Bug fix: removed artifact when a partially covered pixel color was extrapolated between two supersample ranges.

Author: dmester

lib/rasterization/rasterizer.js

@@ -59,7 +59,10 @@ function rasterize(edgeTable, backColor) {
     edgeTable.sort();
 
     var writer = new BitmapWriter(backColor, edgeTable.width, edgeTable.height);
-    var superSampleBuffer = new SuperSampleBuffer(edgeTable.width, SAMPLES_PER_PIXEL_X);
+    
+    // Allocate an extra slot in the super sample buffer to calculate the 
+    // color that will be forwarded until the next supersample range.
+    var superSampleBuffer = new SuperSampleBuffer(edgeTable.width + 1, SAMPLES_PER_PIXEL_X);
 
     var layers = [];
     var color = 0;
@@ -101,9 +104,14 @@ function rasterize(edgeTable, backColor) {
                     color = subScanlineLayers.color;
                 }
 
-                superSampleBuffer.add(color, superSampleRange.width);
+                // Write an extra pixel that will contain the color that
+                // will be forwarded until the next supersample range.
+                superSampleBuffer.add(color, superSampleRange.width + 1);
                 superSampleBuffer.rewind();
             } // /subpixel
+            
+            // Get color to be forwarded
+            color = superSampleBuffer.getColorAt(superSampleRange.width);
 
             // Blend subpixels
             superSampleBuffer.writeTo(writer, superSampleRange.width);

lib/rasterization/superSampleBuffer.js

@@ -106,6 +106,24 @@ SuperSampleBuffer.prototype.writeTo = function SuperSampleBuffer_writeTo(bitmapW
     }
 };
 
+/**
+ * Gets the average color for the pixel at a specified index in the buffer.
+ * @param {number} index  The index of the pixel whose average color will be calculated.
+ * @returns {number} Average color.
+ * @public
+ */
+SuperSampleBuffer.prototype.getColorAt = function SuperSampleBuffer_getColorAt(index) {
+    var sampleCount = this._samples[index * 5 + IDX_COUNT],
+        alphaSum = this._samples[index * 5 + IDX_A];
+    return sampleCount && alphaSum ? 
+            colorUtils.from(
+                Math.floor(alphaSum / sampleCount),
+                Math.floor(this._samples[index * 5 + IDX_R] * 255 / alphaSum),
+                Math.floor(this._samples[index * 5 + IDX_G] * 255 / alphaSum),
+                Math.floor(this._samples[index * 5 + IDX_B] * 255 / alphaSum)
+            ) : 0;
+};
+
 /**
  * Adds a color to the buffer up until the specified x index.
  * @param {number} color  Color to write.