Post-production finishing: colour grading and DI

In short

Colour grading is the controlled alteration of brightness, contrast, colour balance and saturation of each shot so that a film or series reads correctly, cuts together seamlessly and carries the intended mood. The Digital Intermediate (DI) is the whole finishing pipeline around it — scanning or ingesting camera originals at full resolution, conforming them to the offline edit, grading, adding titles and finishing effects, and mastering out to theatrical, broadcast and OTT deliverables. It matters because the DI master is the single legal and commercial source from which every release copy, including the CBFC censor copy, is made. In Indian universities it is examined in post-production and digital cinematography papers; in industry it is the core skill of colourists and DI assistants at Prasad, Annapurna, Prime Focus/DNEG, Famous Studios, Pixion, Qube and every OTT finishing house.

Explained

What DI is, and the shape of the pipeline

• DI = finishing the picture digitally at full resolution and mastering from it, instead of grading photochemically on an answer print. Universal in India since about 2005; film-out to 35 mm negative on a laser recorder is now rare.
• Offline edit is done on lightweight proxies (ProRes Proxy, DNxHR LB, H.264) in Premiere Pro, Avid or FCP; online/conform rebuilds that edit from the camera originals.
• Conform handover is an EDL (CMX 3600, simple cuts), AAF (Avid, carries effects) or XML/FCPXML/OTIO, plus a cut list and a reference QuickTime with burnt-in timecode.
• Handles: 8–24 extra frames each side of every shot are pulled so the colourist and VFX can slip the cut.
• Order of a typical DI day: ingest → conform → check against reference → shot-match (pre-grade) → creative grade → VFX insert and final-grade → titles and subtitles → QC → master → deliverables.
• Dailies grade (on set/next morning, usually a CDL + show LUT) is a *look*, not a grade; it travels through the offline as metadata so the director sees roughly the right image from day one.

Colour fundamentals a colourist must hold

• Luminance vs chrominance: brightness vs hue+saturation. Rec.709 luma Y = 0.2126R + 0.7152G + 0.0722B; Rec.601 (SD) uses 0.299/0.587/0.114; Rec.2020 uses 0.2627/0.6780/0.0593.
• Colour space = primaries + white point + transfer function. Rec.709/sRGB primaries with D65 for HD; DCI-P3 (wider red/green) for cinema; Rec.2020 for UHD/HDR containers; ACES AP0/AP1 as working spaces.
• Transfer function: camera OETF (log or Rec.709), display EOTF (BT.1886 ≈ gamma 2.4 for SDR, ST 2084 PQ and HLG for HDR, gamma 2.6 for DCI projection).
• Log encoding (S-Log3, V-Log, Log C/Log C4, RedLogFilm, Canon Log 2/3, Cineon) packs 12–17 stops into 10 bits by storing roughly density, not light. Log footage looks flat and milky — it must never be judged ungraded.
• Linear scene-referred data (EXR, ACES) is proportional to light; grading maths behaves predictably (an exposure change is a multiply).
• Bit depth: 8-bit = 256 steps/channel (banding under grading), 10-bit = 1,024, 12-bit = 4,096, 16-bit half-float EXR for VFX. Grade in 32-bit float internally even from 10-bit sources.
• Chroma subsampling: 4:4:4 full chroma (grading, keying), 4:2:2 (broadcast acquisition), 4:2:0 (delivery). Keys and windows pulled from 4:2:0 tear at edges.
• Legal vs full range: video range 64–940 in 10-bit, data/full range 0–1023. Mismatched range is the commonest crushed-blacks/washed-whites error in Indian DI.

The grading tools, in the order they are used

1. Primary grade — affects the whole frame: lift/shadows, gamma/midtones, gain/highlights, offset (printer lights), contrast + pivot, saturation, colour temperature/tint, log wheels (shadow, midtone, highlight) which are more surgical on log material.
2. Curves — custom tone curve, plus Hue vs Hue, Hue vs Sat, Hue vs Lum, Lum vs Sat; the standard tool to desaturate Indian skin-tone yellows or tame a saffron that clips.
3. Secondary grade — isolates a region: HSL qualifier (a key on hue/sat/luma, used for skin, sky, costume), power window (circle, linear, polygon, curve, gradient) with softness and tracking across the shot, and matte/alpha from VFX.
4. Node graph / layer stack — Resolve uses nodes (serial, parallel, layer, key, outside); Baselight uses layers. Order matters: a saturation change before a qualifier shifts the key.
5. Effects inside the grade — grain (film grain plates or synthetic), halation/bloom, glow, vignette, sharpening, denoise (temporal/spatial), blur, sky replacement, beauty/skin softening, lens distortion, stabilisation, speed changes.
6. LUTs — 1D LUT (per-channel tone only) and 3D LUT (full colour mapping, typically 17³, 33³ or 65³ lattice with tetrahedral interpolation), as .cube or .3dl. Use a LUT as a *starting* transform, not as a grade; it clips what lies outside its input range.
7. ASC CDL — a ten-number interchange format (slope, offset, power per channel + saturation) that moves a look between on-set, editorial and DI without carrying a LUT.

