integer lerp, integer premultiply and straighten alpha

src/pixie/common.nim

@@ -9,20 +9,43 @@ proc fractional*(v: float32): float32 {.inline.} =
   result = abs(v)
   result = result - floor(result)
 
+proc lerp*(a, b: ColorRGBA, t: float32): ColorRGBA {.inline.} =
+  let x = round(t * 255).uint32
+  result.r = ((a.r.uint32 * (255 - x) + b.r.uint32 * x) div 255).uint8
+  result.g = ((a.g.uint32 * (255 - x) + b.g.uint32 * x) div 255).uint8
+  result.b = ((a.b.uint32 * (255 - x) + b.b.uint32 * x) div 255).uint8
+  result.a = ((a.a.uint32 * (255 - x) + b.a.uint32 * x) div 255).uint8
+
+proc premultiplyAlpha*(c: ColorRGBA): ColorRGBA {.inline.} =
+  ## Converts a color to premultiplied alpha from straight alpha.
+  result.r = ((c.r.uint16 * c.a.uint16) div 255).uint8
+  result.g = ((c.g.uint16 * c.a.uint16) div 255).uint8
+  result.b = ((c.b.uint16 * c.a.uint16) div 255).uint8
+  result.a = c.a
+
+proc straightenAlpha*(c: ColorRGBA): ColorRGBA {.inline.} =
+  ## Converts a color to from premultiplied alpha to straight alpha.
+  result = c
+  if result.a != 0 and result.a != 255:
+    let multiplier = ((255 / c.a.float32) * 255).uint32
+    result.r = ((result.r.uint32 * multiplier) div 255).uint8
+    result.g = ((result.g.uint32 * multiplier) div 255).uint8
+    result.b = ((result.b.uint32 * multiplier) div 255).uint8
+
 func lerp*(a, b: Color, v: float32): Color {.inline.} =
   result.r = lerp(a.r, b.r, v)
   result.g = lerp(a.g, b.g, v)
   result.b = lerp(a.b, b.b, v)
   result.a = lerp(a.a, b.a, v)
 
-proc toAlphy*(c: Color): Color =
+proc toAlphy*(c: Color): Color {.inline.} =
   ## Converts a color to premultiplied alpha from straight.
   result.r = c.r * c.a
   result.g = c.g * c.a
   result.b = c.b * c.a
   result.a = c.a
 
-proc fromAlphy*(c: Color): Color =
+proc fromAlphy*(c: Color): Color {.inline.} =
   ## Converts a color to from premultiplied alpha to straight.
   if c.a == 0:
     return

src/pixie/images.nim

@@ -250,23 +250,22 @@ proc invert*(image: Image) =
     image.data[j] = rgba
 
 proc getRgbaSmooth*(image: Image, x, y: float32): ColorRGBA {.inline.} =
-  ## Gets a pixel as (x, y) floats.
   let
     minX = x.floor.int
     difX = x - x.floor
     minY = y.floor.int
     difY = y - y.floor
 
-    vX0Y0 = image[minX, minY].color().toAlphy()
+    vX0Y0 = image[minX, minY].premultiplyAlpha()
-    vX1Y0 = image[minX + 1, minY].color().toAlphy()
+    vX1Y0 = image[minX + 1, minY].premultiplyAlpha()
-    vX0Y1 = image[minX, minY + 1].color().toAlphy()
+    vX0Y1 = image[minX, minY + 1].premultiplyAlpha()
-    vX1Y1 = image[minX + 1, minY + 1].color().toAlphy()
+    vX1Y1 = image[minX + 1, minY + 1].premultiplyAlpha()
 
     bottomMix = lerp(vX0Y0, vX1Y0, difX)
     topMix = lerp(vX0Y1, vX1Y1, difX)
     finalMix = lerp(bottomMix, topMix, difY)
 
-  return finalMix.fromAlphy().rgba()
+  finalMix.straightenAlpha()
 
 proc resize*(srcImage: Image, width, height: int): Image =
   result = newImage(width, height)

tests/benchmark_images.nim

@@ -5,12 +5,10 @@ let a = newImage(2560, 1440)
 timeIt "fill":
   a.fill(rgba(255, 255, 255, 255))
   doAssert a[0, 0] == rgba(255, 255, 255, 255)
-  keep(a)
 
 timeIt "fill_rgba":
   a.fill(rgba(63, 127, 191, 191))
   doAssert a[0, 0] == rgba(63, 127, 191, 191)
-  keep(a)
 
 timeIt "subImage":
   keep a.subImage(0, 0, 256, 256)
@@ -34,3 +32,19 @@ timeIt "toAlphy":
 
 timeIt "fromAlphy":
   a.fromAlphy()
+
+timeIt "lerp integers":
+  for i in 0 ..< 100000:
+    let c = a[0, 0]
+    var z: int
+    for t in 0 .. 100:
+      z += lerp(c, c, t.float32 / 100).a.int
+    doAssert z > 0
+
+timeIt "lerp floats":
+  for i in 0 ..< 100000:
+    let c = a[0, 0]
+    var z: int
+    for t in 0 .. 100:
+      z += lerp(c.color, c.color, t.float32 / 100).rgba().a.int
+    doAssert z > 0