How to identify shapes in Canvas? - html

Question

If I define a canvas and draw few shapes onto it, then how can I pinpoint each of the shape or image so as to declare events and other properties on the each shape. Consider I have a Rectangle and a triangle. SO can I have some mechanism so as to define them as specific entity and can I deal with them individually. I know about KineticJS but I would like to implement this functionality on my own(For learning purpose). Can anyone pinpoint the way to do it. Or may be an algorithmic approach??

Answer 1

I would like explain pinpoint using mouse events
First of all you have to implement a method to get mouse position
function getMousePos(canvas, evt){
// get canvas position
var obj = canvas;
wrapper = document.getElementById('wrapper');
var top = 0;
var left = 0;
while (obj && obj.tagName != 'BODY') {
top += obj.offsetTop;
left += obj.offsetLeft;
obj = obj.offsetParent;
}
// return relative mouse position
var mouseX = evt.clientX - left + window.pageXOffset+wrapper.scrollLeft;
var mouseY = evt.clientY - top + window.pageYOffset+wrapper.scrollTop;
return {
x: mouseX,
y: mouseY
};
}
Rectangle
Say, we have a rectangle with following values x1, y1, w, h
$(canvas).mousemove(function(e){
//Now call the method getMousePos
var mouseX, mouseY;
var mousePos = getMousePos(canvas, e);
mouseX=mousePos.x;
mouseY=mousePos.y;
// check if move on the rect
if(mouseX>x1 && mouseX<x1+w && mouseY>y1 && mouseY<y1+h)
{
alert('mouse on rect')
}
});
Circle
Say, we have a circle with following values x, y, r
$(canvas).mousemove(function(e){
//Now call the method getMousePos
var mouseX, mouseY;
var mousePos = getMousePos(canvas, e);
mouseX=mousePos.x;
mouseY=mousePos.y;
// check if move on the rect
if(Math.pow(mouseX-x,2)+Math.pow(mouseY-y,2)<Math.pow(r,2))
{
alert('mouse on circle')
}
});
By this way we can pinpoint a object of canvas

Answer 2

You can't use any existing functionality in the DOM for that. So you have to write it yourself. You could start by making an object model like this:
function Shape(x, y) {
var obj = {};
obj.x = x;
obj.y = y;
obj.paint = function(canvasTarget) {
}
return obj;
}
function Rectangle(x, y, width, height) {
var obj = Shape(x, y);
obj.width = width;
obj.height = height;
obj.paint = function(canvasTarget) {
//paint rectangle on the canvas
}
return obj;
}
function Canvas(target) {
var obj = {};
obj.target = target
obj.shapes = [];
obj.paint = function() {
//loop through shapes and call paint
}
obj.addShape(shape) {
this.shapes.push(shape);
}
}
After making the object model you could use it to draw the shapes like this:
var cnv = new Canvas();
cnv.addShape(new Rectangle(10,10,100,100));
cnv.paint();
Then you can handle the onclick event on the canvas and determine which shape is clicked on.