Question

Alpha invisibility.

I currently define circular regions on some images as “hot spots”. For instance, I could have my photo on screen and overlay a circle on my head. To check for interaction with my head in realtime, I would returnOverlaps and do some manipulation on all objects overlapping the circle. For debugging, I make the circle yellow with alpha 0.5, and for release I decrease alpha to 0, making the circle invisible (as it should be).

Does this slow down the program? Is there another way to make the circle itself invisible while still remaining capable of interaction? Is there some way to color it “invisible” without using a (potentially) costly alpha of 0? Cache as bitmap matrix? Or some other efficient way to solve the “hot spot” detection without using masks?

Answer 1

Having just a few invisible display objects should not slow it down that much, but having many could. I think a more cleaner option may be to just handle it all in code, rather then have actual invisible display objects on the stage.

For a circle, you would define the center point and radius. Then to get if anyone clicked on it, you could go:

var xDist:Number = circle.x - mousePoint.x;
var yDist:Number = circle.y - mousePoint.y;

if((xDist * xDist) + (yDist * yDist) <= (circle.radius * circle.radius)){
    // mousePoint is within circle
} else {
    // mousePoint is outside of circle
}

If you insist on using display objects to set these circular hit areas (sometimes it can be easier visually, then by numbers), you could also write some code to read those display objects (and remove them from being rendered) in to get their positions and radius size.

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added method:

// inputX and inputY are the hotspot's x and y positions, and inputRadius is the radius of the hotspot
function hitTestObj(inputA:DisplayObject, inputX:int, inputY:int, inputRadius:int):Boolean {
    var xDist:Number = inputX - inputA.x;
    var yDist:Number = inputY - inputA.y;
    var minDist:Number = inputRadius + (inputA.width / 2);
    return (((xDist * xDist) + (yDist * yDist)) =< (minDist * minDist))
}