.offset || -2;
 
     var output;
 
@@ -17,8 +18,11 @@ module.exports = function Dynamic(options, UI, util) {
 
         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(-2);
+        var priorStep = this.getStep(options.offset);
 
         getPixels(priorStep.output.src, function(err, pixels) {
             options.firstImagePixels = pixels;

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.");