Multipass Keying Workflow

[Blackvfx2018 (Omar Brown)]

This response may be an overkill of information but here goes. Most chromakeying or pulling keys issues will have to do with lighting and channel noise. Noise is most apparent in the Red and Blue channels. Like I had posted before, using nodes like lp_CleanScreen1 or PxF_ScreenClean1 can help flatten the color range of the green screen. Now there are times when your green screen surface is noisy because of bad lighting period. Then the DenoiseSharpen1 node will come in handy the smooth out the channels to get a decent key. After you have done all of that, applying a MultiPass Chromakeying workflow should follow in the pipeline. See attached image for visual reference. I had used the screen clean node, DenoiseSharpen1 node and 3 ChromaKeyer nodes in a multipass piping.

The Chromakey nodes need to generate inner core mattes and then additional keys. Each time you need to apply an additional ChromaKeyer node, the Output Mode needs the “Intermediate” option selected Source Alpha “None” selected. This generates the Core Matte (AKA Hard Edges). The second ChromaKeyer Output Mode can be premultiplied if it is the last keyer in the chain or Intermediate for more multipass keying. Rather ChromaKeyer node is the last keyer or use for more multipass keying, the Source Alpha “Add to inside mask” must be selected. Every ChromaKeyer node after the first ChromaKeyer in the pipeline for multipass keying must have these options selected:Output Mode needs the “Intermediate” option selected Source Alpha “Add to inside mask” selected. Only the last ChromaKeyer node in the chain Output Mode should be premultiplied.

Sorry for the redundancy. Multipass keying gets a little complex once you need to go pass 3 layers.

[groslaine (lecointre leny)]

Very very nice workflow indeed. Thx a lot for sharing. I just dont understand the point of having 3 or more of this. Should it be the same has having a good key for the edge and another one for the inside (solid matte). What are the intermediate keying are doing exactly ?

[PhotoPhysicsGuy (Bob)]

@Blackvfx2018

Multipass keying gets a little complex once you need to go pass 3 layers

My question would be similar to the one from @groslaine : What advantages do you see for this technique compared to IBK/PIK (or image-difference based stuff in general) keying of core and edges seperately? Would you say this method is easier to apply to complex keys?

It’s good to have options! So thanks for this!

[Blackvfx2018 (Omar Brown)]

Thanks for the question. The answer is in the name itself as MultiPass Keying. Not all keyers are equal in any commercial or open-source compositor. I have demoed this multipass keying workflow example because 90% of the time you will not even green screens because of lighting or keys colors. One node does not fit all based on what the scene or shot requires. See attached images below to see different lighting, colors of wrinkled fabrics/materials, or layers for ground and wall backings. Some keyers may get all of the green but at the risk of damaging your core matte edges, wrinkled fabrics, hair, and flesh tones if not careful. You can use roto nodes to Gmask out your screen garbage and that is fine. You will have to do a lot of roto keyframing if there is a lot of movement in the shot.

The IBK/PIK nodes are very powerful nodes but can sometimes be an overkill for basic keying jobs and they too can be used for multipass keying. The PIK Color node that is the complement to the PIK node performs some clean matte generation which is sometimes not very good on certain green screen saturation. It is always good practice to use multiple keyers when needed to protect the integrity of your alpha channels.

[PhotoPhysicsGuy (Bob)]

@Blackvfx2018

It is always good practice to use multiple keyers when needed to protect the integrity of your alpha channels.

Certainly! There is no one-fits-all solution (at least I 'm not aware of one) so using multiple keys is almost always necessary. So the same priciple of combining keys is the main takeaway, got it.

[devernay (Frédéric Devernay)]

Sending some love to Omar. Thank you @Blackvfx2018 for all you did for Natron, and I hope you find the same resources to help Natron in the future! We need people like you, I love people like you.

[Blackvfx2018 (Omar Brown)]

Cool Breeze Fred and thanks. I love Natron. I hope that it will be fully Big Sir compatible because the node graph has a major delay when moving nodes.

By the way, will there be any more updates to the tracker?