4 files changed, 21 insertions, 7 deletions
diff --git a/libdimension/color.c b/libdimension/color.c
index 0b74adf..0af063f 100644
--- a/libdimension/color.c
+++ b/libdimension/color.c
@@ -326,8 +326,8 @@ dmnsn_Luv_from_color(dmnsn_color color, dmnsn_CIE_XYZ white)
   return ret;
 }
 
-/** Greyscale color intensity. */
-static double
+/* Greyscale color intensity */
+double
 dmnsn_color_intensity(dmnsn_color color)
 {
   dmnsn_sRGB sRGB = dmnsn_sRGB_from_color(color);
diff --git a/libdimension/dimension/color.h b/libdimension/dimension/color.h
index c9d777b..887c2c7 100644
--- a/libdimension/dimension/color.h
+++ b/libdimension/dimension/color.h
@@ -121,6 +121,8 @@ dmnsn_CIE_Luv dmnsn_Luv_from_color(dmnsn_color color, dmnsn_CIE_XYZ white);
 
 /* Perceptual color manipulation */
 
+/** Greyscale color intensity. */
+double dmnsn_color_intensity(dmnsn_color color);
 /** Add two colors together. */
 dmnsn_color dmnsn_color_add(dmnsn_color color1, dmnsn_color color2);
 /** Multiply a color's intensity by \p n. */
diff --git a/libdimension/gl.c b/libdimension/gl.c
index 77eefcd..857c7de 100644
--- a/libdimension/gl.c
+++ b/libdimension/gl.c
@@ -112,8 +112,14 @@ dmnsn_gl_write_canvas(const dmnsn_canvas *canvas)
         pixel[2] = sRGB.B*UINT16_MAX;
       }
 
-      /* color.filter + color.trans is in [0.0, 1.0] by definition */
-      pixel[3] = (color.filter + color.trans)*UINT16_MAX;
+      double alpha = dmnsn_color_intensity(color)*color.filter + color.trans;
+      if (alpha <= 0.0) {
+        pixel[3] = 0;
+      } else if (alpha >= 1.0) {
+        pixel[3] = UINT16_MAX;
+      } else {
+        pixel[3] = alpha*UINT16_MAX;
+      }
     }
   }
 
@@ -201,6 +207,12 @@ dmnsn_gl_optimizer_fn(const dmnsn_canvas *canvas,
     pixel[2] = sRGB.B*UINT16_MAX;
   }
 
-  /* color.filter + color.trans is in [0.0, 1.0] by definition */
-  pixel[3] = (color.filter + color.trans)*UINT16_MAX;
+  double alpha = dmnsn_color_intensity(color)*color.filter + color.trans;
+  if (alpha <= 0.0) {
+    pixel[3] = 0;
+  } else if (alpha >= 1.0) {
+    pixel[3] = UINT16_MAX;
+  } else {
+    pixel[3] = alpha*UINT16_MAX;
+  }
 }
diff --git a/libdimension/png.c b/libdimension/png.c
index 9ff2afc..7a327d6 100644
--- a/libdimension/png.c
+++ b/libdimension/png.c
@@ -94,7 +94,7 @@ dmnsn_png_optimizer_fn(const dmnsn_canvas *canvas,
     pixel[2] = sRGB.B*UINT16_MAX;
   }
 
-  double alpha = color.filter + color.trans;
+  double alpha = dmnsn_color_intensity(color)*color.filter + color.trans;
   if (alpha <= 0.0) {
     pixel[3] = 0;
   } else if (alpha >= 1.0) {