Make the Frosted Glass effect deterministic

- Extracted out some shared code into the RandomSeed class, for computing a random seed for each tile being processed.

Author: cameronwhite

CHANGELOG.md

@@ -21,7 +21,7 @@ Thanks to the following contributors who worked on this release:
 
 ### Changed
 - When building Pinta using the Makefile, 'dotnet publish' is now run during the build step rather than the install step.
-- Added a "Reseed" button for the Add Noise Effect, which generates a new noise pattern. The noise pattern will otherwise remain the same while other parameters are adjusted.
+- Added a "Reseed" button for the random noise used by several effects ("Add Noise" and "Frosted Glass").  Previously, the noise pattern changed every time the effect was computed (including when other parameters were changed).
 
 ### Fixed
 - Fixed an issue where the Pan tool's cursor could show up as a missing icon ([#2013047](https://bugs.launchpad.net/pinta/+bug/2013047))

Pinta.Core/Effects/RandomSeed.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Numerics;
+using System.Runtime.CompilerServices;
 
 namespace Pinta.Core;
 
@@ -27,4 +29,45 @@ public override readonly bool Equals (object? obj)
 	public override readonly int GetHashCode () => Value.GetHashCode ();
 	public static bool operator == (RandomSeed left, RandomSeed right) => left.Equals (right);
 	public static bool operator != (RandomSeed left, RandomSeed right) => !left.Equals (right);
+
+	/// <summary>
+	/// Produces a new random seed based on <see cref="Value"/>  and the specified region.
+	/// This can be useful for tiled effects.
+	/// </summary>
+	public int GetValueForRegion (in RectangleI region)
+	{
+		// Note that HashCode.Combine() can't be used because it is random per-process and would
+		// produce inconsistent results for unit tests.
+		// This is the same implementation from HashCode.cs, but without the randomization.
+		const uint Prime2 = 2246822519U;
+		const uint Prime3 = 3266489917U;
+		const uint Prime4 = 668265263U;
+
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
+		static uint MixFinal (uint hash)
+		{
+			hash ^= hash >> 15;
+			hash *= Prime2;
+			hash ^= hash >> 13;
+			hash *= Prime3;
+			hash ^= hash >> 16;
+			return hash;
+		}
+
+		[MethodImpl (MethodImplOptions.AggressiveInlining)]
+		static uint QueueRound (uint hash, uint queuedValue)
+		{
+			return BitOperations.RotateLeft (hash + queuedValue * Prime3, 17) * Prime4;
+		}
+
+		uint hash = 374761393U;
+		hash += 12;
+
+		hash = QueueRound (hash, (uint) Value);
+		hash = QueueRound (hash, (uint) region.Left);
+		hash = QueueRound (hash, (uint) region.Top);
+
+		hash = MixFinal (hash);
+		return (int) hash;
+	}
 }

Pinta.Effects/Effects/AddNoiseEffect.cs

@@ -9,8 +9,6 @@
 
 using System;
 using System.Collections.Immutable;
-using System.Numerics;
-using System.Runtime.CompilerServices;
 using Cairo;
 using Pinta.Core;
 using Pinta.Gui.Widgets;
@@ -101,49 +99,11 @@ private static ImmutableArray<int> CreateLookup ()
 		return result.ToImmutable ();
 	}
 
-	private static int CreateSeedForRegion (int global_seed, int rect_left, int rect_top)
-	{
-		// Note that HashCode.Combine() can't be used because it is random per-process and would
-		// produce inconsistent results for unit tests.
-		// This is the same implementation from HashCode.cs, but without the randomization.
-		const uint Prime2 = 2246822519U;
-		const uint Prime3 = 3266489917U;
-		const uint Prime4 = 668265263U;
-
-		[MethodImpl (MethodImplOptions.AggressiveInlining)]
-		static uint MixFinal (uint hash)
-		{
-			hash ^= hash >> 15;
-			hash *= Prime2;
-			hash ^= hash >> 13;
-			hash *= Prime3;
-			hash ^= hash >> 16;
-			return hash;
-		}
-
-		[MethodImpl (MethodImplOptions.AggressiveInlining)]
-		static uint QueueRound (uint hash, uint queuedValue)
-		{
-			return BitOperations.RotateLeft (hash + queuedValue * Prime3, 17) * Prime4;
-		}
-
-		uint hash = 374761393U;
-		hash += 12;
-
-		hash = QueueRound (hash, (uint) global_seed);
-		hash = QueueRound (hash, (uint) rect_left);
-		hash = QueueRound (hash, (uint) rect_top);
-
-		hash = MixFinal (hash);
-		return (int) hash;
-	}
-
 	public override void Render (ImageSurface src, ImageSurface dst, ReadOnlySpan<RectangleI> rois)
 	{
 		int intensity = Data.Intensity;
 		int color_saturation = Data.ColorSaturation;
 		double coverage = 0.01 * Data.Coverage;
-		int global_seed = Data.Seed.Value;
 
