Merge pull request #295 from Sorunome/soru/fix-gif

fix: Some gifs not thumbnailing correctly

thumbnailing/i/gif.go

@@ -49,15 +49,6 @@ func (d gifGenerator) GenerateThumbnail(b []byte, contentType string, width int,
 			disposal = g.Disposal[i]
 		}
 
-		// if disposal type is 1 (preserve previous frame) we can get artifacts from re-scaling.
-		// as such, we re-render those frames to disposal type 0 (start with a transparent frame)
-		// see https://www.w3.org/Graphics/GIF/spec-gif89a.txt
-		if disposal == 1 {
-			g.Disposal[i] = 0
-		} else {
-			draw.Draw(frameImg, frameImg.Bounds(), image.Transparent, image.Point{X: 0, Y: 0}, draw.Src)
-		}
-
 		// Copy the frame to a new image and use that
 		draw.Draw(frameImg, frameImg.Bounds(), img, image.Point{X: 0, Y: 0}, draw.Over)
 
@@ -94,6 +85,17 @@ func (d gifGenerator) GenerateThumbnail(b []byte, contentType string, width int,
 			}, nil
 		}
 
+		// if disposal type is 0 or 1 (preserve previous frame) we can get artifacts from re-scaling.
+		// as such, we re-render those frames to disposal type 1 (do not dispose)
+		// Importantly, we do not clear the previous frame buffer canvas
+		// see https://www.w3.org/Graphics/GIF/spec-gif89a.txt
+		// This also applies to frame disposal type 0, https://legacy.imagemagick.org/Usage/anim_basics/#none
+		if disposal == 1 || disposal == 0 {
+			g.Disposal[i] = 1
+		} else {
+			draw.Draw(frameImg, frameImg.Bounds(), image.Transparent, image.Point{X: 0, Y: 0}, draw.Src)
+		}
+
 		g.Image[i] = targetImg
 	}