fix: Semi transparent colors normalzation (#93)

* Normalize alpha before blending

* Add unit tests for blendSemiTransparentColor

* Clamp rgb values and distance instead of range assertion

* Update tests

* Correctly fix alpha decoding

* Optimize color blending

* Unstage test artifacts

---------

Co-authored-by: Dmitriy Kovalenko <dmtr.kovalenko@outlook.com>

.gitignore

@@ -6,4 +6,6 @@ _release
 *.byte
 *.native
 *.install
-images/diff.png
+images/diff.png
+test/test-images/_*.png
+

io/png/ReadPng.c

@@ -63,7 +63,8 @@ CAMLprim value read_png_file(value file) {
     free(out);
   };
 
-  result = spng_decode_image(ctx, out, out_size, SPNG_FMT_RGBA8, 0);
+  result =
+      spng_decode_image(ctx, out, out_size, SPNG_FMT_RGBA8, SPNG_DECODE_TRNS);
   if (result) {
     spng_ctx_free(ctx);
     free(out);

src/ColorDelta.ml

@@ -1,16 +1,29 @@
-let blend color alpha = 255. +. ((color -. 255.) *. alpha)
+let blend_channel_white color alpha = 255. +. ((color -. 255.) *. alpha)
+let white_pixel = (255., 255., 255., 0.)
 
 let blendSemiTransparentColor = function
+  | r, g, b, 0. -> white_pixel
+  | r, g, b, 255. -> (r, g, b, 1.)
   | r, g, b, alpha when alpha < 255. ->
-      (blend r alpha, blend g alpha, blend b alpha, alpha /. 255.)
-  | colors -> colors
+      let normalizedAlpha = alpha /. 255. in
+      let r, g, b, a =
+        ( blend_channel_white r normalizedAlpha,
+          blend_channel_white g normalizedAlpha,
+          blend_channel_white b normalizedAlpha,
+          normalizedAlpha )
+      in
+      (r, g, b, a)
+  | _ ->
+      failwith
+        "Found pixel with alpha value greater than uint8 max value. Aborting."
 
 let convertPixelToFloat pixel =
   let pixel = pixel |> Int32.to_int in
   let a = (pixel lsr 24) land 255 in
   let b = (pixel lsr 16) land 255 in
   let g = (pixel lsr 8) land 255 in
   let r = pixel land 255 in
+
   (Float.of_int r, Float.of_int g, Float.of_int b, Float.of_int a)
 
 let rgb2y (r, g, b, a) =
@@ -25,10 +38,13 @@ let rgb2q (r, g, b, a) =
 let calculatePixelColorDelta _pixelA _pixelB =
   let pixelA = _pixelA |> convertPixelToFloat |> blendSemiTransparentColor in
   let pixelB = _pixelB |> convertPixelToFloat |> blendSemiTransparentColor in
+
   let y = rgb2y pixelA -. rgb2y pixelB in
   let i = rgb2i pixelA -. rgb2i pixelB in
   let q = rgb2q pixelA -. rgb2q pixelB in
-  (0.5053 *. y *. y) +. (0.299 *. i *. i) +. (0.1957 *. q *. q)
+
+  let delta = (0.5053 *. y *. y) +. (0.299 *. i *. i) +. (0.1957 *. q *. q) in
+  delta
 
 let calculatePixelBrightnessDelta pixelA pixelB =
   let pixelA = pixelA |> convertPixelToFloat |> blendSemiTransparentColor in

test/Test_Core.ml

@@ -37,8 +37,8 @@ let _ =
           let _, diffPixels, diffPercentage, _ =
             PNG_Diff.compare img1 img2 ~threshold:0.5 ()
           in
-          (expect.int diffPixels).toBe 222;
-          (expect.float diffPercentage).toBeCloseTo 0.19))
+          (expect.int diffPixels).toBe 25;
+          (expect.float diffPercentage).toBeCloseTo 0.02))
 
