From c3c062672a859426c8583e197795e67150a01a89 Mon Sep 17 00:00:00 2001
From: nfnt <jan.schlicht@gmail.com>
Date: Tue, 12 May 2015 15:19:11 +0200
Subject: [PATCH] Reverse alpha-premultiplication for image.RGBA, image.RGBA64
 and generic images types.

---
 converter.go | 59 +++++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 51 insertions(+), 8 deletions(-)

diff --git a/converter.go b/converter.go
index f4d9d0f..84bd284 100644
--- a/converter.go
+++ b/converter.go
@@ -64,6 +64,17 @@ func resizeGeneric(in image.Image, out *image.NRGBA64, scale float64, coeffs []i
 						xi = maxX
 					}
 					r, g, b, a := in.At(xi+in.Bounds().Min.X, x+in.Bounds().Min.Y).RGBA()
+
+					// reverse alpha-premultiplication.
+					if a != 0 {
+						r *= 0xffff
+						r /= a
+						g *= 0xffff
+						g /= a
+						b *= 0xffff
+						b /= a
+					}
+
 					rgba[0] += int64(coeff) * int64(r)
 					rgba[1] += int64(coeff) * int64(g)
 					rgba[2] += int64(coeff) * int64(b)
@@ -112,10 +123,26 @@ func resizeRGBA(in *image.RGBA, out *image.NRGBA, scale float64, coeffs []int16,
 					default:
 						xi = 0
 					}
-					rgba[0] += int32(coeff) * int32(row[xi+0])
-					rgba[1] += int32(coeff) * int32(row[xi+1])
-					rgba[2] += int32(coeff) * int32(row[xi+2])
-					rgba[3] += int32(coeff) * int32(row[xi+3])
+
+					r := uint32(row[xi+0])
+					g := uint32(row[xi+1])
+					b := uint32(row[xi+2])
+					a := uint32(row[xi+3])
+
+					// reverse alpha-premultiplication.
+					if a != 0 {
+						r *= 0xffff
+						r /= a
+						g *= 0xffff
+						g /= a
+						b *= 0xffff
+						b /= a
+					}
+
+					rgba[0] += int32(coeff) * int32(r)
+					rgba[1] += int32(coeff) * int32(g)
+					rgba[2] += int32(coeff) * int32(b)
+					rgba[3] += int32(coeff) * int32(a)
 					sum += int32(coeff)
 				}
 			}
@@ -192,10 +219,26 @@ func resizeRGBA64(in *image.RGBA64, out *image.NRGBA64, scale float64, coeffs []
 					default:
 						xi = 0
 					}
-					rgba[0] += int64(coeff) * int64(uint16(row[xi+0])<<8|uint16(row[xi+1]))
-					rgba[1] += int64(coeff) * int64(uint16(row[xi+2])<<8|uint16(row[xi+3]))
-					rgba[2] += int64(coeff) * int64(uint16(row[xi+4])<<8|uint16(row[xi+5]))
-					rgba[3] += int64(coeff) * int64(uint16(row[xi+6])<<8|uint16(row[xi+7]))
+
+					r := uint32(uint16(row[xi+0])<<8 | uint16(row[xi+1]))
+					g := uint32(uint16(row[xi+2])<<8 | uint16(row[xi+3]))
+					b := uint32(uint16(row[xi+4])<<8 | uint16(row[xi+5]))
+					a := uint32(uint16(row[xi+6])<<8 | uint16(row[xi+7]))
+
+					// reverse alpha-premultiplication.
+					if a != 0 {
+						r *= 0xffff
+						r /= a
+						g *= 0xffff
+						g /= a
+						b *= 0xffff
+						b /= a
+					}
+
+					rgba[0] += int64(coeff) * int64(r)
+					rgba[1] += int64(coeff) * int64(g)
+					rgba[2] += int64(coeff) * int64(b)
+					rgba[3] += int64(coeff) * int64(a)
 					sum += int64(coeff)
 				}
 			}