From 1e6070c1508995039ecbef1a11789c1da2f7477f Mon Sep 17 00:00:00 2001
From: Ryan Oldenburg <ryan@guzba.com>
Date: Tue, 14 Jun 2022 17:54:02 -0500
Subject: [PATCH] much faster toStraightAlpha

---
 src/pixie/internal.nim | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/src/pixie/internal.nim b/src/pixie/internal.nim
index e5cb53e..3f309a6 100644
--- a/src/pixie/internal.nim
+++ b/src/pixie/internal.nim
@@ -81,16 +81,23 @@ proc fillUnsafe*(
     for j in i ..< start + len:
       data[j] = rgbx
 
+const straightAlphaTable = block:
+  var table: array[256, array[256, uint8]]
+  for a in 0 ..< 256:
+    let multiplier = if a > 0: (255 / a.float32) else: 0
+    for c in 0 ..< 256:
+      table[a][c] = min(round((c.float32 * multiplier)), 255).uint8
+  table
+
 proc toStraightAlpha*(data: var seq[ColorRGBA | ColorRGBX]) {.raises: [].} =
   ## Converts an image from premultiplied alpha to straight alpha.
   ## This is expensive for large images.
-  for c in data.mitems:
-    if c.a == 0 or c.a == 255:
-      continue
-    let multiplier = ((255 / c.a.float32) * 255).uint32
-    c.r = ((c.r.uint32 * multiplier) div 255).uint8
-    c.g = ((c.g.uint32 * multiplier) div 255).uint8
-    c.b = ((c.b.uint32 * multiplier) div 255).uint8
+  for i in 0 ..< data.len:
+    var c = data[i]
+    c.r = straightAlphaTable[c.a][c.r]
+    c.g = straightAlphaTable[c.a][c.g]
+    c.b = straightAlphaTable[c.a][c.b]
+    data[i] = c
 
 proc toPremultipliedAlpha*(data: var seq[ColorRGBA | ColorRGBX]) {.raises: [].} =
   ## Converts an image to premultiplied alpha from straight alpha.