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

1. 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.
2. 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.
3. With ‘Add mix’ node, we can merge alphas and can set how much alpha we want to see.
4. 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.
5. 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.
6. 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.
7. 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.
8. 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.

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.