improve gif thumbnail generation

thumbnails/pkg/thumbnail/generator.go

@@ -3,6 +3,7 @@ package thumbnail
 import (
 	"errors"
 	"image"
+	"image/color"
 	"image/draw"
 	"image/gif"
 	"strings"
@@ -35,22 +36,44 @@ func (g SimpleGenerator) GenerateThumbnail(size image.Rectangle, img interface{}
 type GifGenerator struct{}
 
 func (g GifGenerator) GenerateThumbnail(size image.Rectangle, img interface{}) (interface{}, error) {
+	// Code inspired by https://github.com/willnorris/gifresize/blob/db93a7e1dcb1c279f7eeb99cc6d90b9e2e23e871/gifresize.go
+
 	m, ok := img.(*gif.GIF)
 	if !ok {
 		return nil, ErrInvalidType2
 	}
-	var bounds image.Rectangle
-	for i := range m.Image {
-		img := imaging.Resize(m.Image[i], size.Dx(), size.Dy(), imaging.Lanczos)
-		bounds = image.Rect(0, 0, size.Dx(), size.Dy())
-		m.Image[i] = image.NewPaletted(bounds, m.Image[i].Palette)
-		draw.Draw(m.Image[i], bounds, img, image.Pt(0, 0), draw.Src)
+	// Create a new RGBA image to hold the incremental frames.
+	srcX, srcY := m.Config.Width, m.Config.Height
+	b := image.Rect(0, 0, srcX, srcY)
+	tmp := image.NewRGBA(b)
+
+	for i, frame := range m.Image {
+		bounds := frame.Bounds()
+		prev := tmp
+		draw.Draw(tmp, bounds, frame, bounds.Min, draw.Over)
+		scaled := imaging.Resize(tmp, size.Dx(), size.Dy(), imaging.Lanczos)
+		m.Image[i] = g.imageToPaletted(scaled, frame.Palette)
+
+		switch m.Disposal[i] {
+		case gif.DisposalBackground:
+			tmp = image.NewRGBA(b)
+		case gif.DisposalPrevious:
+			tmp = prev
+		}
 	}
-	m.Config.Height = bounds.Dy()
-	m.Config.Width = bounds.Dx()
+	m.Config.Width = size.Dx()
+	m.Config.Height = size.Dy()
+
 	return m, nil
 }
 
+func (g GifGenerator) imageToPaletted(img image.Image, p color.Palette) *image.Paletted {
+	b := img.Bounds()
+	pm := image.NewPaletted(b, p)
+	draw.FloydSteinberg.Draw(pm, b, img, image.Point{})
+	return pm
+}
+
 // GeneratorForType returns the generator for a given file type
 // or nil if the type is not supported.
 func GeneratorForType(fileType string) (Generator, error) {