Colour management: ACES and the alternatives

• Problem: five cameras, graphics, archive and stock footage must meet in one space and go out to cinema, HD, HDR and OTT.
• ACES (Academy Colour Encoding System, SMPTE ST 2065): Input Transform (IDT) per camera → working space ACEScct (log, with a toe, for grading) or ACEScg (linear, for VFX) → Output Transform (RRT+ODT) per display. ACES 2.0 with redesigned output transforms is now being adopted by facilities.
• Resolve Colour Management (RCM) and Baselight's Truelight CM are vendor equivalents; a show LUT workflow is the older, simpler and less future-proof method.
• Benefits: one grade, many deliverables; predictable VFX round-trip; archival master in a device-independent space. Limitations: the output transform imposes a look some cinematographers dislike; wrong IDT ruins the grade silently.

Mastering, deliverables and the Indian release chain

• Reference environment: a grading theatre matching the release — DCI projector at 48 cd/m² (14 fL) peak white on a calibrated screen for theatrical; a grade-1 reference monitor at 100 cd/m² D65 with a dim neutral surround (ITU-R BT.2035 conditions) for SDR video; 1,000 cd/m² class monitor for HDR. Calibrate to ΔE below about 2 and re-probe regularly.
• Scopes are the ground truth: waveform (luma/RGB parade levels), vectorscope (hue and saturation, skin-tone line at about 123° — the I-line, roughly the 11 o'clock direction on the graticule), histogram, false colour. Eyes adapt; scopes do not.
• Theatrical: DI master → DCP — JPEG 2000 frames in 12-bit X′Y′Z′ with gamma 2.6, SMPTE ST 428/429 packaging, max 250 Mbit/s, encrypted with KDM keys per screen and date. Indian distribution runs largely over Qube (Real Image) and UFO Moviez satellite/hard-disk networks.
• Broadcast: Rec.709, 1080i50 or 1080p25 for India (PAL heritage, 25 fps), ProRes 422 HQ/XDCAM, with loudness to ITU-R BS.1770 / EBU R128 as the client specifies.
• OTT (Netflix, Prime Video, JioHotstar, ZEE5, SonyLIV): IMF package (SMPTE ST 2067) or ProRes 4444 XQ, usually both an SDR Rec.709 and an HDR (Dolby Vision or HDR10) master, plus textless elements and separate M&E audio.
• Archive: ACES/EXR or DPX image sequence plus the project database and LUTs — a graded MP4 is not an archive.
• Certification: the picture is certified by the CBFC under the Cinematograph Act, 1952 as amended in 2023, which introduced the age-based sub-categories UA 7+, UA 13+ and UA 16+, made certificates perpetually valid, and added penal provisions against unauthorised camcording and transmission of a film in a cinema. Censor cuts and the certificate card are inserted in the DI before the release DCP is built.

Grading as craft: matching, looks and limits

• Shot matching first, look second. Neutralise and match every shot in a scene (skin tone, black level, white point) before applying the creative look, or the look hides the mismatch.
• Continuity: the audience reads a cut, not a frame; a half-stop jump across a cut is more visible than a wrong-but-consistent scene.
• Skin tone is the anchor. Indian skin covers a wide range; keep it on or near the vectorscope skin line and protect it with a qualifier while pushing the rest of the frame.
• Day-for-night, flashback, dream, period looks are built with desaturation, channel mixing, split-toning and crushed or lifted blacks — all reversible, which is why nothing destructive is baked on the plate.
• Hard limits: you cannot recover clipped highlights, add resolution, remove motion blur or fix a wrong white balance baked into 8-bit 4:2:0 without artefacts. Grading rewards exposure discipline on set — expose for the sensor, protect the highlights with log.
• Round-trip with VFX: send plates in the working space with the grade *off* (or as a CDL alongside), receive comps back at the same space, re-apply the grade.