 let _ =
   describe "CORE: Ignore Regions" (fun { test; _ } ->
@@ -80,3 +80,50 @@ let _ =
               "test/test-images/png/diff-of-diff-green.png");
           (expect.int diffOfDiffPixels).toBe 0;
           (expect.float diffOfDiffPercentage).toBeCloseTo 0.0))
+
+let _ =
+  describe "CORE: blendSemiTransparentColor" (fun { test; _ } ->
+      test "blend 255. alpha" (fun { expect; _ } ->
+          let r, g, b, a =
+            Odiff.ColorDelta.blendSemiTransparentColor (0., 128., 255., 255.)
+          in
+          (expect.float r).toBeCloseTo 0.;
+          (expect.float g).toBeCloseTo 128.;
+          (expect.float b).toBeCloseTo 255.;
+          (expect.float a).toBeCloseTo 1.);
+
+      test "blend 0. alpha" (fun { expect; _ } ->
+          let r, g, b, a =
+            Odiff.ColorDelta.blendSemiTransparentColor (0., 128., 255., 0.)
+          in
+          (expect.float r).toBeCloseTo 255.;
+          (expect.float g).toBeCloseTo 255.;
+          (expect.float b).toBeCloseTo 255.;
+          (expect.float a).toBeCloseTo 0.);
+
+      test "blend 5. alpha" (fun { expect; _ } ->
+          let r, g, b, a =
+            Odiff.ColorDelta.blendSemiTransparentColor (0., 128., 255., 5.)
+          in
+          (expect.float r).toBeCloseTo 250.;
+          (expect.float g).toBeCloseTo 252.51;
+          (expect.float b).toBeCloseTo 255.;
+          (expect.float a).toBeCloseTo 0.02);
+
+      test "blend 51. alpha" (fun { expect; _ } ->
+          let r, g, b, a =
+            Odiff.ColorDelta.blendSemiTransparentColor (0., 128., 255., 51.)
+          in
+          (expect.float r).toBeCloseTo 204.;
+          (expect.float g).toBeCloseTo 229.6;
+          (expect.float b).toBeCloseTo 255.;
+          (expect.float a).toBeCloseTo 0.2);
+
+      test "blend 128. alpha" (fun { expect; _ } ->
+          let r, g, b, a =
+            Odiff.ColorDelta.blendSemiTransparentColor (0., 128., 255., 128.)
+          in
+          (expect.float r).toBeCloseTo 127.;
+          (expect.float g).toBeCloseTo 191.25;
+          (expect.float b).toBeCloseTo 255.;
+          (expect.float a).toBeCloseTo 0.5))

test/Test_IO_PNG.ml

@@ -1,16 +1,17 @@
 open TestFramework
+open Png.IO
 open ODiffIO
 module Diff = Odiff.Diff.MakeDiff (Png.IO) (Png.IO)
 
 let _ =
   describe "IO: PNG" (fun { test; _ } ->
-      let open Png.IO in
       test "finds difference between 2 images" (fun { expect; _ } ->
           let img1 = loadImage "test/test-images/png/orange.png" in
           let img2 = loadImage "test/test-images/png/orange_changed.png" in
           let _, diffPixels, diffPercentage, _ = Diff.compare img1 img2 () in
-          (expect.int diffPixels).toBe 1430;
-          (expect.float diffPercentage).toBeCloseTo 1.20);
+          (expect.int diffPixels).toBe 1366;
+          (expect.float diffPercentage).toBeCloseTo 1.14);
+
       test "Diff of mask and no mask are equal" (fun { expect; _ } ->
           let img1 = loadImage "test/test-images/png/orange.png" in
           let img2 = loadImage "test/test-images/png/orange_changed.png" in
@@ -24,6 +25,7 @@ let _ =
           in
           (expect.int diffPixels).toBe diffPixelsMask;
           (expect.float diffPercentage).toBeCloseTo diffPercentageMask);
+
       test "Creates correct diff output image" (fun { expect; _ } ->
           let img1 = loadImage "test/test-images/png/orange.png" in
           let img2 = loadImage "test/test-images/png/orange_changed.png" in
@@ -36,4 +38,15 @@ let _ =
             saveImage diffOutput "test/test-images/png/diff-output.png";
             saveImage diffMaskOfDiff "test/test-images/png/diff-of-diff.png");
           (expect.int diffOfDiffPixels).toBe 0;
-          (expect.float diffOfDiffPercentage).toBeCloseTo 0.0))
+          (expect.float diffOfDiffPercentage).toBeCloseTo 0.0);
+
+      test "Correctly handles different encodings of transparency"
+        (fun { expect; _ } ->
+          let img1 =
+            loadImage "test/test-images/png/extreme-alpha.png"
+          in
+          let img2 =
+            loadImage "test/test-images/png/extreme-alpha-1.png"
+          in
+          let _, diffPixels, _, _ = Diff.compare img1 img2 () in
+          (expect.int diffPixels).toBe 0))