Passes – Nira's VFX Blog

This week, we learnt the 3D compositing process to put together plates and CG, to do clean-ups, to regrain, and to do a beauty rebuild with a multipass comp.

The 3D compositing general process looks like the following:

Main plate clean-up and roto work. After finishing the clean-up, it is recommended to render the cleaned plate so we can use this pre-rendered version to do the rest of the comp. This is done so Nuke has less nodes to calculate each time and the preview of the work done goes quicker.
CG compositing. In this part, we can move on with the beauty rebuild, adjusting the AOVs or passes with subtle grade and/or colour correction
- Basic grade match. With a ‘grade’ node, we first do the white balance of the CG that we are going to integrate in the plate, measuring the ‘whitepoint’ (whitest part of the CG) and the ‘blackpoint’ (darkest part of the CG) while holding ‘ctrl+shift+alt’. Subsequently, we go to our background plate and measure ‘gain’ (for the whites) and the ‘lift’ (for the darks) while holding ‘ctrl+shift’. This will balance both plate and CG’s darks and shadows and will integrate them together.
- Multipass comp. In this technique, we need first to ‘unpremult (all)’ our CG so we can start splitting the AOVs or passes. This split is made using ‘shuffle’ node and setting it to the desired pass we want to correct. Before editing the passes, we need to make sure to structure the nodes from the CG plate to a ‘copy’ node with all passes merged together, and double check that the CG plate looks exactly the same from initial point (original plate) to the ‘copy’ node. Sometimes this may look different as some of the passes could have been exported wrong. Once we split our passes we can proceed to ‘grade’ them individually. We ‘merge (plus)’ the light passes and we ‘merge (multiply)’ the shadows. We can also select an ID to create a colour map with ‘Keylight’ node. With this node we can select a specific area of the model that we want to adjust as its features will be separated in different saturated colour mattes. This way we could then re-texture a part of the model using a ‘ST-map’ node connected to the texture source. We can then re-light with ‘position pass’ and ‘normal pass’, followed by a ‘grade’ of the master CG plate. We can finish our beauty rebuild with a ‘copy (alpha-alpha)’ to copy the original alpha to the one created, and we we ‘premult’.

Motion Blur. Motion blur will add more realism and dynamism to the movement of the CG added as in 3D everything looks sharp and in focus so it is not as realistic. We can add motion blur following two methods:
- Method 1: adding a ‘vector blur (rgba)’ node, then link it to ‘camera’, and adjust ‘motion amount’ in the ‘vector blur’ node as desired.
- Method 2: ‘remove (keep)’ node linked to ‘motion blur 3d’ nodes, and adjust this last one’s ‘motion amount’ as desired.

Chroma aberration and defocus. We can add an ‘aberration’ node to match the original camera aberration of the live-footage plate, so we make the scene more credible. Also, with ‘defocus’ node we can add depth to the scene to be able to differentiate between sharp image and out of focus image (depth of field). After adjusting these, we need to add a ‘remove (keep)’ node connected to an ‘ST map’ node to put the original distortion back to the scene.
Regrain. We also could add some grain to the scene with ‘grain’ node. Then with ‘key mix (all)’ node linked to previous changes and ‘grain’, we can mix channels and add a mask to the previous changes made in the comp.
Effect card. We can add effects like smoke with a ‘card’ node. We will need to connect it to ‘shuffle (rgba to rgba with R to alpha)’ node to ‘card’, and ‘grade’ it. Then we ‘copy (alpha to alpha)’ and ‘premult’ to create the alpha of the effect and then we ‘defocus’. This will be projected on a ‘card’ (connected to ‘scene’, ‘scanline render’, and ‘camera’). Finally, we add the ‘ST map’ to unfreeze the frame and ‘multiply’ to show alpha created.
Lightwrap. We use this to add light to the edges, which could be adjusted with ‘diffuse’ and ‘intensity’. Then we will ‘merge (plus)’ as this is light feature.
QC. Using the ‘merge (difference)’ node, we can see and assess the changes made and there is any error. The ‘colour space’ node with the ‘output’ set as ‘HSV’ can be used to check the colours hue (R), saturation (G), and luminance (B) quality.
Final colour correction.
Export. The main preferred format to export our comp would be EXR. Some companies will also want a photo ‘JPEG’, or ‘AppleProRes’, or even ‘Avid DNxHD’, but that depends of the pipeline of each company.

