Keyer – Nira's VFX Blog

Week 7: Despill Corrections Tips, Creating Gizmos in Nuke, & Garage Homework WIP

In this lesson, we learnt to make despill corrections when removing green/blue screen, also we saw how to create our own personalised gizmos in Nuke, and lastly, we asked questions we had related to our garage homework WIP.

Despill correction tips

When keying to remove green screen and then remove saturation, we can roto some parts of the shot and then link this roto to ‘invert’ node so the despill does not affect that specific part.
We can also correct edges with ‘IBK Colour’ node set to ‘blue’ colour only, then we add a ‘Grade (alpha)’ so it only affects the alpha, then we correct the edge with ‘Filter Erode’ and ‘Blur’, and lastly, we ‘Merge (screen)’. We could also add an ‘Edge Blur’ to soften sharp edges and a ‘Clamp’ to make sure all merged alphas value is 0.
With ‘Add mix’ node, we can merge alphas and can set how much alpha we want to see.
Additive key: after a ‘Merge (minus)’ we desaturate and grade, and then we add a ‘Merge (plus)’ to ‘Constant’ node with the green colour as reference.
Divide/Mult key: we rerplace spill with ‘Merge (divide)’ from both the chroma plate and the chroma reference plate, to then ‘Merge (multiply)’ with the background plate.
When the green/blur screen have different luminance along the shot, we correct it taking a ‘Constant’ node with the darkest part colour of the green/blue screen connected to a ‘Merge (average)’ node so we create a ‘Constant’ with a colour with the same luminance. Then we ‘Merge (minus)’ with a ‘Keylight’ for despill.
We could add a ‘Light wrap’ node to add a light glow around specific areas. We will ‘Merge (plus)’ to the background in this case.
An inverted matte can be used to delete light from an outside edge. We just ‘Invert’ the matte, then ‘Roto’ the required parts, and ‘Merge (mask)’ to the matte. We could also add a ‘Grade’ node with a mask link to this ‘Merge (mask)’ node so we colour correct that specific edge.

How to create gizmos

First, we select the nodes we want in the gizmo and group them (ctrl + G). Then in the creed node options, we click the ‘edit’ button and drag and drop the features that we want (controllers). We can label these controllers by clicking on the little circle next to it. We then link each controller with the node controller (hold ctrl + drag and drop from main node to grouped node).

Green screen and despill homework

The homework for this week was to remove the green screen of a hero shot girl scene and add it to a snowing forest background, as well as add some of the background snow to the foreground.

First, I use a ‘Keylight’ node set to detect just the green colour and ‘Merge (minus)’ to the main plate to only see the greens of the shot. Then I aded a ‘Roto’ to the eyes of he girl and ‘Invert’ it linked as a mask to he saturation node to preserve the little amount of green of her eyes. Then I linked a ‘Merge (multiply)’ node from the background plate to the foreground plate to take some of the luminance of the background to the girl. This also was ‘Merge (plus)’ to the foreground to add that luminance to the scene.

Separately in another block of nodes, I used an ‘IBK Colour’ node to key the green screen of the foreground. Then I desaturated it, and added another ‘Grade’ with ‘Filter erode’ and ‘Blur’ and ‘Merge (Screen)’ to previous ‘Grade’ so I get more details and luminance from the girls hair. Then I ‘Copy’ this alpha to the main foreground alpha, to then ‘Add mix’ these alphas to the background plate.

In the background plate, I used a luminance ‘Keyer (alpha)’ node, to select only the colour of the snowflakes falling. then I ‘Copy’ this alpha to the background plate and also ‘Premult’ to create the alpha that will be added to the foreground with ‘Merge (over)’ node.

Finally, I colour corrected and graded the overall result and rendered the alpha and the final comp.

Final hero shot

Alpha version

I am not totally sure about the amount of hair detail that is visible in this version so will ask the professor on the next class (corrected version added to Advanced Nuke – Week 9 post).

Garage homework WIP

Lastly, I asked Gonzalo about my issue with the shadows not showing in my garage comp. He found out that the ‘Grade’ node that was after the ‘Shuffle’ node to create the shadows alpha, had ‘black clamp’ option ticked, so I had to deselect this and select ‘white clamp’ option instead so the blacks of the shadows started to show. However, despite the shadows were finally showing, I feel like they are too harsh and saturated and I could not figure out how to soften them. I tried to grade them and desaturate them but still looked to black and unnatural to me. Also, the hole in the wall is receiving the shadow from the chain hang on the wall and it looks like there is a plane receiving this shadow. This issue is due to the card added on that wall to receive the cast shadow so I tried adding a ‘Merge (stencil)’ from the roto I have from that wall but it did not work for some reason. I will have to ask Gonzalo in the next class.

Week 6: HSV Correction Process & Chroma Key in Nuke, & Garage Homework WIP

In this lecture, we learnt how to correct Hue, Saturation, and Value (Luminance), and how to use chroma key in Nuke. We also reviewed our garage comp WIP.

The meaning of HSV break down is the following:

H – Hue. Applies to colour (R, red value)
S – Saturation. Applies to the intensity of the hue/colour (G, green value)
V – Value. Applies to luminance or brightness (B, blue value)

This is important to understand as the quality of a HSV correction in a comp depends of our understanding of this elements.