Key points

• DI — full-resolution digital finishing chain: conform, grade, titles, master; the DI master is the source of every deliverable.
• Offline/online — creative cut on proxies, finish on originals; linked by EDL/AAF/XML plus handles.
• Log encoding — compresses wide dynamic range into 10 bits; flat by design, never delivered ungraded.
• Linear/scene-referred — values proportional to light; the correct space for VFX and exposure maths.
• Primary grade — lift, gamma, gain, offset, contrast+pivot, saturation, temperature/tint over the whole frame.
• Secondary grade — HSL qualifier, power window, tracking; targets a region or a colour.
• ASC CDL — ten portable numbers (slope, offset, power ×3 + saturation) carrying a look between departments.
• 3D LUT — lattice colour mapping (17³/33³/65³); a transform or a look, not a substitute for grading.
• ACES — Input Transform → ACEScct/ACEScg → Output Transform; one grade, many display targets.
• Colour space — primaries + white point + transfer function; Rec.709, DCI-P3, Rec.2020, ACES AP0/AP1.
• EOTF — BT.1886 (SDR ≈ γ2.4), ST 2084 PQ (absolute nits), HLG (relative, broadcast-friendly), γ2.6 (DCI).
• Scopes — waveform, RGB parade, vectorscope, histogram: the only objective check on a grade.
• Reference levels — 48 cd/m² cinema white, 100 cd/m² SDR reference white, ~203 cd/m² HDR diffuse white.
• DCP — JPEG 2000, 12-bit X′Y′Z′, γ2.6, ≤250 Mbit/s, KDM-encrypted; Qube and UFO distribute it in India.
• Legal vs full range — 64–940 vs 0–1023 in 10 bits; a mismatch crushes or washes the image.
• Bit depth and subsampling — 10-bit 4:4:4 minimum for serious grading; 8-bit 4:2:0 bands and tears on keys.
• Round-trip discipline — VFX plates go out ungraded in the working space and come back to the same space.
• Certification — CBFC under the Cinematograph Act, 1952 (amended 2023): UA 7+/13+/16+ sub-categories, perpetual validity, anti-camcording provisions.

Formulas and examples

Example 1 — applying an ASC CDL value. Pixel red channel in = 0.500, slope = 1.20, offset = −0.05, power = 0.90. 0.500 × 1.20 = 0.600 → 0.600 − 0.05 = 0.550 → 0.550^0.90. ln 0.550 = −0.5978; × 0.90 = −0.5381; e^−0.5381 = 0.584. The pixel is lifted from 0.500 to 0.584.

Example 2 — saturation without shifting luma. R = 0.60, G = 0.40, B = 0.30, saturation = 1.40. Y = 0.2126(0.60) + 0.7152(0.40) + 0.0722(0.30) = 0.1276 + 0.2861 + 0.0217 = 0.4353. R′ = 0.4353 + 1.4(0.60 − 0.4353) = 0.4353 + 0.2306 = 0.666 G′ = 0.4353 + 1.4(−0.0353) = 0.386 B′ = 0.4353 + 1.4(−0.1353) = 0.246 Check: 0.2126(0.666) + 0.7152(0.386) + 0.0722(0.246) = 0.435 — luma preserved, as intended.

Example 3 — reading a Cineon/log scan. A face sits at code value 445; 90% white is 685. ΔCV = 685 − 445 = 240 → ΔD = 240 × 0.002 = 0.480 density. Stops below white = 0.480 ÷ 0.301 = 1.59 stops, i.e. about 30% of white — a correctly exposed mid-tone face. Total 0–1023 range = 1023 × 0.002 = 2.046 D ÷ 0.301 ≈ 6.8 stops carried by the encoding.

Example 4 — DI storage for an Indian feature. 2K DPX (2048 × 1556, 10-bit RGB) = 2048 × 1556 × 3 × 10 ÷ 8 = 11.95 MB per frame. At 24 fps: 11.95 × 24 = 286.8 MB/s ≈ 1.03 TB per hour. A 150-minute feature = 2.5 × 1.03 = 2.58 TB for one pass of the conformed DI — and a real project needs camera originals, VFX versions, renders and backup, so 4–6× that is budgeted. Frame count = 150 × 60 × 24 = 216,000 frames; at an average shot length of 4 s that is about 2,250 shots. At 25 shots graded per hour, the first pass alone is 90 suite-hours — roughly eleven 8-hour days before client changes.

Example 5 — white balance and HDR headroom. Tungsten source 3200 K, daylight target 5600 K: mired 10⁶/3200 = 312.5; 10⁶/5600 = 178.6. Shift needed = 178.6 − 312.5 = −133.9 mired, which is approximately a full CTB correction — a grade can finish the last few mired, but a 134-mired error baked into 8-bit footage will show noise in the blue channel. HDR master at 1,000 cd/m² peak with BT.2408 diffuse white at 203 cd/m²: headroom = log₂(1000 ÷ 203) = log₂(4.93) = 2.30 stops reserved for speculars and light sources above white.

