I'm currently trying to rotoscope a video and the background is currently white and I don't know how to make it transparent.
Here is a screenshot:
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Image when the background is first changed to solid green.
As a first step you can use the keying node in the Compositor. I assume you are removing the background for a large sequence of images. If this is a a single image I would use GIMP .
The key color was taken from the image you provided, some white color, which should be replaced by green. Note the white outline around the head and the chest area has become transparent. This is not a tutorial about how to use the Keying node. Please search here at BSE and other videos for the eduction regarding the Keying Node. This is a first step in the right direction. Blurring helps to remove patches of colors that match the background color. I assume you control the background color. An uncommon solid green rather than white will reduce work. Above.
The mask is improved by adding a box mask making the chest area of the mask more white. The mask was added, located and sized quickly and imprecisely.
You see more chest area visible opaque which is desired. This was to show that objects can be placed in problem areas to bring back opaqueness visibility. Sometimes animating masks and objects in black or white in the 3D View can help you in problem areas.
There is no need to make the background white (or green) when you can make it transparent without more processing. The idea is to use the mask information to determine which parts of the image are opaque and which are transparent.
Don't use the mix node.
Use Set Alpha, so that your mask controls the transparency of the video: Whatever is white on the mask will remain opaque, the black areas will be transparent.
If you need to re-position or re-scale the elements do so after the set alpha node, so that the transformation happen as a single combined element.