When making hue corrections, we can use the ‘Hue correct‘ node to mute (colour channel as ‘0’), suppress (suppressed colour channel as ‘0’), or desaturate (saturation channel as ‘0’) a specific colour. This is useful for example to remove a green screen with the green colour channel suppressed.

With the ‘Keyer‘ node set with ‘luminance key‘ operation, we can determine the quantity of luminance we want to remove from the alpha of an image (black parts are not affected, but white parts only). We could also set the operation for ‘red keyer’, ‘blue keyer’, etc. Then we could ‘shuffle’ to green only, for example, and the colour correction will only affect saturation. We can also use this node to add, subtract, and multiply elements:

We can remove a colour channel with a ‘Merge (minus)‘ node linked to the colour we want as background and the colour we want to remove.
With ‘Add (Math)‘ node we can add colour when linked to a ‘merge (minus)’ node.
We could also use ‘Roto‘ to add or remove colour of a specific area.

Texturing can also be made with a ‘keyer (luminance)‘ so we use the alpha of the texture to adjust the luminance. Then this would be blurred, graded, and merged (minus). Moreover, we could also use the keyer to add noise or denoise in certain areas.

Some extra nodes and techniques can be used to create some effects that will give more credibility to our image:

‘Volume rays‘ node. Used to create rays effects or motion effect.
‘Edge detect‘ node. To select the edges of an image’s alpha and colour correct those specific edges.
‘Ramp‘ node. To balance image with gradient (used with ‘Merge (minus)’ node).
To add a new channel in ‘Roto’. We create a new ‘output’ (name it, click ‘rgba’ and ‘ok’), so when adding different features like blur or grade, we can link that change in the node and it would only affect that new channel created. We could also use ‘Add (channel)‘ node instead, select the channel as ‘Matte’, and choose that it will only affect to certain colour. We could also add a ‘Rotopaint’ to this and add shapes linked to different channels.

We can use keying nodes and techniques for chroma key such as:

‘IBK‘ (Image Based Keyer). We can subtract or difference with this node. It is considered the best option for getting detail out of little areas like hair or severely motion blurred edges:
- ‘IBK colour‘ node. Frame by frame rebuilding background taking blue or green colour.
- ‘IBK Gizmo‘ node. Can select specific colour.
‘Chroma key‘ node. First we can unselect ‘use GPU if available’ if our computer starts lagging. This node works better with evenly lit screens and with more saturated colour. We could use it for despill, but better not to as what we want is to extract the alpha.
‘Key light‘ node. This is used for colour spill.
‘Primate‘ node. This is a 3D keyer that puts colour into 3D colour space and creates a 3D geometric shape to select colours from it. We first select ‘Smart select background colour’, we pick a colour while holding ctrl+shift, then we change to ‘Clean background noise’, and holding ctrl+shift, we pick the colour parts that are still showing in the alpha (and need to be removed). We could also click on ‘Auto compute’ to create an automatic alpha and then retouch areas back to alpha with ‘Clean foreground noise’.
‘Ultimate‘ node. This is used for fine detail, and to pull shadows and transparency from the same image.
Green/Blue despill technique. We create an alpha with ‘Keylight’ node and ‘Merge (difference)’ to plate. Then we desaturate the background with a ‘Colour correct’ node and ‘Merge (plus)’ with the ‘Keylight’ node. Then we ‘Shuffle’ and put ‘alpha’ in black. Additionally, we could reduce the light in the background (saturate and grade) with ‘IBK Gizmo/Colour’. There are some companies that have created their own gizmo with all the required presets to despill.
‘Edge Extend‘ node. This is used to extend edges so we can correct the darken bits (smoother edges and not as pixelated).

A standard chroma key process would have the following steps:

Denoise plate
White balance
Alpha pipe
1. Core matte
2. Base matte
3. Hair matte
4. Gamma matte
5. Edges alpha
Despill pipe
1. Edges despill (specific parts)
2. Core despill (overall)
QC
Light wrap
Regrain alpha and background
‘Addmix’ to link alpha and background

Green screen homework

The homework of this week is to improve the garage comp and to make a chroma key of a sequence provided by the professor so we can put in practice all the techniques learnt in class.

Denoise, white balance, core matte, base matte, and hair matte

Final green screen replacement

Alpha version

Garage comp WIP

Regarding my garage comp work in progress, the professor also sent us a Nuke comp with examples of how to set lighting and shadows with projections or geometry. I tried to follow the example with the geometry as I only had 3D objects in my comp, however, I had some problems with the shadows as they were not showing at all in the final result. I could see them showing in the alpha created with the ‘shuffle’ node, however, since I could not see them in the final output, I guess I have something wrong with the ‘merge’ node or with the concatenation of the comp. I will try to ask Gonzalo in the next lecture about this. I also added a texture to the right wall so it looks like it was previously painted but the paint is being degraded and it is peeling off the wall. I roto the part of the texture that I was interested in showing and then used a projection of the texture on a card in a 3D scene.

References

Cini, A (2023). Color Theory HSV or Hue, Saturation, Value Brightness Illustration Chart Vector (online). Available at: https://www.dreamstime.com/color-theory-hsv-hue-saturation-value-brightness-illustration-chart-color-theory-hsv-hue-saturation-value-brightness-image237125365 [Accessed 19 February 2023]

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