gamnit: use premultiplied alpha

[nit.git] / lib / gamnit / textures.nit

diff --git a/lib/gamnit/textures.nit b/lib/gamnit/textures.nit
index 53e88a5..4fb98ac 100644 (file)
--- 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