Fix grayscale PNG images with alpha channel

Author: blikblum

lib/image/png.js

@@ -109,17 +109,18 @@ class PNGImage {
   splitAlphaChannel() {
     return this.image.decodePixels(pixels => {
       let a, p;
-      const colorByteSize = (this.image.colors * this.image.bits) / 8;
+      const colorCount = this.image.colors;
+      const colorByteSize = (colorCount * this.image.bits) / 8;
       const pixelCount = this.width * this.height;
       const imgData = new Buffer(pixelCount * colorByteSize);
       const alphaChannel = new Buffer(pixelCount);
 
       let i = p = a = 0;
       const len = pixels.length;
       while (i < len) {
-        imgData[p++] = pixels[i++];
-        imgData[p++] = pixels[i++];
-        imgData[p++] = pixels[i++];
+        pixels.copy(imgData, p, i, i + colorCount);
+        p += colorCount;
+        i += colorCount;
         alphaChannel[a++] = pixels[i++];
       }