Rendering rec709 in Fusion

This is part of a series of articles dealing with color workflows in Fusion. Other parts:
Linear Gamma Workflow in Fusion Part 1 and Part 2.

I’ve noticed a couple of Google hits for this topic. Maybe you’re interested in this because Fusion seems to lack a color space selector like Nuke has in its Write node? In Fusion the workflow boils down to using the Gamut tool where rec709 is called by its official name “ITU-BT R.709”. But as with everything in compositing, it makes sense to understand what you’re actually doing since in the end the “how to output rec709” question is just a matter of giving your client what he expects.

The rec709 standard encompasses much more than just the gray-scale gamma curve. It also specifies color primaries, white point and so on which would also affect the hue of your pixels. But usually, if somebody hands you his footage, telling you it’s in “rec709” he probably means that he exported it from his black box editing thingy and he wants the result to look neither brighter nor darker. So I’m mostly dealing with the proper gamma curve for rec709 in this post, not colorimetric mambo-jumbo that I’m still figuring out myself and that mostly concerns DOPs or colorists.

Option 1:

Source footage is a supposed rec709 mov and you’re not using a linear gamma workflow. Fusion won’t do anything to footage it loads in or saves out, so your workflow will simply be this:

Your end result will look the same as your source footage (except for the effect you’ve added). rec709 in, rec709 out. sRGB in, sRGB out. If it doesn’t, then it might be because you’re writing out movs in different codecs and Quicktime does its own unpredictable color shifting. Use file sequences instead. Also, if you’re comparing Fusion’s output to something else in Nuke, make sure both Read nodes are set to the same color space! By default, image sequences are interpreted as sRGB but movs as something else.

Option 2:

Same as before, but you’re working with linear gamma:

The first gamut tool allows you to bring in footage from a different color space as well (sRGB jpegs from a camera come to mind). This is the proper workflow for compositing and also the one used in Nuke.

Note: Fusion 6.4 added a new display-referred rec709 space that is used by ARRI’s Alexa color science. The regular rec709 color space has now been renamed to ITU-R BT.709 (scene).

Option 3:

Linear CGI to rec709. First graph is for a linear workflow, the second one for compositing in rec709 directly. The latter isn’t correct, but useful if you’re afraid of this whole LUT thing…

Option 4:

Source footage is a dpx sequence in a logarithmic format (film or something new like Alexa Log C). Your client wants rec709 for reviews and editing (regardless of what the deliverable will be). You really should be working in a linear gamma workflow here:

To convert from logarithmic to linear you’ll have to set the Loader’s conversion values to those specified by the guy who made the dpx files. If you’ve received Alexa Log C there’s a special conversion that needs to be done. Fusion 6.4 supports it in the Loader and CineonLog tools. In earlier versions you have to bypass Fusion’s log-to-lin conversion that happens in the Loader. Check this forum thread for an implementation of the gamma curve that Nuke is suggesting for Log C or create your own conversion LUTs on ARRI’s site.

The result of the loader will be linear, so a Gamut tool with output space set to ITU-R BT.709 and add gamma enabled results in rec709. Alexa footage will look less saturated than what an editor might see using his ARRI LUT. To fix this, set the Gamut tool’s source color space to Alexa’s wide gamut (requires Fusion 6.4 or later).

Option 4b:

You have also been given a film LUT which the editor would have used to convert logarithmic footage to rec709 on his side. He’s doing more than just a gamma correction so if you’re not doing the same he’ll think you screwed up because saturation and highlights look different. This flow assumes that his LUT expects a logarithmic input which gets converted to rec709:

Don’t use his LUT to convert your footage to rec709 before linearizing it. You’ll lose the dynamic range you could get from the dpx. Better guess some settings for the log2lin conversion in the Loader. As long as you’re using the same values in the CineonLog tool (which goes lin to log), you’re kinda safe.

Update: Ken Turner has more on how to use LUTs in Fusion.



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