generate animated thumbnails for animated gifs

thumbnails/pkg/thumbnail/generator.go

@@ -0,0 +1,65 @@
+package thumbnail
+
+import (
+	"errors"
+	"image"
+	"image/draw"
+	"image/gif"
+	"strings"
+
+	"github.com/disintegration/imaging"
+)
+
+var (
+	// ErrInvalidType represents the error when a type can't be encoded.
+	ErrInvalidType2 = errors.New("can't encode this type")
+	// ErrNoGeneratorForType represents the error when no generator could be found for a type.
+	ErrNoGeneratorForType = errors.New("no generator for this type found")
+)
+
+type Generator interface {
+	GenerateThumbnail(image.Rectangle, interface{}) (interface{}, error)
+}
+
+type SimpleGenerator struct{}
+
+func (g SimpleGenerator) GenerateThumbnail(size image.Rectangle, img interface{}) (interface{}, error) {
+	m, ok := img.(image.Image)
+	if !ok {
+		return nil, ErrInvalidType2
+	}
+
+	return imaging.Thumbnail(m, size.Dx(), size.Dy(), imaging.Lanczos), nil
+}
+
+type GifGenerator struct{}
+
+func (g GifGenerator) GenerateThumbnail(size image.Rectangle, img interface{}) (interface{}, error) {
+	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)
+	}
+	m.Config.Height = bounds.Dy()
+	m.Config.Width = bounds.Dx()
+	return m, nil
+}
+
+// GeneratorForType returns the generator for a given file type
+// or nil if the type is not supported.
+func GeneratorForType(fileType string) (Generator, error) {
+	switch strings.ToLower(fileType) {
+	case typePng, typeJpg, typeJpeg:
+		return SimpleGenerator{}, nil
+	case typeGif:
+		return GifGenerator{}, nil
+	default:
+		return nil, ErrNoEncoderForType
+	}
+}