Where it is used

• Feature DI houses: Prasad EFX (Chennai, Hyderabad), Annapurna Studios, Prime Focus/DNEG, Famous Studios and Pixion (Mumbai), Phantom FX, Qube's post services — roles of colourist, DI assistant, conform artist, QC operator, data wrangler.
• OTT and broadcast finishing: SDR + HDR masters, IMF packages and localisation versions for Netflix India, Prime Video, JioHotstar, ZEE5 and SonyLIV; broadcast grading for Star, Zee and Sun network channels at 1080i50/25p.
• Advertising and music video: fast one- to three-day grades in Resolve or Baselight; brand colour accuracy (a precise corporate red or a product's packaging colour) is contractually checked.
• Wedding, corporate and event post in regional markets: Resolve Studio grading as a paid add-on; the commonest first job for a Tier-2/3 city graduate.
• Theatrical delivery and exhibition: DCP building, KDM issuance and screen calibration through Qube and UFO Moviez; re-mastering and restoration of archive titles at NFDC-National Film Archive of India.
• Games, animation and virtual production: LUT/ACES pipelines for in-engine look development and LED-wall shoots, where the on-set look must match the final DI.

Interview questions

• Q: What is the difference between colour correction and colour grading? A: Correction is technical — neutralising casts, fixing exposure and matching shots so a scene is consistent; grading is creative — building a look that serves the story.
• Q: Why does log footage look washed out? A: Log encoding maps a very wide scene dynamic range into a small code range, so contrast and saturation are deliberately low until a transform or grade expands them for the display.
• Q: What exactly does an ASC CDL carry? A: Ten numbers — slope, offset and power for R, G and B plus one saturation value — applied as (in × slope + offset)^power, giving a portable look between set, editorial and DI.
• Q: When would you use a 1D LUT instead of a 3D LUT? A: A 1D LUT only remaps each channel's tone independently, so it suits gamma or range conversions; a 3D LUT is needed when hue and saturation must change too, as in a camera-to-display transform or a show look.
• Q: What is the advantage of an ACES workflow on a multi-camera show? A: Every camera is converted to a common scene-linear space by its Input Transform, so the grade is camera-agnostic and a single master can be output to cinema, SDR and HDR through different Output Transforms.
• Q: Your blacks look crushed only on the client's monitor. What do you check first? A: Range handling — whether the file is being interpreted as legal (64–940) versus full (0–1023) range — then the monitor's EOTF and calibration.
• Q: What are the DCP image specifications? A: JPEG 2000 compressed frames in 12-bit X′Y′Z′ with a 2.6 gamma, SMPTE packaging, up to 250 Mbit/s, encrypted and unlocked by a KDM tied to a specific projector and date range.
• Q: How do you grade a shot where the sky is blown out? A: You cannot recover clipped data; you match the rest of the frame, then either roll off the highlight with a soft window and added texture or grain, or request a sky replacement from VFX.
• Q: Why are handles requested at conform? A: They give extra frames beyond the cut so the editor can adjust the cut point, and so dissolves, stabilisation and motion effects have material to work with.
• Q: What is the skin-tone line on a vectorscope? A: A reference axis at roughly 123° of phase (the I-line, pointing to about 11 o'clock on the graticule) along which human skin tone of any complexion falls; it is used to check that faces are not pushed too green or magenta.
• Q: How does HDR grading differ from SDR grading in practice? A: HDR uses an absolute EOTF with far more headroom above diffuse white, so highlights are placed in nits rather than percentages, and an SDR trim pass is graded separately rather than derived automatically.
• Q: A director wants the grade changed after the DCP is built. What is involved? A: Re-grade the affected shots, re-render those reels, rebuild and re-hash the DCP, re-issue KDMs and re-QC on a projector — which is why picture lock before DI matters.

Common mistakes

• Judging a grade on an uncalibrated laptop, a TV in "vivid" mode, or a room with daylight on the screen.
• Grading by eye alone and ignoring scopes, then finding illegal levels or a cast at QC.
• Applying a creative LUT first and grading on top of it, so clipped values are already lost and the look cannot be reversed.
• Confusing legal and full range on import or export — the most frequent cause of crushed blacks in student projects.
• Over-saturating skin and sky, or pushing a single "teal and orange" look onto every scene regardless of the script.
• Starting the look before shot-matching, so the look masks mismatches that reappear on a bigger screen.
• Forgetting deliverable differences — delivering the cinema P3 grade as the OTT Rec.709 master, or ignoring the HDR trim pass and textless elements.

Related topics

Video production and editing · VFX: compositing, rotoscoping and matchmoving · Subtitling and dubbing · Cinematography: exposure, lighting and lenses · Digital cinema formats, codecs and DCP mastering · Sound mixing and loudness standards for delivery