I have lots of points. How can I tell which ones touch the line and which do not?
Curve Closest Point component. Then test if distance output is larger than a tolerance (something like 0.001) with the Larger Than component.
It sounds like you need to sort your lines based on how “dark” they are, then remove the upper (i.e. lightest) n%
I’d start out with just dividing each line into -say- one hundred points, sampling the brightness at all those points, and just adding up the total brightness using Mass Addition. You should now have for each line a value between zero (all black) and 100 (all white). Use those values (after flattening) to sort your lines from darkest to lightest and split this list at some index. The lines to the left of the index are the ones you want.
Wait no, don’t divide into a fixed number of points, your lines have different lengths. You need to divide them based on length, so that the sampling density is roughly equal for all of them.
It’s doesn’t work…
I don’t see any closest point search there. However now that you’ve decided to give us all the information (why not provide it in the first place?) you should follow David’s suggestion.