I think I did the luminance key for this homework correctly, but I don’t understand how to make this alpha back into the footage as a mask after the stage in this moment. Should I use a roto? A shuffle node to put alpha back into RGB ? I don’t know what to do 🙁

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Colorspace of HSV

Using the Colorspace node to shuffle the colorspace input and output is useful for selecting the details in alpha, which allows us to control easier in the lum key.
However, our eyes need some times to adjust the relationship between HSV and the black and white data.

I found this blog explain HSV quite clearly. Indeed, the Python coding stuff is entirety another master degree, but the image in this blog clear my minds with thinking HSV with black and white pixels showing on screen.

How to Get HSV Values From RGB Colorspace and vice-versa In Python

Published on : August 27, 2020  Published by : Syed Faizunnabi

In shorts, a full range of color is transformed into number 0 to 360 or 0 to 100, and using black = 0 and white =1 for representation.

R = HUE:
Hue literally means colour.
As Red to Magenta = 0 to 360, related to Black to White = 0 to 1.

G = Saturation:
Saturation pertains the amount of white light mixed with a hue.
As plain to rich color = 0 to 100, related to Black to White = 0 to 1.

B = Luminance/ Value
Luminance is a measure to describe the perceived brightness of a colour.
As dark to bright color = 0 to 100, related to Black to White = 0 to 1.