A Reminder: Avoid Log

For HDR projects, steer clear of recording XAVC S-I 4:2:2 internal if at all possible. Art Adams explains:

“The Y’CbCr encoding model is popular because it conceals subsampling artifacts vastly better than does RGB encoding. Sadly, while Y’CbCr works well in Rec 709, it doesn’t work very well for HDR. Because the Y’CbCr values are created from RGB values that have been gamma corrected, the luma and chroma values are not perfectly separate: subsampling causes minor shifts in both. This isn’t noticeable in Rec 709’s smaller color gamut, but it matters quite a lot in a large color gamut. Every process for scaling a wide color gamut image to fit into a smaller color gamut utilizes desaturation, and it’s not possible to desaturate Y’CbCr footage to that extent without seeing unwanted hue shifts.

My recommendation: always use RGB 4:4:4 codecs or capture raw when shooting for HDR, and avoid Y’CbCr 4:2:2 codecs. If a codec doesn’t specify that it is “4:4:4” then it uses Y’CbCr encoding, and should be avoided”.

This is the consensus of industry professionals, including post production houses, who prefer to see RAW for HDR projects. This is also why streaming platforms like Netflix use the ICtCp color space. For those wanting to future-proof their material or who might need to create a Dolby Vision master in the future:

“Dolby Laboratories recommends using certain types of source material for creating high quality Dolby Vision content. In general, the best source material for Dolby Vision best retains the native color gamut and original dynamic range of the content at the time of its origination.

Original camera footage

For mastering first run movies in Dolby Vision, the original camera raw files or original film scans are ideal.

  • Film: Original Camera Negative (OCN)
  • Digital: Camera RAW”

Matthew Bilodeau provides further grounds for preferring 12-bit RAW to 10-bit Log:

“However, if you’re considering which codecs to use as intermediates for HDR work, especially if you’re planning on an SDR down-grade from these intermediates, 12 bits per channel as a minimum is important.  I don’t want to get sidetracked into the math behind it, but just a straight cross conversion from PQ HDR into SDR loses about ½ bit of precision in data scaling, and another ¼ – ½ bit precision in redistributing the values to the gamma 2.4 curve, leaving a little more 1 bit of precision available for readjusting the contrast curve (these are not uniform values).  So, to end up with an error-free 10 bit master (say, for UHD broadcast) you need to encode 12 bits of precision into your HDR intermediate.”

“Converting one 10-bit format into another 10-bit format will reduce the performance of the signal and should be avoided whenever possible”. – HDR: A Guide to High Dynamic Range Operation for Live Broadcast Applications, Grass Valley

In our own experience, and from watching countless videos uploaded by others, RAW will give significantly better results and there’s no way we’d go back to shooting Log for HDR. Not to mention that the a7s III 4.2K ProRes RAW files contain nearly 25% more resolution, with no baked-in denoising, lens corrections, in-camera sharpening or other enhancements applied.

Click on image below to enlarge.

HDR tends to amplify existing noise in the image, particularly in shadow areas. De-noising in post with either Resolve’s own built-in denoising software or Neat Video will give better results.

And here’s the same ProRes 4444 clip with a small amount of spatial noise reduction in addition to the temporal. Mode faster – radius small – Luma 2 – Chroma 2.

Sony a7s III XAVC S-I versus ProRes RAW HQ transcoded to ProRes 4444:

There’s also more color information to work with in the RAW clip than in the XAVC S-I internal clip:

Presumably, one is shooting HDR in order to obtain the very best image quality possible, which begins with using the highest quality format. Incidentally, ProRes RAW transcoded to ProRes 4444 and graded in Resolve will yield superior results to grading the original RAW footage in Final Cut.

“If you’re using a prosumer or entry level professional camera, taking a few preparatory steps to set up how you’ll actually be capturing the image can mean the difference between getting footage that can be used in HDR mastering, vs. footage that can’t.

  1. 16 bpc > 14 bpc > 12 bpc > 10 bpc > 8 bpc: 10 bpc should be your minimum spec.
  2. RAW > LOG > Linear > Gamma 2.4: avoid baking in your gamma at all costs!
  3. Camera Native / BT.2020 > DCI-P3 > Rec. 709 color primaries
  4. RAW > Compressed RAW > Intraframe compression (ProRes, DNxHR, AVC/HEVC Intra) > Interframe compressed (AVC/H.264, HEVC)”. – Samuel Bilodeau, Mystery Box

Mark Spencer demonstrates how, with the same extreme grade applied, the Sony internal XAVC file falls apart much more easily than the ProRes RAW one:

It should be mentioned here that traditional lift/gamma/gain wheels are unsuitable for grading HDR, just one reason why DaVinci Resolve, with its HDR Color Palette, is preferable to Final Cut Pro.

“It is also worth bearing in mind that the traditional lift, gamma, and gain operators are particularly unsuitable for acting on ST 2084 image data, where 0-1 represent zero to 10 000 cd/m2. Highlights, in particular, can swing uncontrollably into extreme values as a result of small changes. Modern grading systems offer alternate grading operators tailored to the needs of HDR grading.” – Nick Shaw

5 thoughts on “A Reminder: Avoid Log

Add yours

  1. Unless you’re shooting hours upon hours of footage, let’s say stock footage of people walking on a busy street, there’s not many cons of shooting RAW other than the required recorder.

    I have a 500GB drive and PRR HQ 4K UHD 23.98p gives me about 27min of recording time. Which is quite a lot if you think about taking multiple minute clips here and there. 1 minute of recording is quite a while if you think about it.

    Sharing a video of a scene with a lot of color and brightness can’t be done justice by SDR. If you can’t think about anything just pulling down the highlights a bit will make the footage look great.

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