How do I make a Sepia Filter?

if you go by the color values for sepia shown here:
https://www.colorhexa.com/704214
(seems to be universally agreed upon values as confirmed by wikipedia and other sites:
https://en.wikipedia.org/wiki/Sepia_(color))
you can use the percentages for the op values, to be exact i'm dividing the 'rgb decimal' values they list there by 255 to get more exact values you can use with [jit.op @op *]:
R - 0.439216
G - 0.258824
B - 0.078431

sepia itself is pretty dark, so you'll probably want to brighten it from there(one way is to just multiply a scaling factor by all of those... for example, multiply all those RGB values by 2 to get twice as bright, etc... if you multiply too high, though, it might accentuate the lesser values more since they can't go beyond 1 in this format, so you could also try applying the jit.brcosa object to the output for better control overall)

that would just be my way of figuring it out from the online info. available, but there might be some more savvy photography/visual experts here who can give more exact details on how to make it look like old chemically toned photographs. hope it helps get ya started, though 🍻