Bluring specific parts of a video

I wanted to blur part on a video and cover it with something else but I don't want to blur everything on screen only a certain part of it does anyone know how to do that?
(I would also like to be able to track the blur to the object if that's possible)

Almost every tool in Fusion can be limited through use of an alpha channel connected to the blue EffectMask input of the node. You can use the alpha of an element, or you can make a custom alpha using one of the Mask nodes such as Polygon, BSpline, Rectangle or Ellipse, or you can use a Keyer node or a Bitmap node to create an alpha from an image or footage.

This is the rough draft of a chapter from a book about compositing with Fusion:
http://www.bryanray.name/wordpress/blac ... -tracking/

There's some information specifically about tracking and using masks on a color correction. It works the same way for most other tools, too.

Using alpha wouldn't blur it though it would just partially cover it up wouldn't it?

Although in a normal RGBA image, Alpha is thought of as a transparency channel, it might be better to think of it as the mask channel. When you just apply it to an image, it masks that image off, hiding the parts where the mask/alpha is black. If you apply the same channel instead to a color correction, it prevents the correction from taking place where the mask is black.

In fact, you can use any channel in Fusion to mask a tool. Alpha happens to be the one we use most often, but if you had a Puzzle Matte from Redshift, for instance:

https://cdnb.artstation.com/p/assets/im ... 1480450193

you could use the red channel to mask the image to affect only the skyscrapers using the options in the Common Controls tab of the tool:

Untitled.png (19.32 KiB) Viewed 1758 times

Or you might want to just think of it as the "free channel" for alignment purposes. 32 bits is 4 channels x 8 bits, 128 bits is 4 channels x 32 bits. Fusion doesn't use 3 channel images because it would just fill the extra bits with empty data. You can also have an image that is one channel, like mask outputs, and that's still aligned but saves 75% of your memory (and bandwidth).

I wanted to blur part on a video and cover it with something else but I don't want to blur everything on screen only a certain part of it does anyone know how to do that?
(I would also like to be able to track the blur to the object if that's possible)

Open the video editor to blur a section of your video. Sign in to YouTube Studio. From the left menu, select Content.
...
Add face blur
Select Blur , then select Face Blur.
Once processing is complete, select the faces that you'd like to blur, then click APPLY.
Click and drag the box to adjust the blur.
Click SAVE.

There are tons of ways. Create a power window in color and blur that. Create an adjustment layer in edit and blur that. Add a blur node in Fusion and select the area to blur using a poly shape. I can't think of a way to do it in Fairlight LOL.

I can't think of a way to do it in Fairlight LOL.

Ha! I was about to chide someone for necroing a 3.5 year-old-thread, and doing so to give a completely inappropriate answer to boot, but it was worth it for that quip.