更新缩略图

backend/internal/service/thumbnail.go

@@ -0,0 +1,71 @@
+package service
+
+import (
+	"bytes"
+	"image"
+	"image/jpeg"
+	"strings"
+
+	"golang.org/x/image/draw"
+
+	_ "image/gif"
+	_ "image/jpeg"
+	_ "image/png"
+
+	_ "golang.org/x/image/webp"
+)
+
+// thumbSuffix is appended to an image's object key to form its thumbnail key
+// ("u/x.png" → "u/x.png.thumb.jpg"). List views load the thumbnail; preview and
+// download always use the original.
+const thumbSuffix = ".thumb.jpg"
+
+// thumbMaxDim bounds the thumbnail's longest side. 512px is crisp for grid
+// cards / table rows while staying ~20-50 KB as JPEG.
+const thumbMaxDim = 512
+
+// ThumbKey returns the thumbnail object key for an image key.
+func ThumbKey(rel string) string { return rel + thumbSuffix }
+
+// IsThumbKey reports whether name refers to a thumbnail object, and OrigKey
+// maps a thumbnail key back to its original image key.
+func IsThumbKey(name string) bool { return strings.HasSuffix(name, thumbSuffix) }
+func OrigKey(name string) string  { return strings.TrimSuffix(name, thumbSuffix) }
+
+// makeThumbnail downscales an image to thumbMaxDim (longest side) and encodes
+// it as JPEG. Images already small enough are re-encoded as-is (so the thumb
+// object always exists once generated). Returns an error for undecodable input
+// (e.g. video bytes) — callers treat thumbnailing as best-effort.
+func makeThumbnail(b []byte) ([]byte, error) {
+	src, _, err := image.Decode(bytes.NewReader(b))
+	if err != nil {
+		return nil, err
+	}
+	bounds := src.Bounds()
+	w, h := bounds.Dx(), bounds.Dy()
+	tw, th := w, h
+	if w > thumbMaxDim || h > thumbMaxDim {
+		if w >= h {
+			tw = thumbMaxDim
+			th = h * thumbMaxDim / w
+		} else {
+			th = thumbMaxDim
+			tw = w * thumbMaxDim / h
+		}
+		if tw < 1 {
+			tw = 1
+		}
+		if th < 1 {
+			th = 1
+		}
+	}
+	// JPEG has no alpha — composite onto white so transparent PNGs don't go black.
+	dst := image.NewRGBA(image.Rect(0, 0, tw, th))
+	draw.Draw(dst, dst.Bounds(), image.White, image.Point{}, draw.Src)
+	draw.CatmullRom.Scale(dst, dst.Bounds(), src, bounds, draw.Over, nil)
+	var out bytes.Buffer
+	if err := jpeg.Encode(&out, dst, &jpeg.Options{Quality: 78}); err != nil {
+		return nil, err
+	}
+	return out.Bytes(), nil
+}