Add thumbnailing for photo grids

thumb_image.go

@@ -0,0 +1,81 @@
+package main
+
+import (
+	"image"
+	"image/color"
+	_ "image/gif"
+	"image/jpeg"
+	_ "image/png"
+	"io"
+)
+
+func init() {
+	thumbnailers = append(thumbnailers, &imageThumbnailer{})
+}
+
+type imageThumbnailer struct{}
+
+func (it *imageThumbnailer) CanHandle(ext string) bool {
+	switch ext {
+	case ".jpg", ".jpeg", ".png", ".gif":
+		return true
+	}
+	return false
+}
+
+func (it *imageThumbnailer) Generate(src io.Reader, dst io.Writer, width int) error {
+	img, _, err := image.Decode(src)
+	if err != nil {
+		return err
+	}
+	return jpeg.Encode(dst, resizeBox(img, width), &jpeg.Options{Quality: 80})
+}
+
+// resizeBox downsamples src to the requested width using a box filter.
+// Aspect ratio is preserved. Upscaling is a no-op (returns src unchanged).
+// Each source pixel is visited exactly once; alpha is discarded.
+func resizeBox(src image.Image, width int) image.Image {
+	b := src.Bounds()
+	srcW, srcH := b.Dx(), b.Dy()
+	if srcW <= width {
+		return src
+	}
+	dstW := width
+	dstH := srcH * width / srcW
+	if dstH < 1 {
+		dstH = 1
+	}
+	dst := image.NewRGBA(image.Rect(0, 0, dstW, dstH))
+
+	for y := 0; y < dstH; y++ {
+		sy0 := y * srcH / dstH
+		sy1 := (y + 1) * srcH / dstH
+		if sy1 == sy0 {
+			sy1 = sy0 + 1
+		}
+		for x := 0; x < dstW; x++ {
+			sx0 := x * srcW / dstW
+			sx1 := (x + 1) * srcW / dstW
+			if sx1 == sx0 {
+				sx1 = sx0 + 1
+			}
+			var r, g, bl, n uint64
+			for sy := sy0; sy < sy1; sy++ {
+				for sx := sx0; sx < sx1; sx++ {
+					sr, sg, sb, _ := src.At(b.Min.X+sx, b.Min.Y+sy).RGBA()
+					r += uint64(sr >> 8)
+					g += uint64(sg >> 8)
+					bl += uint64(sb >> 8)
+					n++
+				}
+			}
+			dst.SetRGBA(x, y, color.RGBA{
+				R: uint8(r / n),
+				G: uint8(g / n),
+				B: uint8(bl / n),
+				A: 255,
+			})
+		}
+	}
+	return dst
+}