Absolute, and why that is unusual
Every transfer curve before PQ was relative. In BT.709 the brightest code value means "as bright as this display goes", whatever that happens to be. PQ breaks with that: code value X means X nits, and a 4,000-nit reference monitor and a 500-nit laptop are both expected to render the same code as the same intended brightness, with the laptop tone-mapping what it cannot reach.
The curve is not arbitrary. It is derived from Barten's model of human contrast sensitivity, allocating code values where the eye can actually distinguish steps and spending fewer where it cannot. That is what allows a 10-bit signal to cover a 0 to 10,000 nit range without visible banding.
The failure mode you will actually hit
A player that does not recognise PQ reads the code values as if they were ordinary gamma-encoded SDR. The picture that comes out is not "less bright", it is wrong in a specific way: blacks lift toward grey and colour drains out of the whole frame.
This is worth knowing because it is the single most common reason someone concludes an HDR conversion "made it look worse". The file is usually fine; the playback path is not applying the curve. Verify on a display that reports HDR support, or check the file's own metadata rather than trusting what the preview window shows.
In a file
PQ reports color_transfer=smpte2084. HLG, the relative alternative, reports arib-std-b67.
Questions people ask
Why is it called the perceptual quantizer?
Because the curve allocates code values according to a model of human contrast sensitivity rather than spreading them evenly. Steps land where the eye can tell them apart and are spent sparingly where it cannot, which is how 10 bits cover a 0 to 10,000 nit range without visible banding.
Is PQ the same thing as HDR10?
No. PQ is the transfer curve; HDR10 is a format that uses it alongside BT.2020 primaries, 10-bit depth and static metadata. Dolby Vision also uses PQ. A file can carry the PQ curve and still not be a conforming HDR10 file.
What happens to PQ content on an SDR screen?
A player that recognises it tone-maps the signal down and shows a reasonable picture. A player that does not recognise it reads the code values as ordinary gamma, and blacks lift while colour drains. The second case is the washed-out look people mistake for a bad conversion.
Related: converting HDR to SDR without the washed-out result is the same problem in the opposite direction.