I am messing around with HTML5 right now and I wanted to program a little browser games - just for fun. I created a canvas in which a background image is loaded - its kind of a hexagonal grid. To find out which hexagon is clicked I use circles an their centers/radius. I explain this, because this way you can understand the code I post more quickly.
This all works really fine until I use the scrollbars - then everything is messed up. It just seems that I does't get the coordinates right. Because the grid is a little bit bigger, I have to use scrollbars especially on smaller screens.
So here is the code
The Website
<html>
<head>
<script type="text/javascript" src="worldmap.js"></script>
<script type="text/javascript" src="worldmapMouseEvents.js"></script>
</head>
<body onload="draw()">
Xcoord: <div id="xcoord">0</div>
Ycoord: <div id="ycoord">0</div><br>
Spalte: <div id="column">0</div><br>
Reihe: <div id="row">0</div><br>
<canvas id="canvas"
width="1100"
height="1100"
style="border: 1px solid black;"
onmousemove="getMousePosition()"
onmousedown="getCell()">
</canvas>
</body>
</html>
The file worldmap.js which displays the grid
function draw(){
var c=document.getElementById("canvas");
var ctx=c.getContext("2d");
var img = new Image();
img.onload = function(){
ctx.drawImage(img,0,0);
ctx.stroke();
};
img.src = './pics/grid.jpg';
}
And here the more source file worldmapMouseEvents.js which acts strangely after scrolling
function getMousePosition() {
var x = new Number();
var y = new Number();
var canvas = document.getElementById("canvas");
if (event.x != undefined && event.y != undefined) {
x = event.x;
y = event.y;
} else
{
x = event.clientX + document.body.scrollLeft
+ document.documentElement.scrollLeft;
y = event.clientY + document.body.scrollTop
+ document.documentElement.scrollTop;
}
x -= canvas.offsetLeft;
y -= canvas.offsetTop;
return [x,y];
}
Here the function that calculates which hexagon is clicked by using circles (The math is correct and its pretty simple to (xCoord - xCenter)^2 + (yCoord -yCenter)^2. If the radius^2 is equal or smaller its the point is within the circle. Those arrays in the beginning are just center coordinates of the grid.)
function getCell(){
var xOddRows = new Array(25, 65, 105, 145, 185, 225, 265, 305, 345, 385, 425, 465, 505, 545, 585, 625, 665, 705, 745, 785, 825, 865, 905, 945, 985);
var yOddRows = new Array(35, 115, 195, 275, 355, 435, 515, 595, 675, 755, 835, 915, 995);
var xEvenRows = new Array(45, 84, 125, 165, 205, 245, 285, 325, 365, 405, 445, 485, 525, 565, 605, 645, 685, 725, 765, 805, 845, 885, 925, 965, 1005);
var yEvenRows = new Array(75, 155, 235, 315, 395, 475, 555, 635, 715, 795, 875, 955);
var position = getMousePosition();
var x = position[0];
var y = position[1];
for(var yOddRowsRunner = 0; yOddRowsRunner < 13; yOddRowsRunner++){
for( var xOddRowsRunner = 0; xOddRowsRunner < 25; xOddRowsRunner++){
var circle = (x - xOddRows[xOddRowsRunner])*(x - xOddRows[xOddRowsRunner]) + (y - yOddRows[yOddRowsRunner])*(y - yOddRows[yOddRowsRunner]);
var radius = (20 * 20);
if(radius >= circle){
yPos = (2 * yOddRowsRunner) +1 ;//This is just for nicer output
return [yPos, xOddRowsRunner];
}
}
}
for(var yEvenRowsRunner = 0; yEvenRowsRunner < 12; yEvenRowsRunner++){
for( var xEvenRowsRunner = 0; xEvenRowsRunner < 25; xEvenRowsRunner++){
var circle = (x - xEvenRows[xEvenRowsRunner])*(x - xEvenRows[xEvenRowsRunner]) + (y - yEvenRows[yEvenRowsRunner])*(y - yEvenRows[yEvenRowsRunner]);
var radius = (20 * 20);
if(radius >= circle){
yPos = (2 * yEvenRowsRunner) +2; //This is just for nicer output
return [yPos, xEvenRowsRunner];
}
}
}
return null;
}
So this is the complete code and like I said, it works fine as long as you don't use the scrollbars. I really appreciate your help - this whole thing drives me nuts.
I've experienced something like this before; I eventually figured it out by using something similar to what you had attempted; I replaced .x and .y with .pageX and .pageY
As far as I am aware, this is because .x and .y do not grab the coordinates relative to the element, but rather relative to the body; in other words, they do not account for scrolling, while pageX and pageY do.
Test case with .x and .y
Test case with .pageX and .pageY