Circle rect collision detection not working in Corona SDK

Question

I needed to get rid of physics in my game so I needed to implement a custom collision detection function (the collision detection function is being called every frame). I want to check if a rectangle has intersected a circle. I took the code from here (the answer by e.James):

bool intersects(CircleType circle, RectType rect)
{
    circleDistance.x = abs(circle.x - rect.x - rect.width/2);
    circleDistance.y = abs(circle.y - rect.y - rect.height/2);

    if (circleDistance.x > (rect.width/2 + circle.r)) { return false; }
    if (circleDistance.y > (rect.height/2 + circle.r)) { return false; }

    if (circleDistance.x <= (rect.width/2)) { return true; } 
    if (circleDistance.y <= (rect.height/2)) { return true; }

    cornerDistance_sq = (circleDistance.x - rect.width/2)^2 +
                         (circleDistance.y - rect.height/2)^2;

    return (cornerDistance_sq <= (circle.r^2));
}

And changed it to this:

--Collision detection between circle and rect
local function intersects(circle, rect)
    if circle ~= nil then
        local circleDistance_x = math.abs(circle.x - rect.x - rect.width/2);
        local circleDistance_y = math.abs(circle.y - rect.y - rect.height/2);

        if (circleDistance_x > (rect.width/2 + circle.r)) then 
            return false
        end
        if (circleDistance_y > (rect.height/2 + circle.r)) then  
            return false
        end

        if (circleDistance_x <= (rect.width/2)) then
            return true
        end

        if (circleDistance_y <= (rect.height/2)) then
            return true
        end

        cornerDistance_sq = (circleDistance_x - rect.width/2)^2 +
                             (circleDistance_y - rect.height/2)^2;

        return (cornerDistance_sq <= (circle.r^2));
        else
            return false
    end
end

I made sure to give the circle a r property and made it 20. I am not getting any errors. And the rectangles are removed when they get near the circle (it seems like the function returns true when the rectangle hits the center of the circle but I may be wrong). Am I doing something wrong in my conversion?

score 1 · Accepted Answer · answered Aug 22 '11 at 00:39

1

Changing these two lines:

    local circleDistance_x = math.abs(circle.x - rect.x - rect.width/2);
    local circleDistance_y = math.abs(circle.y - rect.y - rect.height/2);

To:

    local circleDistance_x = math.abs(circle.x+circle.r - rect.x - rect.width/2)
    local circleDistance_y = math.abs(circle.y+circle.r - rect.y - rect.height/2)

Seems to do the trick. Hurray for random guessing!

answered Aug 22 '11 at 00:39

Dair

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Additionally, there is a special case that is not handled: If the whole rectangle is inside the circle. If this case must be handled, i would see if distRC <= radius where distRC is the distance from the center of the circle to the center of the rectangle. – Gareve May 04 '13 at 23:39

Circle rect collision detection not working in Corona SDK

1 Answers1