The homework for this week was to start to put together the elements that would form part of our garage comp, and also, include the machine provided by the professor following all the steps we have learnt today.

Following the reference pictures we got with the brief, I started to research for 3D objects I could include such as tools, tyres, a table, etc.

Objects to add to the comp (downloaded from Quixel)

I also decided to re-watch this week’s recording of the lecture to make sure I followed step by step the compositing process. This way, I started to understand the functionality of each node and technique, and to become more confident at the time of creating a whole comp by myself without having to look at references in other comps. The first thing I added was the machine in the back room. I did a beauty rebuilt with the separation of the passes and added a smoke effect with a card 3D projection. I feel like this part went really well as I did not have any issues along the process and the final look is pretty realistic.

Garage comp WIP with machine

After my back machine was fully set, I continued to add the 3D geometry to the comp with its textures. One problem that I had with the objects is the fact that they were really heavy and really jumpy when following the movement of the scene so it was hard to work with.

My work in progress comp looks like the following:

Garage comp WIP with 3D objects

This week, we studied how to do a CG beauty rebuild, using channels or passes of our CG to see its layers to then adjust them separately, relight them, and put them back together.

To start with the CG beauty rebuild, first we need our CG layers (usually the CG has already been exported like this). We can see all these layers separated in the ‘layer contact sheet‘ which contains a view of passes in EXR (e.g. diffuse, specular, reflection, etc). The separation of the EXR in layers or passes (channels) is used for adjusting each pass separately to match the lighting and colour conditions of the background. In order to adjust each pass, we first need a ‘shuffle‘ node set with the specific pass (input layer) we need to then ‘merge (plus)‘ (+) for the lights (diffuse, indirect, specular, and reflections) and ‘merge (multiply)‘ (*) for shadows (AO or ambient occlusion, and shadow). Every pass must be graded separately and then we could add a final ‘grade’ or/and ‘colour correct’ to the entire asset if needed.

There are several types of ‘render passes’ or ‘AOVs’ (Arbitrary Output Variable):

Beauty Rebuilt Passes:
- Material AOVs. To adjust material attributes (shader).
- Light Groups. To adjust individual lights of a scene.
Data Passes:
- Utilities. Combined with tools to get various effects (e.g. motion blur, defocus, etc.).
- IDs. To create alphas or mattes for different areas of the render.

There are some elements that can be used to double check or improve our CG beauty rebuild quality:

Cryptomatte. To see different parts of the scene colours.
KeyID. To create a mask of the ID pass.
AO pass. It creates a fake shadow, produced by proximity of geometry to other geometry or background.
Motion pass. It let us see the blur of the motion clearly.

The process to subtract a pass to edit it is the following:

Unpremult (all)
Link to ‘shuffle’ node (set with pass needed)
‘Grade’ and make adjustments needed
Add back with ‘merge (plus)’ or ‘merge (multiply)’
‘Remove (keep)’ node
‘Permult’

Once we have our colour correction and grading made, we can relight the scene with ‘position pass’ which is the 3D scene but in colour values (red=X, green=Y, blue=Z). In order to have a reference of the 3D space, we could use a ‘position to points’ node set with ‘surface point’ to ‘position’ and ‘surface normal’ to ‘normal’. We then adjust the point size how we want and we will see a 3D representation of colour values. Once the representation is made we can start to add lights with ‘points’ nodes linked to the ‘scene’ node to put them together. This scene is then connected to a ‘relight’ node which puts light, colour, material, and camera together (use alpha, and link ‘normal vector’ to ‘normal’ and ‘point positions’ to ‘point’). To merge over original background, we then ‘shuffle’ and ‘merge’.

As a homework of the week, we need to composite a 3D modelled car in a background of out choice:

I feel like this practice was simpler than last week’s homework, however, I still encountered some challenges that I would like to research and study, such as the addition of ‘fake’ lights to the car lights to look like they are turned on, and also to get rid of a specific area glow like the one on the right door of the car which does not really make sense it shows there.

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