2x faster convert to straight alpha

src/pixie/images.nim

@@ -218,13 +218,15 @@ proc toAlphy*(image: Image) =
     image.data[j] = c
 
 proc fromAlphy*(image: Image) =
-  ## Converts an image to from premultiplied alpha to straight.
+  ## Converts an image from premultiplied alpha to straight alpha.
+  ## This is expensive for large images.
   for c in image.data.mitems:
-    if c.a == 0:
+    if c.a == 0 or c.a == 255:
       continue
-    c.r = ((c.r.uint32 * 255) div c.a.uint32).uint8
-    c.g = ((c.g.uint32 * 255) div c.a.uint32).uint8
-    c.b = ((c.b.uint32 * 255) div c.a.uint32).uint8
+    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
 
 proc draw*(a, b: Image, mat: Mat3, blendMode = bmNormal)
 proc draw*(a, b: Image, pos = vec2(0, 0), blendMode = bmNormal) {.inline.}

tests/benchmark_images.nim

@@ -8,8 +8,8 @@ timeIt "fill":
   keep(a)
 
 timeIt "fill_rgba":
-  a.fill(rgba(63, 127, 191, 255))
-  doAssert a[0, 0] == rgba(63, 127, 191, 255)
+  a.fill(rgba(63, 127, 191, 191))
+  doAssert a[0, 0] == rgba(63, 127, 191, 191)
   keep(a)
 
 timeIt "subImage":
@@ -26,3 +26,11 @@ timeIt "applyOpacity":
 timeIt "sharpOpacity":
   a.sharpOpacity()
   keep(a)
+
+a.fill(rgba(63, 127, 191, 191))
+
+timeIt "toAlphy":
+  a.toAlphy()
+
+timeIt "fromAlphy":
+  a.fromAlphy()

tests/test_images.nim

@@ -26,7 +26,7 @@ block:
   let image = newImage(10, 10)
   image.fill(rgba(128, 0, 0, 128))
   image.fromAlphy()
-  doAssert image[9, 9] == rgba(255, 0, 0, 128)
+  doAssert image[9, 9] == rgba(254, 0, 0, 128)
 
 block:
   let