Quickly apply LUTs (color grading) with ffmpeg

This is a quick post, mostly for my own reference.

I've avoided LUTs and 'Log' video footage for years¹, mostly because of the extra tiny bit of workflow involved. Like RAW photos, 'Log' footage retains the video sensor's full dynamic range, so you can pull more color and luminance information out of the footage later.

But unlike photography, where RAW has been a thing for decades, and many workflows 'just work' without me having to 'grade' every individual photo, in video precious few consumer apps handle Log footage gracefully. You generally end up with a muddy grey mess.

I don't want to launch DaVinci Resolve or Final Cut Pro to process every video I shoot. Especially if I am recording in Log for a particular reason, but forget to switch back to 'normal' mode and shoot more clips later.

The difference is stark², as illustrated by this clip (above) I was shooting with my kids around the house with my DJI Osmo Pocket 4. I just picked it up and was testing it, and I realized I still had it in '10-bit D-Log' mode after I imported the footage to my computer.

I didn't want to go through launching Final Cut Pro just to grade a half-dozen clips to put them into our family video library, so I turned to ffmpeg, after seeing this mention of the built-in lut3d filter.

ffmpeg for LUTs

The full ffmpeg command I'm using is:

ffmpeg \
  -i /Users/jgeerling/Downloads/DJI_20000219193052_0005_D.MP4 \
  -vf "lut3d=osmo-vivid.cube" \
  -c:v hevc_videotoolbox \
  -profile:v main10 \
  -b:v 30M \
  -pix_fmt p010le \
  -tag:v hvc1 \
  -c:a copy \
  DJI_0005_D_converted.mp4

Some of those flags will only work on a Mac, where Apple's hardware H.265 video encoder is present. The main thing is -vf "lut3d=osmo-vivid.cube", which is the path to a .cube file for your specific camera/footage.

In this case, I'm using the DJI Osmo Pocket 4 'vivid' LUT³.

If you'd like to modify the ffmpeg flags to target your own hardware's H.264 or H.265 acceleration, just grab the command above and run it through your favorite LLM. After a couple decades using ffmpeg, it still feels like AP Calculus 2 trying to remember all the various flags and abbreviations :D

I'd love there to be a standard for 'LUT sidecars'. Apple, BlackMagic, Sony, and others do embed LUT data in video files... but there's no standard, so you need DaVinci Resolve for Blackmagic camera clips, QuickTime for iPhone clips... and in the end, most non-pro apps will just show a muddy video file if you don't process it like I'm doing with ffmpeg. Sigh.

Batch-converting Log footage with ffmpeg

Given a particular .cube file, here's how you can apply the LUT to all the MP4 files in a given folder:

#!/bin/bash
CUBE="osmo-vivid.cube"
DIR="video_files"

files=("$DIR"/*.MP4)
total=${#files[@]}
count=0

for input in "${files[@]}"; do
    [ -e "$input" ] || continue
    ((count++))
    echo "Processing file $count of $total: $(basename "$input")"
    
    ffmpeg -i "$input" -vf "lut3d=$CUBE" -c:v hevc_videotoolbox -profile:v main10 -b:v 30M -pix_fmt p010le -tag:v hvc1 -c:a copy "${input%.MP4}_converted.mp4"
done

Put the path to your .cube file in CUBE, and the path to a directory with a bunch of ungraded .MP4 files in DIR. Save the script, make it executable, and run it!