HTML5 canvas: find out if click co-ordinates are inside a given rectangle

May be some one has had similar experiences on this dilemma and can help me out here...

Basically, I have a canvas element on which I draw several rectangles in a loop using

context.fillRect (x, y, width, height)

Now, I want some of the rectangles to be hotspots and respond to click events. I can find out the exact (x,y) of a click event using event.layerX and event.layerY.

Given that I know the following:

• the exact x,y of the click
• the x,y,width and height of every rectangle

how do I find out if the click event occurred inside the perimeter a certain rectangle or not?

and,

which rectangle the click event occurred 0n?

Is there like a mathematical formula for this?

Any help would be much appreciated, and if I'm not clear enough, let me know...

Thanks

EDIT

Is t开发者_高级运维here no better way than to loop through all rectangles and check their position and dimensions?

This is a general topology problem frequently encountered when using maps.

The algorithm for deciding whether a point is within ANY polygon (rectangles, circles, irregular shapes) is as follows:

• From the point being checked construct any line in any direction until the edge of the screen area in which your polygons are located

• if the line intersects any polygon boundary at an odd number of places then it is within that polygon

• if the line intersects any polygon boundary an even number of places it is outside that polygon.

         ------------
         |           |
         |        o--|-------------------    1 intersection: inside
         |           |
         -------------
         ------------
         |           |
  o------|-----------|-------------------    2 intersections: outside
         |           |
         -------------
  

Notes:

• The direction of the line is irrelevant

• Will not work if a polygon is cut at the side of the screen without closure

• If the polygon cuts over itself then the algorithm can fail if the line happens to pass through the cutpoint (eg in the figure 8 considered as a single polygon where the line passes exactly through where upper and lower parts connect of the figure connect)

I hate to post a ton of code here, but here's a js class that might help you...

function Point(x,y){
    this.x=x;
    this.y=y;
}

function Polygon(){
    this.points=[];
    this.x_min=undefined;
    this.x_max=undefined;
    this.y_min=undefined;
    this.y_max=undefined;

    this.add = function(p){
        this.points=this.points.concat(p);
        if (p.x<this.x_min){this.x_min=p.x;}
        if (p.x>this.x_max){this.x_max=p.x;}
        if (p.y<this.y_min){this.y_min=p.y;}
        if (p.y>this.y_min){this.y_max=p.y;}
    }

    this.pointInPoly = function(p){
        var j=(this.points.length-1);  //start by testing the link from the last point to the first point
        var isOdd=false;

        //check the bounding box conditions
        if (p.x < this.x_min || p.x > this.x_max || p.y < this.y_min || p.y > this.y_max){
            return false;
        }

        //if necessary use the line crossing algorithm
        for(var i=0; i<this.points.length; i++){
            if ((this.points[i].y<p.y && this.points[j].y>=p.y) ||  
                (this.points[j].y<p.y && this.points[i].y>=p.y)) {
                    if (this.points[i].x+(p.y-this.points[i].y)/(this.points[j].y-
                        this.points[i].y)*(this.points[j].x-this.points[i].x)<p.x)
                    { isOdd=(!isOdd);} }
            j=i;
        }
        return isOdd;
    }
}

PS: you'll need to convert your click event to local coordinates using something like the following function (where e is your click event):

function getPosition(e){
    var p = new Point();
    if (e.pageX != undefined && e.pageY != undefined) {
        p.x = e.pageX;
        p.y = e.pageY;
     }
     else {
        p.x = e.clientX + document.body.scrollLeft +
                document.documentElement.scrollLeft;
        p.y = e.clientY + document.body.scrollTop +
                document.documentElement.scrollTop;
    }
    p.x -= gCanvas.offsetLeft;
    p.y -= gCanvas.offsetTop;


    return p;
}

Roughly, you can do it like this:

var click_x = event.layerX;
var click_y = event.layerY;

for ( var i = 0; i < rects.length; i++ ) {
    var rect = rects[i];
    if ( click_x >= rect.x && click_x <= rect.x + rect.width
    &&   click_y >= rect.y && click_y <= rect.y + rect.height ) {
        // The click was inside the rectange
    }
}

Assuming we're dealing with non-negative widths and heights :)

This may be overkill, but Cake JS will do the job for rectangles and other shapes. Check out the demo.

I have recently created a API for detecting mouse clicks on rectangles. It's very basic but but it should solve your problem
You can find it here.
Note: It currently doesn't have a text support for the buttons but it does have support for some basic styles
Documentation: Here Code:

function canvasButton(x, y, w, h, style , text) {
  this.style = style
  this.x = x || 0;
  this.y = y || 0;
  this.w = w || 1;
  this.h = h || 1;
  this.fill = style.default.fill ;
  this.text = text || undefined;
  this.onclick = function() {alert("Click! Make sure to add the onclick listener to your button!")}
  this.mouseup = function() {alert("Mouseup! Make sure to add the mouseup listener to your button!")}
  this.addListener = function() {
    var buttonX = this.x;
    var buttonY = this.y;
    var buttonWidth = this.w;
    var buttonHeight = this.h;
    var onclick = this.onclick
    var mouseup = this.mouseup
    var styles = this.style
    var draw = this.draw
    var ctx = this.ctx
    this.ctx.canvas.addEventListener('mousedown',function(e){
        var click_x = e.clientX;
        var click_y = e.clientY;
        if ( click_x >= buttonY && click_x <= buttonX + buttonWidth && click_y >= buttonY && click_y <= buttonY + buttonHeight ) {  
            onclick()
            ctx.fillStyle = style.active.fill
            ctx.fillRect(buttonX, buttonY, buttonWidth, buttonHeight);
        }
    })
    this.ctx.canvas.addEventListener('mouseup',function(e) {
        var click_x = e.clientX;
        var click_y = e.clientY;
        if ( click_x >= buttonY && click_x <= buttonX + buttonWidth && click_y >= buttonY && click_y <= buttonY + buttonHeight ) {  
            mouseup()
            ctx.fillStyle = style.default.fill
            ctx.fillRect(buttonX, buttonY, buttonWidth, buttonHeight);

        }

    })
  }

}

// Draws this shape to a given context
canvasButton.prototype.draw = function(ctx) {
  this.ctx = ctx
  ctx.fillStyle = this.fill;
  ctx.font = this.font
  ctx.fillRect(this.x, this.y, this.w, this.h);
  ctx.fillText(this.text,this.x,this.y)
  this.addListener();
}


//Use it like this:
var start = new canvasButton(x,y,height,width,{ // Styles
    "default": {
        "fill":"#765",

    }, 
    "active": {
        "fill":"#876",

    } 
})
//Event Listeners
start.mousedown = function() {

}
start.mouseup = function() {

}
start.draw(document.getElementById("canvas").getContext("2d")); // Draw to canvas