DPX log encoding limits

What is the maximum encoded linear light value in a DPX file? Depends on the log encoding format.

When working in ACES, you should use the file format OpenEXR to store image data.

There are some workflow issues, for example when exporting EXR files from Resolve – the ReelName metadata is not present in the file.

Or the Colorist decides not to grade in ACES and prefers to receive DPX Log encoded files instead of ACES 2065-1 EXR files.

One way of working is to convert all camera media to ACES EXR files, do your compositing and for grading export again the log format of the camera media. This ensures that the colorist can start with the same image before the comp and with the comp. (Note: I am referring here to work with Nuke & Resolve)

So just write out DPX files from Nuke for example. Each flavor of Camera-Log format will encode numbers between 0-1 in 10 or 16 Bit. Not only the maximum linear light value is different in each log encoding, but also where ranges of values will be stretched and quashed to fit best for each camera.

The moment you have material from Alexa, RED and Sony, etc. Venice cameras on one show, this doesn’t work anymore or this is not at least not ideal – that’s why it is recommended to use ACES-EXR files instead.

Here is an overview of the different IDTs and how high is the maximum linear light value they can decode:

  • ACEScc / ACEScct ≈ 221.9
  • Alexa V3LogC ≈ 55.1
  • Canon Log3 ≈ 14.7
  • REDlogFilm ≈ 13.5
  • REDLog3G10 ≈ 184.3
  • Panasonic V-Log ≈ 46.1
  • Sony S-Log3 ≈ 38.4
  • sRGB – Texture = 1.0 (this is not a log encoding)
  • Output Rec.709 ≈ 16.3 (this is not a log encoding)

The highest value should be the best, but these are camera log encoding formats so the sensor has to give you actually useful information in the specular highlights that you still use. In most Arri Alexas files that I saw the “soft-clip” value is actually around 35-39, but the format allows values up to around 55.1.

When Resolve is internally working in floating point precision, there is no much “damage” happening converting from scene linear to ACEScct, do the grade and convert back to scene-linear.

But in the case of 10-16Bit DPX files, there is more data loss, but actually not really visible.

sRGB-Texture and Output Rec.709 are not log encodings and the use of both in Nuke for textures and references are a topic for another time.