From a3b0831472601bacf2eb946a211968070c8015f6 Mon Sep 17 00:00:00 2001
From: Hajime Hoshi <hajimehoshi@gmail.com>
Date: Mon, 22 Dec 2014 21:39:25 +0900
Subject: [PATCH] Change the texture pixel format to premultiplied alpha (#36)

---
 image.go                                  |  2 +-
 internal/math.go                          | 10 +++++-----
 internal/opengl/internal/shader/shader.go | 17 ++++++++---------
 internal/opengl/texture.go                |  6 +++---
 4 files changed, 17 insertions(+), 18 deletions(-)

diff --git a/image.go b/image.go
index 3a71a7093..3373af5db 100644
--- a/image.go
+++ b/image.go
@@ -101,7 +101,7 @@ type syncer interface {
 }
 
 // Image represents an image.
-// The pixel format is non alpha-premultiplied.
+// The pixel format is alpha-premultiplied.
 type Image struct {
 	syncer syncer
 	inner  *innerImage
diff --git a/internal/math.go b/internal/math.go
index 8e59d7421..b37743a16 100644
--- a/internal/math.go
+++ b/internal/math.go
@@ -37,11 +37,11 @@ func NextPowerOf2Int(size int) int {
 }
 
 func RGBA(clr color.Color) (r, g, b, a float64) {
-	clr2 := color.NRGBA64Model.Convert(clr).(color.NRGBA64)
+	cr, cg, cb, ca := clr.RGBA()
 	const max = math.MaxUint16
-	r = float64(clr2.R) / max
-	g = float64(clr2.G) / max
-	b = float64(clr2.B) / max
-	a = float64(clr2.A) / max
+	r = float64(cr) / max
+	g = float64(cg) / max
+	b = float64(cb) / max
+	a = float64(ca) / max
 	return
 }
diff --git a/internal/opengl/internal/shader/shader.go b/internal/opengl/internal/shader/shader.go
index 43574edb4..38f55382e 100644
--- a/internal/opengl/internal/shader/shader.go
+++ b/internal/opengl/internal/shader/shader.go
@@ -84,15 +84,15 @@ varying vec2 vertex_out_tex_coord1;
 void main(void) {
   vec4 color0 = texture2D(texture0, vertex_out_tex_coord0);
   vec4 color1 = texture2D(texture1, vertex_out_tex_coord1);
+  // Un-premultiply alpha
+  color0.rgb /= color0.a;
+  // Apply the color matrix
   color0 = (color_matrix * color0) + color_matrix_translation;
+  // Premultiply alpha
+  color0 = clamp(color0, 0.0, 1.0);
+  color0.rgb *= color0.a;
 
-  // Photoshop-like RGBA blending.
-  //
-  // NOTE: If the textures are alpha premultiplied, this calc would be much simpler,
-  // but the color matrix must be applied to the straight alpha colors.
-  // Thus, straight alpha colors are used in Ebiten.
-  gl_FragColor.a = color0.a + (1.0 - color0.a) * color1.a;
-  gl_FragColor.rgb = (color0.a * color0.rgb + (1.0 - color0.a) * color1.a * color1.rgb) / gl_FragColor.a;
+  gl_FragColor = color0 + (1.0 - color0.a) * color1;
 }
 `,
 	},
@@ -104,8 +104,7 @@ varying vec2 vertex_out_tex_coord0;
 
 void main(void) {
   vec4 color0 = texture2D(texture0, vertex_out_tex_coord0);
-  gl_FragColor.rgb = color0.a * color0.rgb;
-  gl_FragColor.a = 1.0;
+  gl_FragColor = color0;
 }
 `,
 	},
diff --git a/internal/opengl/texture.go b/internal/opengl/texture.go
index 21300571d..96ffd076d 100644
--- a/internal/opengl/texture.go
+++ b/internal/opengl/texture.go
@@ -23,7 +23,7 @@ import (
 	"image/draw"
 )
 
-func adjustImageForTexture(img image.Image) *image.NRGBA {
+func adjustImageForTexture(img image.Image) *image.RGBA {
 	width, height := img.Bounds().Size().X, img.Bounds().Size().Y
 	adjustedImageBounds := image.Rectangle{
 		image.ZP,
@@ -32,11 +32,11 @@ func adjustImageForTexture(img image.Image) *image.NRGBA {
 			internal.NextPowerOf2Int(height),
 		},
 	}
-	if nrgba, ok := img.(*image.NRGBA); ok && img.Bounds() == adjustedImageBounds {
+	if nrgba, ok := img.(*image.RGBA); ok && img.Bounds() == adjustedImageBounds {
 		return nrgba
 	}
 
-	adjustedImage := image.NewNRGBA(adjustedImageBounds)
+	adjustedImage := image.NewRGBA(adjustedImageBounds)
 	dstBounds := image.Rectangle{
 		image.ZP,
 		img.Bounds().Size(),