mosaic

Troubleshooting

This document lists common failures and how to debug them.

FFprobe Not Found

Symptom:

exec: "ffprobe": executable file not found in $PATH

Fix:

Install FFmpeg and ensure ffprobe is on PATH.

ffprobe -version
ffmpeg -version

FFmpeg Not Found

Symptom:

exec: "ffmpeg": executable file not found in $PATH

Fix:

Install FFmpeg and ensure ffmpeg is on PATH.

Remote URL Fails

Symptoms:

Failed to resolve hostname
Input/output error

Likely causes:

• DNS blocked.
• Network egress blocked.
• URL expired.
• FFmpeg build lacks protocol support.

Debug:

ffprobe -v error -show_format -show_streams "https://example.com/video.mp4"

Output Directory Permission Denied

Symptom:

create output dir: mkdir /path: permission denied

Fix:

Use a writable output directory.

mkdir -p /tmp/mosaic-output

Mosaic creates directories automatically, but the process still needs permissions.

Video Is Upside Down

Likely cause:

• Source rotation metadata was interpreted differently by the player.
• Orientation normalization was disabled.

Recommended option:

mosaic.WithNormalizeOrientation()

Debug source metadata:

ffprobe -v error -select_streams v:0 \
  -show_entries stream=width,height:stream_tags=rotate:stream_side_data=rotation \
  -of json input.mp4

Debug output init segment:

ffprobe -v error -select_streams v:0 \
  -show_entries stream=width,height:stream_tags=rotate:stream_side_data=rotation \
  -of json /tmp/output/init_0.mp4

Output rotation metadata should be absent or 0.

Output Aspect Ratio Looks Wrong

Mosaic preserves display aspect ratio when building ladder dimensions.

Check source display dimensions:

ffprobe -v error -select_streams v:0 \
  -show_entries stream=width,height:stream_side_data=rotation:stream_tags=rotate \
  -of json input.mp4

Check output dimensions:

for f in /tmp/output/init*.mp4; do
  echo "$f"
  ffprobe -v error -select_streams v:0 \
    -show_entries stream=width,height \
    -of csv=p=0 "$f"
done

For a square input, outputs should be square.

For a portrait input, outputs should be portrait.

For a non-standard input, dimensions may differ by one or two pixels from the exact mathematical ratio because Mosaic forces even dimensions.

Hardware Encoder Fails

Symptoms:

Unknown encoder 'h264_nvenc'
No capable devices found
Error initializing output stream

Fix:

• Confirm FFmpeg supports the encoder:

ffmpeg -hide_banner -encoders | grep h264

• Confirm drivers and devices are available.
• Try CPU encoding by removing the GPU option.

HLS Player Cannot Load Output

Check files:

find /tmp/output -maxdepth 1 -type f -printf '%f\n' | sort

Expected HLS files:

master.m3u8
stream_0.m3u8
init*.mp4
seg_*.m4s

Serve the directory over HTTP for browser playback. Many browser/player combinations do not reliably load HLS from file://.

DASH Player Cannot Load Output

Expected DASH files:

manifest.mpd
init-stream*.m4s
chunk-stream*.m4s

Use an HTTP server for playback tests.

Tests Fail Because Go Cache Is Read-Only

Symptom:

open /home/user/.cache/go-build/...: read-only file system

Fix:

GOCACHE=/tmp/go-build go test ./...

Collecting a Useful Bug Report

Include:

• input file or URL
• exact code snippet or command
• OS and architecture
• Go version
• FFmpeg version
• full error output
• source FFprobe JSON
• output init segment FFprobe JSON
• expected output dimensions
• actual output dimensions

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