Normalize reserved color_trc in Preview transcoder

Same swscaler failure that broke VideoH264 (7ec46fe) also hit the
lowres preview transcoder on ProRes inputs with reserved/unknown
color_transfer ("Operation not supported", exit 161). Insert the
setparams=color_trc=bt709 filter ahead of scale in both Preview
filter graphs — the video-only branch and the video+audio branch
via buildVUMeterFilters.

Collapse the two NormalizeColorTRCFilter variants into a single
helper that takes a StreamInfo and picks the first video stream
internally, so VideoH264 and Preview share one code path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: KillerX

potential_improvements.md

@@ -0,0 +1,3 @@
+# Potential improvements
+
+- `services/transcode/preview.go::GrowingPreview` has the same `trc:reserved` swscaler-failure exposure that was just fixed in `Preview` and `VideoH264`. The function reads from a pipe (`pipe:0`) so it cannot probe upfront. Options to consider: probe `input.FilePath` once before starting (race-y while the file is still growing), unconditionally insert `setparams=color_trc=bt709` since the workflow is SDR-only, or accept the trc explicitly from the caller.

services/ffmpeg/probe.go

@@ -102,16 +102,19 @@ type FFProbeResult struct {
 	} `json:"format"`
 }
 
-// NormalizeColorTRCFilter returns a setparams filter clause when the source
-// stream's color_transfer is unset/unknown/reserved, or "" when the existing
-// tag is already valid.
+// NormalizeColorTRCFilter returns a setparams filter clause when the first
+// video stream's color_transfer is unset/unknown/reserved, or "" when the
+// existing tag is already valid (or there is no video stream).
 //
 // Why: ProRes files written with reserved/unknown trc make swscaler refuse
 // pix_fmt conversions ("Operation not supported"), which breaks the
-// scale->libx264 pipeline in VideoH264. bt709 is a safe SDR default for the
-// content this repo handles.
-func NormalizeColorTRCFilter(s FFProbeStream) string {
-	switch strings.ToLower(strings.TrimSpace(s.ColorTransfer)) {
+// scale->libx264 pipeline in VideoH264 and the lowres preview transcoder.
+// bt709 is a safe SDR default for the content this repo handles.
+func NormalizeColorTRCFilter(info StreamInfo) string {
+	if len(info.VideoStreams) == 0 {
+		return ""
+	}
+	switch strings.ToLower(strings.TrimSpace(info.VideoStreams[0].ColorTransfer)) {
 	case "", "unknown", "reserved":
 		return "setparams=color_trc=bt709"
 	}

services/ffmpeg/probe_test.go

@@ -25,8 +25,13 @@ func TestNormalizeColorTRCFilter(t *testing.T) {
 
 	for _, tc := range cases {
 		t.Run(tc.name, func(t *testing.T) {
-			got := NormalizeColorTRCFilter(FFProbeStream{ColorTransfer: tc.trc})
+			info := StreamInfo{VideoStreams: []FFProbeStream{{ColorTransfer: tc.trc}}}
+			got := NormalizeColorTRCFilter(info)
 			assert.Equal(t, tc.expected, got)
 		})
 	}
+
+	t.Run("no video stream returns empty", func(t *testing.T) {
+		assert.Equal(t, "", NormalizeColorTRCFilter(StreamInfo{}))
+	})
 }

services/transcode/preview.go

@@ -45,12 +45,12 @@ type audioPreviewData struct {
 }
 
 // buildVUMeterFilters generates ffmpeg filter steps for compact VU meters for each audio track.
-func buildVUMeterFilters(audioTracks int) (string, string) {
+func buildVUMeterFilters(audioTracks int, trcPrefix string) (string, string) {
 	meterW := 200
 	meterH := 20
 	meterAlpha := 0.5
 	spacing := 10
-	parts := []string{"[0:v]scale=1280:720[vmain]"}
+	parts := []string{fmt.Sprintf("[0:v]%sscale=1280:720[vmain]", trcPrefix)}
 	lastVid := "[vmain]"
 	for i := 0; i < audioTracks; i++ {
 		y := 10 + i*(meterH+spacing)
@@ -201,6 +201,11 @@ func Preview(input PreviewInput, progressCallback ffmpeg.ProgressCallback) (*Pre
 		}
 	}
 
+	var trcPrefix string
+	if trcFix := ffmpeg.NormalizeColorTRCFilter(ffmpeg.ProbeResultToInfo(info)); trcFix != "" {
+		trcPrefix = trcFix + ","
+	}
+
 	filename := filepath.Base(input.FilePath)[:len(filepath.Base(input.FilePath))-len(filepath.Ext(input.FilePath))]
 	if hasVideo {
 		filename += "_lowres.mp4"
@@ -225,7 +230,7 @@ func Preview(input PreviewInput, progressCallback ffmpeg.ProgressCallback) (*Pre
 			"-i", input.FilePath,
 			"-ss", "0.0",
 			"-i", watermark,
-			"-filter_complex", "sws_flags=bicubic;[0:v]split=1[VIDEO-main-.mp4];[VIDEO-main-.mp4]scale=-2:540,null[temp];[temp][1:v]overlay=0:0:eof_action=repeat[VIDEO-.mp4]",
+			"-filter_complex", fmt.Sprintf("sws_flags=bicubic;[0:v]%ssplit=1[VIDEO-main-.mp4];[VIDEO-main-.mp4]scale=-2:540,null[temp];[temp][1:v]overlay=0:0:eof_action=repeat[VIDEO-.mp4]", trcPrefix),
 			"-map", "[VIDEO-.mp4]",
 			"-c:v", encoder,
 		)
@@ -238,7 +243,7 @@ func Preview(input PreviewInput, progressCallback ffmpeg.ProgressCallback) (*Pre
 			"-ss", "0.0",
 			"-i", watermark,
 		)
-		vuFilters, lastVid := buildVUMeterFilters(audioTracks)
+		vuFilters, lastVid := buildVUMeterFilters(audioTracks, trcPrefix)
 		// Compose filter graph: scale, vumeters, watermark, stereo audio
 		var audioFilter string
 		if audioTracks == 1 {

services/transcode/preview_test.go

@@ -63,6 +63,15 @@ func Test_AudioAdioPreviewGenerator(t *testing.T) {
 
 }
 
+func TestBuildVUMeterFilters_TRCPrefix(t *testing.T) {
+	head, _ := buildVUMeterFilters(2, "setparams=color_trc=bt709,")
+	assert.Contains(t, head, "[0:v]setparams=color_trc=bt709,scale=1280:720[vmain]")
+
+	head2, _ := buildVUMeterFilters(2, "")
+	assert.Contains(t, head2, "[0:v]scale=1280:720[vmain]")
+	assert.NotContains(t, head2, "setparams")
+}
+
 func TestPreview_VUMeters_MultipleAudioTracks(t *testing.T) {
 	t.Parallel()
 	trackCounts := []int{1, 2, 4, 16}

services/transcode/video.go

@@ -54,10 +54,7 @@ func VideoH264(input common.VideoInput, cb ffmpeg.ProgressCallback) (*common.Vid
 
 	var filterComplex string
 
-	var trcFix string
-	if len(info.VideoStreams) > 0 {
-		trcFix = ffmpeg.NormalizeColorTRCFilter(info.VideoStreams[0])
-	}
+	trcFix := ffmpeg.NormalizeColorTRCFilter(info)
 
 	switch {
 	case input.WatermarkPath != nil && trcFix != "":