improve gif thumbnail generation

changelog/unreleased/gif-thumbnails.md

@@ -0,0 +1,5 @@
+Bugfix: Improve gif thumbnails
+
+Improved the gif thumbnail generation for gifs with different disposal strategies.
+
+https://github.com/owncloud/ocis/pull/3305

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) {