From 2aa37c2e404825aa6706945859991e921d1c53cb Mon Sep 17 00:00:00 2001
From: Jennifer Taylor <dragonminded@dragonminded.com>
Date: Sat, 22 May 2021 23:24:51 +0000
Subject: [PATCH] Fix additive blending on semi-transparent backgrounds.

---
 bemani/format/afp/blend.py         | 12 ++++++------
 bemani/format/afp/blendaltimpl.cxx | 12 ++++++------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/bemani/format/afp/blend.py b/bemani/format/afp/blend.py
index 6704182..019c198 100644
--- a/bemani/format/afp/blend.py
+++ b/bemani/format/afp/blend.py
@@ -35,12 +35,12 @@ except ImportError:
         # Calculate alpha blending.
         srcpercent = src[3] / 255.0
         destpercent = dest[3] / 255.0
-        srcremaineder = 1.0 - srcpercent
-        new_alpha = (srcpercent + destpercent * srcremaineder)
+        srcremainder = 1.0 - srcpercent
+        new_alpha = max(min(0.0, srcpercent + destpercent * srcremainder), 1.0)
         return (
-            clamp(((dest[0] * destpercent * srcremaineder) + (src[0] * srcpercent)) / new_alpha),
-            clamp(((dest[1] * destpercent * srcremaineder) + (src[1] * srcpercent)) / new_alpha),
-            clamp(((dest[2] * destpercent * srcremaineder) + (src[2] * srcpercent)) / new_alpha),
+            clamp(((dest[0] * destpercent * srcremainder) + (src[0] * srcpercent)) / new_alpha),
+            clamp(((dest[1] * destpercent * srcremainder) + (src[1] * srcpercent)) / new_alpha),
+            clamp(((dest[2] * destpercent * srcremainder) + (src[2] * srcpercent)) / new_alpha),
             clamp(255 * new_alpha)
         )
 
@@ -64,7 +64,7 @@ except ImportError:
             clamp(dest[0] + (src[0] * srcpercent)),
             clamp(dest[1] + (src[1] * srcpercent)),
             clamp(dest[2] + (src[2] * srcpercent)),
-            dest[3],
+            clamp(dest[3] + (255 * srcpercent)),
         )
 
     def blend_subtraction(
diff --git a/bemani/format/afp/blendaltimpl.cxx b/bemani/format/afp/blendaltimpl.cxx
index a4a6eeb..6533df7 100644
--- a/bemani/format/afp/blendaltimpl.cxx
+++ b/bemani/format/afp/blendaltimpl.cxx
@@ -92,12 +92,12 @@ extern "C"
         // Calculate alpha blending.
         float srcpercent = src.a / 255.0;
         float destpercent = dest.a / 255.0;
-        float srcremaineder = 1.0 - srcpercent;
-        float new_alpha = (srcpercent + destpercent * srcremaineder);
+        float srcremainder = 1.0 - srcpercent;
+        float new_alpha = fmin(fmax(0.0, srcpercent + destpercent * srcremainder), 1.0);
         return (intcolor_t){
-            clamp(((dest.r * destpercent * srcremaineder) + (src.r * srcpercent)) / new_alpha),
-            clamp(((dest.g * destpercent * srcremaineder) + (src.g * srcpercent)) / new_alpha),
-            clamp(((dest.b * destpercent * srcremaineder) + (src.b * srcpercent)) / new_alpha),
+            clamp(((dest.r * destpercent * srcremainder) + (src.r * srcpercent)) / new_alpha),
+            clamp(((dest.g * destpercent * srcremainder) + (src.g * srcpercent)) / new_alpha),
+            clamp(((dest.b * destpercent * srcremainder) + (src.b * srcpercent)) / new_alpha),
             clamp(255 * new_alpha)
         };
     }
@@ -121,7 +121,7 @@ extern "C"
             clamp(dest.r + (src.r * srcpercent)),
             clamp(dest.g + (src.g * srcpercent)),
             clamp(dest.b + (src.b * srcpercent)),
-            dest.a,
+            clamp(dest.a + (255 * srcpercent)),
         };
     }