I am trying to extract the y channel of a rgb image and asking myself if there are some packages available for that?
What kind of brightness do you mean? There are different types…
The “standard” luminance is
Y = 0.2126R + 0.7152G + 0.0722B
Here is more information: Relative luminance - Wikipedia
To get started with reading the images, you can check https://juliaimages.org/latest/quickstart/#
In Images.jl, there is
imgg = Gray.(img) which does what you want.