Merge pull request #32 from cb1kenobi/flatten-bgcolor

Added bgColor support when removing alpha from image.

README.md

@@ -115,6 +115,8 @@ As input any color type is accepted (grayscale, rgb, palette, grayscale with alp
 - `filterType` - png filtering method for scanlines (default: -1 => auto, accepts array of numbers 0-4)
 - `colorType` - the output colorType - see constants. 2 = color, no alpha, 6 = color & alpha. Default currently 6, but in the future may calculate best mode.
 - `inputHasAlpha` - whether the input bitmap has 4 bits per pixel (rgb and alpha) or 3 (rgb - no alpha).
+- `bgColor` - an object containing red, green, and blue values between 0 and 255
+that is used when an rgba image is converted to rgb (default: 255,255,255)
 
 
 ### Event "metadata"
@@ -206,6 +208,30 @@ Buffer of image pixel data. Every pixel consists 4 bytes: R, G, B, A (opacity).
 ### Property: gamma
 Gamma of image (0 if not specified)
 
+## Converting RGBA to RGB
+
+When removing the alpha channel from an image, there needs to be a background color to correctly
+convert each pixel's transparency to the appropriate RGB value. By default, pngjs will flatten
+the image against a white background. You can override this in the options:
+
+```js
+var fs = require('fs'),
+    PNG = require('pngjs').PNG;
+
+fs.createReadStream('in.png')
+    .pipe(new PNG({
+        colorType: 2,
+        bgColor: {
+            red: 0,
+            green: 255,
+            blue: 0
+        }
+    }))
+    .on('parsed', function() {
+        this.pack().pipe(fs.createWriteStream('out.png'));
+    });
+```
+
 # Sync API
 
 ## PNG.sync

lib/bitpacker.js

@@ -17,6 +17,17 @@ module.exports = function(data, width, height, options) {
   var inIndex = 0;
   var outIndex = 0;
 
+  var bgColor = options.bgColor || {};
+  if (bgColor.red === undefined) {
+    bgColor.red = 255;
+  }
+  if (bgColor.green === undefined) {
+    bgColor.green = 255;
+  }
+  if (bgColor.blue === undefined) {
+    bgColor.blue = 255;
+  }
+
   for (var y = 0; y < height; y++) {
     for (var x = 0; x < width; x++) {
       var red = data[inIndex];
@@ -26,6 +37,12 @@ module.exports = function(data, width, height, options) {
       var alpha;
       if (options.inputHasAlpha) {
         alpha = data[inIndex + 3];
+        if (!outHasAlpha) {
+          alpha /= 255;
+          red = Math.min(Math.max(Math.round((1 - alpha) * bgColor.red + alpha * red), 0), 255);
+          green = Math.min(Math.max(Math.round((1 - alpha) * bgColor.green + alpha * green), 0), 255);
+          blue = Math.min(Math.max(Math.round((1 - alpha) * bgColor.blue + alpha * blue), 0), 255);
+        }
       }
       else {
         alpha = 255;
@@ -44,4 +61,4 @@ module.exports = function(data, width, height, options) {
   }
 
   return outData;
-};
+};