Does a line contain a point

Question

I want the user to be able to drag the edges of a square around the canvas. With my current solution it works but has glitches, sometimes an edge cannot be selected. Is ther

Answer 1

use the line equation y = mx + b to find out if the point is on a line

float EPSILON = 0.001f;

public boolean isPointOnLine(Point linePointA, Point linePointB, Point point) {
    float m = (linePointB.y - linePointA.y) / (linePointB.x - linePointA.x);
    float b = linePointA.y - m * linePointA.x;
    return Math.abs(point.y - (m*point.x+b)) < EPSILON);
}

Answer 2

You could define 8 Rect to check against - the 4 sides and 4 corners (so you can move 2 edges at once). The lines of the edge should have a width for the touchable area.

Define a Point centred on your touch event, there are then methods for checking if a rect contains a point.

Answer 3

Great piece of code by @tyczj !
I added a use-case to handle vertical lines, which gives me the following code fragment:

public boolean isPointOnLine(PointF lineStaPt, PointF lineEndPt, PointF point) {
    final float EPSILON = 0.001f;
    if (Math.abs(staPt.x - endPt.x) < EPSILON) {
        // We've a vertical line, thus check only the x-value of the point.
        return (Math.abs(point.x - lineStaPt.x) < EPSILON);
    } else {
        float m = (lineEndPt.y - lineStaPt.y) / (lineEndPt.x - lineStaPt.x);
        float b = lineStaPt.y - m * lineStaPt.x;
        return (Math.abs(point.y - (m * point.x + b)) < EPSILON);
    }
}

Also a piece of code to check if a point lies on a line-segment:

public boolean isPointOnLineSegment(PointF staPt, PointF endPt, PointF point) {
    final float EPSILON = 0.001f;
    if (isPointOnLine(staPt, endPt, point)) {
        // Create lineSegment bounding-box.
        RectF lb = new RectF(staPt.x, staPt.y, endPt.x, endPt.y);
        // Extend bounds with epsilon.
        RectF bounds = new RectF(lb.left - EPSILON, lb.top - EPSILON, lb.right + EPSILON, lb.bottom + EPSILON);
        // Check if point is contained within lineSegment-bounds.
        return bounds.contains(point.x, point.y);
    }
    return false;
}