For the line starting off the cube issue, you could easily start watching the input when the player presses the mouse button, and then watch the distance between the coordinates one frame to the next, using that. Also, I dunno if I missed keyboard controls, but lacking a button to swap between draw and erase on the fly was annoying. I didn't like having to click the button at the top left of the screen.
Viewing post in Cubenomics - A Cube and Ball Puzzle Game jam comments
There was its 'X'. But it's only mentioned on the itch page and never in the game, nor is there anyway to change the controls in game. Its something I need to add into the tutorial or somehow let the player know, but all the buttons on the top left have a key assigned to them. For the line I don't want a line appearing off the cube so I'm thinking of adding a point before the first one gets added where the edge of the cube and the path you took from off the cube to the first detected point on the cube intersect. The only issue is I haven't figured out how to determine the position of the edge of the cube especially since you can now draw while the cube is rotated but nothing a few google searches and banging your head against a keyboard can't solve