Rec.709 and HDR — the basics for creators
SDR and HDR don’t differ only in brightness. The colour primaries, the transfer function, the reference display and the way the picture is evaluated all change.
Rec.709 is more than a ‘colour space’
When someone says ‘export in Rec.709’, they usually mean the whole package of HDTV parameters from the ITU-R BT.709 recommendation: a resolution of 1920 × 1080, frame rates, colour primaries (how saturated red, green and blue can be), the D65 white point, the coefficients for converting RGB to Y′CbCr and the camera transfer curve (OETF).
How the picture should look on a display is defined by a separate recommendation, ITU-R BT.1886: the reference curve of a flat panel display with an exponent of 2.4. The reference conditions for evaluating HDTV are described in ITU-R BT.2035 — reference white should correspond to 100 cd/m² and black to less than 0.01 cd/m², in a dimly lit room.
What this means for a creator: an SDR picture is designed for a display with white at around 100 cd/m². A brighter TV only makes the picture brighter — it doesn’t add detail in the highlights.
What HDR brings
ITU-R BT.2100 (currently in its BT.2100-3 edition from February 2025) defines the parameters of HDR television:
- A wider gamut: primaries from BT.2020 — much more saturated colours than BT.709 covers.
- Higher bit depth: 10 or 12 bits (at 8 bits a large range of brightness would break up into bands).
- A new transfer function: either PQ or HLG.
- Reference display: a peak brightness of at least 1,000 cd/m² (for small bright areas, not a full white screen) and black of no more than 0.005 cd/m².
DIAGRAM · LUMINANCE IN CD/M² (NITS)
PQ vs. HLG
| PQ (Perceptual Quantizer) | HLG (Hybrid Log-Gamma) | |
|---|---|---|
| Principle | Absolute: a code value = a specific luminance in cd/m² | Relative: the signal describes the scene and the display adapts it to its own brightness |
| Range | Encodes luminance up to 10,000 cd/m² (also SMPTE ST 2084) | Nominally for a 1,000 cd/m² display; the system gamma adjusts to the display brightness |
| SDR compatibility | Not without conversion | Designed with older displays in mind (the lower part of the curve is close to a conventional gamma) |
| Typical use | Streaming and film, mastering to HDR10 and formats with dynamic metadata | Live broadcasting, where one signal goes to both SDR and HDR receivers |
BT.2100 explicitly recommends using one of the two curves consistently within a production and not mixing them.
Why 203 cd/m²
BT.2100 defines HDR reference white: the level produced by a 100% reflectance white card at correct exposure is 203 cd/m² on a PQ display (or on an HLG display with a nominal peak of 1,000 cd/m²). This doesn’t mean the whole HDR picture is brighter. Skin tones and ordinary scenes stay at levels similar to SDR; what you gain is room above them — for the sun, reflections, lamps and fires. The most common beginner’s mistake is to ‘push’ the whole picture up, which looks unnatural and tires the eyes.
What you need for HDR
- A source with range: log or RAW recording, ideally 10-bit or more. You can’t make HDR from 8-bit Rec.709 footage — the highlights are already clipped.
- Colour management in your grading software, with the input (the camera’s curve and gamut) and output (PQ or HLG, BT.2020) set correctly.
- A reference HDR monitor. An ordinary ‘HDR’ monitor with low brightness or an inaccurate PQ curve shows something different from what the viewer will see. This is the most expensive part and can’t be replaced by guesswork.
- Delivery specifications. Platforms and broadcasters have their own requirements for format, metadata and QC. Read them before grading, not after.
- An SDR version. Most viewers still watch SDR. Either create it as a separate trim pass, or check how the platform converts HDR to SDR.
For SDR uploads YouTube recommends the BT.709 colour space and assumes BT.709 when no values are given. For HDR it lists higher recommended bitrates than for SDR — for example 44–56 Mb/s instead of 35–45 Mb/s for 2160p at 24–30 fps.