Video thumbnail generation

thumb_video.go

@@ -0,0 +1,136 @@
+package main
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
+	"time"
+)
+
+func init() {
+	thumbnailers = append(thumbnailers, &videoThumbnailer{})
+}
+
+// ffmpegPath is resolved once at startup. Empty means ffmpeg is not installed,
+// in which case the video thumbnailer disables itself and videos keep their
+// icon. ffmpeg is a graceful soft-dependency, not a hard requirement: the
+// binary still runs standalone without it.
+var ffmpegPath, _ = exec.LookPath("ffmpeg")
+
+type videoThumbnailer struct{}
+
+func isVideoExt(ext string) bool {
+	switch ext {
+	case ".mp4", ".mkv", ".mov", ".avi", ".webm", ".m4v":
+		return true
+	}
+	return false
+}
+
+// isVideoFile reports whether name is a video container we generate previews
+// for. Independent of ffmpeg availability — used to tag tiles with a play badge.
+func isVideoFile(name string) bool {
+	return isVideoExt(strings.ToLower(filepath.Ext(name)))
+}
+
+func (vt *videoThumbnailer) CanHandle(ext string) bool {
+	return ffmpegPath != "" && isVideoExt(ext)
+}
+
+// Generate satisfies Thumbnailer but is never used: handleThumb routes video
+// through the PathThumbnailer branch (GenerateFromPath), since ffmpeg needs a
+// seekable file path and we must not read large videos into memory.
+func (vt *videoThumbnailer) Generate(src io.Reader, dst io.Writer, width int) error {
+	return fmt.Errorf("video thumbnails require a file path; use GenerateFromPath")
+}
+
+// GenerateFromPath extracts a single frame and writes it to dst as JPEG. It
+// prefers a frame from the middle of the clip (see seekOffsets) to avoid the
+// black/fade-in frames common at the very start, falling back to earlier
+// offsets. The (small) JPEG is buffered first so a failed attempt never leaves
+// partial bytes in dst.
+func (vt *videoThumbnailer) GenerateFromPath(srcPath string, dst io.Writer, width int) error {
+	for _, seek := range seekOffsets(srcPath) {
+		var buf bytes.Buffer
+		if err := runFFmpegFrame(srcPath, &buf, width, seek); err == nil && buf.Len() > 0 {
+			_, err = dst.Write(buf.Bytes())
+			return err
+		}
+	}
+	return fmt.Errorf("ffmpeg produced no frame for %s", srcPath)
+}
+
+// seekOffsets returns seek targets (in seconds) to try in order. When the clip
+// duration is known the midpoint comes first — a representative frame that
+// dodges black intros and end credits. "1" and "0" are fallbacks for when the
+// duration is unknown or the midpoint seek yields nothing (very short clips, or
+// a keyframe gap at the midpoint).
+func seekOffsets(srcPath string) []string {
+	if d := videoDurationSec(srcPath); d > 2 {
+		return []string{strconv.FormatFloat(d/2, 'f', 2, 64), "1", "0"}
+	}
+	return []string{"1", "0"}
+}
+
+// videoDurationSec parses the clip duration from ffmpeg's own stderr, so it
+// depends only on ffmpeg (ffprobe is not installed everywhere we deploy).
+// `ffmpeg -i <file>` with no output exits non-zero but prints the container
+// metadata, including a "Duration: HH:MM:SS.ss" line. Returns 0 when the
+// duration can't be determined, so the caller falls back to a fixed offset.
+func videoDurationSec(srcPath string) float64 {
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, ffmpegPath, "-i", srcPath)
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
+	_ = cmd.Run() // expected to "fail": no output file specified. We want stderr.
+	return parseFFmpegDuration(stderr.String())
+}
+
+// parseFFmpegDuration extracts seconds from an ffmpeg log containing a line like
+// "  Duration: 00:01:23.45, start: 0.000000, bitrate: 1234 kb/s".
+func parseFFmpegDuration(log string) float64 {
+	_, after, ok := strings.Cut(log, "Duration:")
+	if !ok {
+		return 0
+	}
+	field := strings.TrimSpace(after)
+	if c := strings.IndexByte(field, ','); c >= 0 {
+		field = field[:c]
+	}
+	parts := strings.Split(strings.TrimSpace(field), ":")
+	if len(parts) != 3 {
+		return 0
+	}
+	h, err1 := strconv.ParseFloat(parts[0], 64)
+	m, err2 := strconv.ParseFloat(parts[1], 64)
+	s, err3 := strconv.ParseFloat(parts[2], 64)
+	if err1 != nil || err2 != nil || err3 != nil {
+		return 0
+	}
+	return h*3600 + m*60 + s
+}
+
+func runFFmpegFrame(srcPath string, dst io.Writer, width int, seek string) error {
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, ffmpegPath,
+		"-ss", seek,
+		"-i", srcPath,
+		"-frames:v", "1",
+		// Cap width at the requested size but never upscale (matches the image
+		// thumbnailer); -2 keeps the height even. The comma in min() is escaped
+		// so ffmpeg does not read it as a filter separator.
+		"-vf", "scale='min(iw\\,"+strconv.Itoa(width)+")':-2",
+		"-f", "mjpeg",
+		"-q:v", "5",
+		"pipe:1",
+	)
+	cmd.Stdout = dst
+	return cmd.Run()
+}