Video Color Grading

Color grading

Script: scripts/grade.py IN OUT --look NAME [--strength 0.8] [--lut file.cube] [--compare grid.png] Lists looks with --list. Applies a look as an ffmpeg filter chain; --compare renders before/after stills of several looks so you (and the user) can choose by eye.

1. Order of operations (always)

1. Input transform: rotation/HDR/log → Rec.709 (Color Space Transform in Resolve, LUT or tonemap in ffmpeg). Never grade log footage directly by eye.
2. Correction (make it neutral and consistent): white balance → exposure → contrast → saturation. Goal: whites white, blacks black, skin natural, every shot matching.
3. Look / creative grade: hue shifts, split toning, curves, LUT at reduced strength.
4. Finishing: vignette, grain, halation, sharpening last.

Grade per segment before concat so you encode once. Same look for every clip in a sequence unless a change is a story beat (e.g. calm warm A → chaotic cold B).

2. Mental model: ASC CDL

Per channel: out = (in × slope + offset) ^ power, then saturation.

• slope → highlights/gain, offset → shadows/lift, power → midtones/gamma.
• Warm highlights: raise R slope, lower B slope slightly. Teal shadows: offset B and G up, R down.
• ffmpeg equivalents: colorbalance (shadows/mids/highs per RGB), colorlevels, curves, eq (contrast/brightness/saturation/gamma), colortemperature, lut3d, colorchannelmixer.

3. Looks (starting points — judge on frames, then adjust)

Look	Feel	Key moves
neutral-punch	clean, social-ready	small S-curve, +8% sat, no hue shift
teal-orange	blockbuster, travel	teal shadows, warm highlights, protect skin (orange is skin's hue)
warm-golden	nostalgic, food, lifestyle	WB +300-600 K, lifted warm shadows, soft contrast
cool-moody	tech, night, drama	cooler WB, desaturated mids, deep but not crushed blacks
film-kodak	2383/Portra-style print	rolled-off highlights, slight green-cyan shadows, warm mids, grain 4-8%
bleach-bypass	gritty, war, sport	high contrast, -40% saturation, silvery highlights
pastel-matte	aesthetic, minimal, fashion	lifted blacks (matte), lower contrast, -15% sat, pink/peach highs
day-for-night	fake night from day footage	-1.5 to -2 stops, blue cast, sat 0.3-0.5, crush sky, keep practical lights warm
vintage-faded	home video, 90s	faded blacks, yellow-green highs, chroma noise, softness
bw-contrast	timeless, documentary	luminance mix (R heavy for skin), strong S-curve, grain
cyberpunk-neon	night city, gaming	magenta-cyan split, high sat in highs, bloom on lights

Day for night reference: DaVinci Resolve's "LMT Day for Night" works in ACES AP1 linear as power 0.5 → slope (0.3, 0.5, 0.8) → power 2.2 → saturation 0.3. To use it on Rec.709 footage, convert Rec.709 → linear AP1, apply, convert back, and bake it into a 33-point .cube. Lift very dark sources a little before the LUT.

4. LUTs

• Apply with lut3d=file.cube:interp=tetrahedral. Use at reduced strength by blending: split[a][b];[b]lut3d=x.cube[g];[a][g]blend=all_expr='A*(1-0.7)+B*0.7'.
• Technical LUTs (log→709) are exact, use 100%. Creative LUTs are seasoning, usually 40-80%.
• Never read a .cube file into context (tens of thousands of numbers). Look at its effect on a frame.
• Generate custom LUTs in Python: build a 33³ grid, apply your colour function with numpy, write LUT_3D_SIZE 33 + rows (R fastest).

5. Skin tones

• Skin lies on the skin-tone line (~123° on a vectorscope, I-line) regardless of ethnicity; only luminance and saturation change.
• When pushing teal/cool looks, isolate skin (HSL key hue 0-50°, mid saturation) and hold it, or the face goes grey/green.
• Check every face shot in the grade at full size.

6. Matching shots

• Match exposure first (waveform/average luma), then WB (neutral object or white), then saturation.
• Quick numeric match: compare per-channel mean and std of a neutral region between shots; adjust slope/offset until within ~2-3%.
• Mixed cameras (phone + action cam + DSLR): convert all to Rec.709 first, then match to the hero camera.

7. Finishing touches

• Grain: noise=alls=6:allf=t (temporal). Add after scaling to the final size. 4K reels: grain survives compression better at slightly stronger values.
• Vignette: vignette=PI/5 subtle. Avoid on content with text near edges.
• Halation / bloom: blur a highlight-only copy (threshold, gblur 20-40), tint red-orange, screen at 15-30%.
• Sharpen: unsharp=5:5:0.6 once, at the end, at final size.
• HDR to SDR: see ffmpeg-pipeline tone-mapping.

8. In DaVinci Resolve

Node tree baseline: CST in → WB/exposure → contrast/sat → secondary (skin) → look/LUT → CST out → grain/vignette. Use Color Space Transform with input = camera log (e.g. Apple Log) and output = Rec.709 Gamma 2.4. Copy grades with stills or Color Groups (pre/post-clip). See davinci-resolve.

9. Verify

Before/after stills of 3+ shots (brightest, darkest, a face) side by side. Check: no clipped highlights on skin, blacks not crushed where detail matters, text overlays still readable, consistent look across cuts.