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Nuke VFX Fundamentals

Week 9: Blur, Defocus, and 2D Clean-up in Nuke

In this session, we learnt how to use ‘Blur’ and ‘Defocus’ in a scene and how to do a 2D clean-up using ‘Roto Paint’, ‘Difference’, ‘Regrain’, and ‘Grain’ tools.

In order to add realism to a scene, it is a good technique to add some ‘Blur’ or ‘Defocus’ to it. However, depending the desired effect, we use one or the other. We use ‘defocus’ to emulate what happens with a real lens when unfocused, so since this is a more realistic and natural effect than ‘blur’, this is more commonly used for a more cinematic and more visible effect. On the other hand, ‘Blur’ is used when we need to defocus a colour or something minimum that is going to be barely visible (more for correcting purposes rather than effect wise).

‘Z Defocus’ and ‘Z Blur’ are used to defocus or blur specific areas of the plate and can be also used to blur or defocus taking in consideration the depth of the shot when the alpha is converted to depth. With these nodes we could also defocus or blur following a shape like a disc, bladed or following a roto we made. These nodes can be used together with ‘Convolve’ node in order to defocus or blur with a roto shape in different forms.

Nuke is also used for cleaning up a scene. This can be made using ‘Roto Paint’ node with which we can paint, clone, blur, dodge, and burn specific areas of he shot. After this, we could add a ‘Difference’ node to subtract the alpha taken from the ‘Roto Paint’ area followed by a ‘Copy’ and’Premult’ nodes. Also, we could add a ‘Frame Hold’ node to freeze the reference frame where we are going to do the roto painting.

Once we added the patch or correction to our shot, it is good practice to add a ‘Grain’ effect to match the grainy texture of the video and the patch blends in. We can use ‘Grain’ node which is applied through the alpha so it does not affect to whole plate but just the alpha area, or the ‘ReGrain’ node which will affect the whole plate as double grain (so it cannot be applied multiple times).

This week’s task was to practice what we learnt today trying to do a clean-up of the school shot provided: removing some papers that are on the wall, adding some roto paint in the side of the lockers, adding something to the background door (in my case I added some animated text), adding something in perspective in the left side doors (I added a video of what it looks like a circular magic portal), and adding something interesting on the floor (I added another magic portal).

Original Plate

To start, I wanted to remove some papers from the pin board on the right. To do so, I added a ‘Roto Paint’ node and used the ‘clone’ tool to paint on top of the papers using the texture of the board. Then with a regular ‘Roto’ node I created the alpha of the painted area followed by a ‘Filter Erode’ to soften the edges and a ‘Premult’ to transform it into an alpha. All of this has been done with a ‘Frame Hold’ node so it is easier to build up the roto. Then I tracked the area with 4 tracker points and created a ‘Transform (Match Move)’ tracker node to match the move of the scene. Finally, I added the ‘Grain’ node to match the grain of the Roto Paint with the scene grain and merged it with the main comp.

Secondly, I added an animated ‘Roto Paint’ to the side of the lockers. I used the already existing tracker node that was used to remove the poster that was in the same place that I wanted to add the new ‘Roto Paint’. I created a ‘Transform (Match Move)’ tracker node and attached it to the ‘Roto Paint’ node with the animation. To animate the painting, I played with the colour and opacity adding key frames in these features. Then, I linked this the the main comp.

Thirdly, I added some text in the back doors tracking the area first and then adding ‘Corner Pin 2D’ first baked to fix the frame and then another one to match move the scene movement. I also added an animation to the text key framing the colour section, and the merge it to the main comp.

For both magic portals I used the same technique that we used last week with the ‘Planar Tracker’ and creating a ‘Corner Pin 2D (Relative)’ to fix the image to the area selected. I reformatted both clips and corrected the saturation and grades. Then I merged them to the main comp using ‘screen’ option so the black background disappears and there is an transparency effect in the colours.

‘Merge (screen)’ node

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