Currently, I am experimenting with an HTML5 canvas element. A major concern of mine is setting up a mousemove event to monitor the movement of the mouse over the canvas for drawing and other purposes. Unfortunately, I have not been able to locate a definitive answer on how to obtain the precise coordinates of the pixel in the canvas that the mouse is positioned over. In searching for guidance, I stumbled upon a tutorial online which included this snippet (I supplemented the background-color attribute to ensure visibility on the displayed page):
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
#myCanvas {
background-color: cornflowerblue;
}
</style>
</head>
<body>
<canvas id="myCanvas" width="578" height="200"></canvas>
// Remaining script code //
</body>
</html>
Upon loading the page, you should see the canvas as a blue rectangle situated at the upper left corner of the page. Hovering your mouse cursor over it will prompt a display within the canvas reflecting the current X and Y coordinates.
For experimentation purposes, I made minor adjustments to the canvas styling by implementing top and left margins while leaving the rest untouched.
#myCanvas {
background-color: cornflowerblue;
margin-left: 30px;
margin-top: 30px;
}
Following these modifications, when previewed, the canvas's blue rectangular shape was indeed offset from the top and left borders on the page as anticipated. However, moving the mouse over the canvas now shows the X and Y positions as decimal numbers instead of integers like before. Upon inspecting the code further, the issue seems to stem from getBoundingClientRect() returning floated values for top and left properties.
I ponder if rectifying this discrepancy involves truncating or rounding the returned values from getBoundingClientRect(), yet such measures strike me as somewhat misguided.
This poses the question of whether my utilization of getBoundingClientRect() is flawed, or are floating point values the expected output?
Is there perhaps a straightforward method to reliably acquire the exact X and Y mouse coordinates when monitoring various mouse events over the canvas?