Fix transparency issues for a few effects

Premultiplied alpha should be used since the colors are from a Cairo surface

Fixes: #1184

CHANGELOG.md

@@ -74,6 +74,7 @@ Thanks to the following contributors who worked on this release:
 - Fixed various artifacts in the shape tools, particularly with larger brush widths (#733, #955)
 - Fixed an issue where the text tool did not immediately redraw after changes to the font or color of unfinalized text (#952, #975)
 - Fixed a bug where cut / paste operations did not behave as expected with complex selections (#951, #978)
+- Fixed transparency behavior for several effects (#1184)
 
 ## [2.1.2](https://github.com/PintaProject/Pinta/releases/tag/2.1.2) - 2024/04/20
 

Pinta.Effects/Effects/InkSketchEffect.cs

@@ -93,7 +93,7 @@ public override void Render (ImageSurface src, ImageSurface dest, ReadOnlySpan<R
 					int right = Math.Min (x + Radius + 1, dest.Width);
 
 					RectangleI adjustedBounds = RectangleI.FromLTRB (left, top, right, bottom);
-					ColorBgra baseRGB = CreateBaseRGB (src_data, width, x, y, adjustedBounds);
+					ColorBgra baseRGB = CreateBaseRGBA (src_data, width, x, y, adjustedBounds);
 					ColorBgra topLayer = CreateTopLayer (baseRGB);
 
 					// Change Blend Mode to Darken
@@ -104,11 +104,12 @@ public override void Render (ImageSurface src, ImageSurface dest, ReadOnlySpan<R
 		}
 	}
 
-	private static ColorBgra CreateBaseRGB (ReadOnlySpan<ColorBgra> src_data, int width, int x, int y, RectangleI adjustedBounds)
+	private static ColorBgra CreateBaseRGBA (ReadOnlySpan<ColorBgra> src_data, int width, int x, int y, RectangleI adjustedBounds)
 	{
 		int r = 0;
 		int g = 0;
 		int b = 0;
+		int a = 0;
 
 		for (int v = adjustedBounds.Top; v < adjustedBounds.Bottom; v++) {
 
@@ -124,13 +125,15 @@ private static ColorBgra CreateBaseRGB (ReadOnlySpan<ColorBgra> src_data, int wi
 				r += src_pixel.R * w;
 				g += src_pixel.G * w;
 				b += src_pixel.B * w;
+				a += src_pixel.A * w;
 			}
 		}
 
-		return ColorBgra.FromBgr (
+		return ColorBgra.FromBgra (
 			b: Utility.ClampToByte (b),
 			g: Utility.ClampToByte (g),
-			r: Utility.ClampToByte (r)
+			r: Utility.ClampToByte (r),
+			a: Utility.ClampToByte (a)
 		);
 	}
 
@@ -141,8 +144,8 @@ private ColorBgra CreateTopLayer (ColorBgra baseRGB)
 
 		// Adjust Brightness and Contrast
 		return
-			(topLayer.R > (Data.InkOutline * 255 / 100))
-			? ColorBgra.FromBgra (255, 255, 255, topLayer.A)
+			(topLayer.ToStraightAlpha ().R > (Data.InkOutline * 255 / 100))
+			? ColorBgra.FromBgra (topLayer.A, topLayer.A, topLayer.A, topLayer.A)
 			: ColorBgra.FromBgra (0, 0, 0, topLayer.A);
 	}
 	#endregion

Pinta.Effects/Effects/RadialBlurEffect.cs

@@ -104,9 +104,9 @@ private static ColorBgra GetFinalPixelColor (
 			X: (pixel.coordinates.X << 16) - settings.fcx,
 			Y: (pixel.coordinates.Y << 16) - settings.fcy);
 
-		int sr = sourcePixel.R * sourcePixel.A;
-		int sg = sourcePixel.G * sourcePixel.A;
-		int sb = sourcePixel.B * sourcePixel.A;
+		int sr = sourcePixel.R;
+		int sg = sourcePixel.G;
+		int sb = sourcePixel.B;
 		int sa = sourcePixel.A;
 		int sc = 1;
 
@@ -126,9 +126,9 @@ private static ColorBgra GetFinalPixelColor (
 
 				ColorBgra sample = sourceData[p1.Y * settings.canvasSize.Width + p1.X];
 
-				sr += sample.R * sample.A;
-				sg += sample.G * sample.A;
-				sb += sample.B * sample.A;
+				sr += sample.R;
+				sg += sample.G;
+				sb += sample.B;
 				sa += sample.A;
 
 				++sc;
@@ -142,9 +142,9 @@ private static ColorBgra GetFinalPixelColor (
 
 				ColorBgra sample = sourceData[p2.Y * settings.canvasSize.Width + p2.X];
 
-				sr += sample.R * sample.A;
-				sg += sample.G * sample.A;
-				sb += sample.B * sample.A;
+				sr += sample.R;
+				sg += sample.G;
+				sb += sample.B;
 				sa += sample.A;
 
 				++sc;
@@ -154,9 +154,9 @@ private static ColorBgra GetFinalPixelColor (
 		return
 			(sa > 0)
 			? ColorBgra.FromBgra (
-				b: Utility.ClampToByte (sb / sa),
-				g: Utility.ClampToByte (sg / sa),
-				r: Utility.ClampToByte (sr / sa),
+				b: Utility.ClampToByte (sb / sc),
+				g: Utility.ClampToByte (sg / sc),
+				r: Utility.ClampToByte (sr / sc),
 				a: Utility.ClampToByte (sa / sc))
 			: ColorBgra.FromUInt32 (0);
 	}

Pinta.Effects/Effects/ZoomBlurEffect.cs

@@ -102,9 +102,9 @@ private static ColorBgra GetFinalPixelColor (
 		int sc = 0;
 
 		ColorBgra src_pixel = sourceData[pixel.memoryOffset];
-		sr += src_pixel.R * src_pixel.A;
-		sg += src_pixel.G * src_pixel.A;
-		sb += src_pixel.B * src_pixel.A;
+		sr += src_pixel.R;
+		sg += src_pixel.G;
+		sb += src_pixel.B;
 		sa += src_pixel.A;
 		++sc;
 
@@ -124,9 +124,9 @@ private static ColorBgra GetFinalPixelColor (
 					settings.size.Width,
 					transformed);
 
-				sr += src_pixel_2.R * src_pixel_2.A;
-				sg += src_pixel_2.G * src_pixel_2.A;
-				sb += src_pixel_2.B * src_pixel_2.A;
+				sr += src_pixel_2.R;
+				sg += src_pixel_2.G;
+				sb += src_pixel_2.B;
 				sa += src_pixel_2.A;
 				++sc;
 			}
@@ -135,9 +135,9 @@ private static ColorBgra GetFinalPixelColor (
 		return
 			(sa != 0)
 			? ColorBgra.FromBgra (
-				b: Utility.ClampToByte (sb / sa),
-				g: Utility.ClampToByte (sg / sa),
-				r: Utility.ClampToByte (sr / sa),
+				b: Utility.ClampToByte (sb / sc),
+				g: Utility.ClampToByte (sg / sc),
+				r: Utility.ClampToByte (sr / sc),
 				a: Utility.ClampToByte (sa / sc))
 			: ColorBgra.FromUInt32 (0);
 	}