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javascriptcanvascreatejs

Canvas - How to find if a point is above or under a diagonale line?

In a 2D game, i need to find if an object is above or under a diagonale line.

Anyone knows how to do this ?

(i use createJS framework)

Solution

  • OK, scrap my previous answer and use line intersection instead. Shoot a line from the point to test straight up. If there is an intersection the point is below, if none, the point is either above or to the side of the line.

    To avoid side cases (no pun), extend the original line using interpolation.

    Here is a function to do line intersection. To do linear interpolation of the original line simply use some extreme values:

    var tx1 = x1 + (x2-x1) * -51000;
var ty1 = y1 + (y2-y1) * -51000;

var tx2 = x1 + (x2-x1) * 53200;
var ty2 = y1 + (y2-y1) * 53200;

    Update I was a bit in a hurry this morning so here's a small update. As blindman67 points out, you can use just the d in the linked intersection function and check s/t if they are in the normalized range (or just use cross product - see his answer it that is a better fit).