From: Alexis Laferrière <alexis.laf@xymus.net>
Date: Mon, 26 Jun 2017 15:44:53 +0000 (-0400)
Subject: gamnit: use premultiplied alpha
X-Git-Url: http://nitlanguage.org

gamnit: use premultiplied alpha

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
---

diff --git a/lib/gamnit/depth/particles.nit b/lib/gamnit/depth/particles.nit
index 3c40ac1..bfe3392 100644
--- a/lib/gamnit/depth/particles.nit
+++ b/lib/gamnit/depth/particles.nit
@@ -258,7 +258,6 @@ class ParticleProgram
 		{
 			if (use_texture) {
 				gl_FragColor = texture2D(texture0, gl_PointCoord) * v_color;
-				if (gl_FragColor.a <= 0.01) discard;
 			} else {
 				gl_FragColor = v_color;
 			}
@@ -301,7 +300,7 @@ class ExplosionProgram
 		gl_Position = center * mvp;
 		gl_PointSize = scale / gl_Position.z * pt;
 
-		if (pt > 0.8) v_color.a = (1.0-pt)/0.2;
+		if (pt > 0.8) v_color *= (1.0-pt)/0.2;
 	"""
 end
 
@@ -318,8 +317,8 @@ class SmokeProgram
 		gl_PointSize = scale / gl_Position.z * (pt+0.1);
 
 		if (pt < 0.1)
-			v_color.a = pt / 0.1;
+			v_color *= pt / 0.1;
 		else
-			v_color.a = 1.0 - pt*0.9;
+			v_color *= 1.0 - pt*0.9;
 	"""
 end
diff --git a/lib/gamnit/display_android.nit b/lib/gamnit/display_android.nit
index e9d1b4a..6d968f8 100644
--- a/lib/gamnit/display_android.nit
+++ b/lib/gamnit/display_android.nit
@@ -62,7 +62,7 @@ redef class TextureAsset
 			return
 		end
 
-		var buf = bmp.copy_pixels
+		var buf = bmp.copy_pixels(unmultiply=not premultiply_alpha)
 		loaded = true
 		width = bmp.width.to_f
 		height = bmp.height.to_f
diff --git a/lib/gamnit/flat.nit b/lib/gamnit/flat.nit
index 56fd402..f3043c3 100644
--- a/lib/gamnit/flat.nit
+++ b/lib/gamnit/flat.nit
@@ -430,7 +430,7 @@ redef class App
 
 		# Enable blending
 		gl.capabilities.blend.enable
-		glBlendFunc(gl_SRC_ALPHA, gl_ONE_MINUS_SRC_ALPHA)
+		glBlendFunc(gl_ONE, gl_ONE_MINUS_SRC_ALPHA)
 
 		# Enable depth test
 		gl.capabilities.depth_test.enable
@@ -695,7 +695,7 @@ private class Simple2dProgram
 			}
 
 			gl_Position = (vec4(c * scale, 1.0) * rotation() + translation)* mvp;
-			v_color = color;
+			v_color = vec4(color.rgb*color.a, color.a);
 		}
 		""" @ glsl_vertex_shader
 
diff --git a/lib/gamnit/textures.nit b/lib/gamnit/textures.nit
index 53e88a5..4fb98ac 100644
--- a/lib/gamnit/textures.nit
+++ b/lib/gamnit/textures.nit
@@ -208,7 +208,7 @@ class CustomTexture
 	end
 end
 
-# Texture with its own pixels
+# Texture with its own pixel data
 class RootTexture
 	super Texture
 
@@ -221,6 +221,18 @@ class RootTexture
 
 	init do all_root_textures.add self
 
+	# Should the pixels RGB values be premultiplied by their alpha value at loading?
+	#
+	# All gamnit textures must have premultiplied alpha, it provides a better
+	# alpha blending, avoids artifacts and allows for additive blending.
+	#
+	# When at `true`, the default, pixels RGB values are premultiplied
+	# at loading. Set to `false` if pixels RGB values are already
+	# premultiplied in the source data.
+	#
+	# This value must be set before calling `load`.
+	var premultiply_alpha = true is writable
+
 	private fun load_from_pixels(pixels: Pointer, width, height: Int, format: GLPixelFormat)
 	do
 		var max_texture_size = glGetIntegerv(gl_MAX_TEXTURE_SIZE, 0)
@@ -229,6 +241,11 @@ class RootTexture
 			return
 		end
 
+		# Premultiply alpha?
+		if premultiply_alpha and format == gl_RGBA then
+			pixels.premultiply_alpha(width, height)
+		end
+
 		glPixelStorei(gl_UNPACK_ALIGNEMENT, 1)
 		var tex = glGenTextures(1)[0]
 		gl_texture = tex
@@ -368,3 +385,20 @@ class TextureSet
 	# Load all texture of this set
 	fun load_all do for t in self do t.load
 end
+
+redef class Pointer
+	# Multiply RBG values by their alpha value
+	private fun premultiply_alpha(width, height: Int) `{
+		uint8_t *bytes = (uint8_t *)self;
+		int x, y, i = 0;
+		for(y = 0; y < height; y ++) {
+			for(x = 0; x < width; x ++) {
+				int a = bytes[i+3];
+				bytes[i  ] = bytes[i  ] * a / 255;
+				bytes[i+1] = bytes[i+1] * a / 255;
+				bytes[i+2] = bytes[i+2] * a / 255;
+				i += 4;
+			}
+		}
+	`}
+end