 		int dev = intensity * intensity / 4;
 		int sat = color_saturation * 4096 / 100;
@@ -158,7 +118,7 @@ public override void Render (ImageSurface src, ImageSurface dst, ReadOnlySpan<Re
 			// Reseed the random number generator for each rectangle being rendered.
 			// This should produce consistent results regardless of the number of threads
 			// being used to render the effect, but will change if the effect is tiled differently.
-			var rand = new Random (CreateSeedForRegion (global_seed, rect.Left, rect.Top));
+			var rand = new Random (Data.Seed.GetValueForRegion (rect));
 
 			int right = rect.Right;
 			for (int y = rect.Top; y <= rect.Bottom; ++y) {

Pinta.Effects/Effects/FrostedGlassEffect.cs

@@ -28,8 +28,6 @@ public sealed class FrostedGlassEffect : BaseEffect
 
 	public FrostedGlassData Data => (FrostedGlassData) EffectData!;
 
-	private readonly Random random = new ();
-
 	public FrostedGlassEffect ()
 	{
 		EffectData = new FrostedGlassData ();
@@ -63,6 +61,8 @@ public override void Render (ImageSurface src, ImageSurface dst, ReadOnlySpan<Re
 		Span<ColorBgra> dst_data = dst.GetPixelData ();
 
 		foreach (var rect in rois) {
+			var random = new Random (Data.Seed.GetValueForRegion (rect));
+
 			for (int y = rect.Top; y <= rect.Bottom; ++y) {
 
 				var dst_row = dst_data.Slice (y * settings.src_width, settings.src_width);
@@ -76,12 +76,12 @@ public override void Render (ImageSurface src, ImageSurface dst, ReadOnlySpan<Re
 					bottom = settings.src_height;
 
 				for (int x = rect.Left; x <= rect.Right; ++x)
-					dst_row[x] = GetFinalPixelColor (settings, src_data, top, bottom, x);
+					dst_row[x] = GetFinalPixelColor (settings, random, src_data, top, bottom, x);
 			}
 		}
 	}
 
-	private ColorBgra GetFinalPixelColor (FrostedGlassSettings settings, ReadOnlySpan<ColorBgra> src_data, int top, int bottom, int x)
+	private static ColorBgra GetFinalPixelColor (FrostedGlassSettings settings, Random random, ReadOnlySpan<ColorBgra> src_data, int top, int bottom, int x)
 	{
 		int intensityChoicesIndex = 0;
 
@@ -133,11 +133,7 @@ private ColorBgra GetFinalPixelColor (FrostedGlassSettings settings, ReadOnlySpa
 			}
 		}
 
-		int randNum;
-		lock (random) {
-			randNum = random.Next (intensityChoicesIndex);
-		}
-
+		int randNum = random.Next (intensityChoicesIndex);
 		byte chosenIntensity = intensityChoices[randNum];
 
 		return ColorBgra.FromBgra (
@@ -153,5 +149,8 @@ public sealed class FrostedGlassData : EffectData
 	{
 		[Caption ("Amount"), MinimumValue (1), MaximumValue (10)]
 		public int Amount { get; set; } = 1;
+
+		[Caption ("Random Noise")]
+		public RandomSeed Seed { get; set; } = new (0);
 	}
 }

tests/Pinta.Effects.Tests/Assets/frostedglass1.png

@@ -102,10 +102,12 @@ public void Fragment2 ()
 	}
 
 	[Test]
-	[Ignore ("Produces non-deterministic results because the random seed is not fixed")]
 	public void FrostedGlass ()
 	{
-		// TODO
+		var effect = new FrostedGlassEffect ();
+		effect.Data.Amount = 7;
+		effect.Data.Seed = new (42);
+		Utilities.TestEffect (effect, "frostedglass1.png");
 	}
 
 	[Test]