How to get Mouse Position on Transformed HTML5 Canvas

Question

I am trying to get the mouse position on a transformed canvas. Here is my resize method:

window.addEventListener('resize', resize);
function resize() {
    screenWidth = window.innerWidth;
    screenHeight = window.innerHeight;
    scaleFillNative = MathMAX(screenWidth / maxScreenWidth, screenHeight / maxScreenHeight);
    mainCanvas.width = screenWidth;
    mainCanvas.height = screenHeight;
    mainContext.setTransform(scaleFillNative, 0, 0, scaleFillNative, Math.floor((screenWidth - (maxScreenWidth * scaleFillNative)) / 2), 
        Math.floor((screenHeight - (maxScreenHeight * scaleFillNative)) / 2));
}

The maxScreenWidth and maxScreenHeight represents the native screen dimensions that the canvas should be transformed to.

The actual resizing works fine. The issue however is that I am trying to render a circle at the mouse position on the canvas. The mouse position is set as follows:

window.addEventListener('mousemove', gameInput, false);
var mouseX, mouseY;
function gameInput(e) {
    e.preventDefault();
    e.stopPropagation();
    mouseX = e.clientX;
    mouseY = e.clientY;
}

And it is then rendered like this:

renderCircle(mouseX / scaleFillNative, mouseY / scaleFillNative, 10);

The x position is rendered correctly. However when I resize the window so that the width is less than the height, it no longer renders at the correct x location. The y position is always offset.

have a look at this http://stackoverflow.com/questions/17130395/real-mouse-position-in-canvas — Derek, Nov 15 '16 at 00:57

score 3 · Accepted Answer · answered Nov 15 '16 at 14:00

I don't know exactly what you have tried so far, but for a basic mouse coordinate to transformed canvas (non skewed), you'll have to do

mouseX = (evt.clientX - canvas.offsetLeft - translateX) / scaleX;
mouseY = (evt.clientY - canvas.offsetTop - translateY) / scaleY;

But canvas.offsetXXX doesn't take scroll amount into account, so this demo uses getBoundingRect instead.

var ctx = canvas.getContext('2d');

window.addEventListener('resize', resize);
// you probably have these somewhere
var maxScreenWidth = 1800,
  maxScreenHeight = 1200,
  scaleFillNative, screenWidth, screenHeight;

// you need to set available to your mouse move listener
var translateX, translateY;

function resize() {
  screenWidth = window.innerWidth;
  screenHeight = window.innerHeight;
  // here you set scaleX and scaleY to the same variable
  scaleFillNative = Math.max(screenWidth / maxScreenWidth, screenHeight / maxScreenHeight);
  canvas.width = screenWidth;
  canvas.height = screenHeight;
  // store these values
  translateX = Math.floor((screenWidth - (maxScreenWidth * scaleFillNative)) / 2);
  translateY = Math.floor((screenHeight - (maxScreenHeight * scaleFillNative)) / 2);

  ctx.setTransform(scaleFillNative, 0, 0, scaleFillNative, translateX, translateY);
}

window.addEventListener('mousemove', mousemoveHandler, false);

function mousemoveHandler(e) {
  // Note : I don't think there is any event default on mousemove, no need to prevent it

  // normalize our event's coordinates to the canvas current transform
  // here we use .getBoundingRect() instead of .offsetXXX 
  //   because we also need to take scroll into account,
  //   in production, store it on debounced(resize + scroll) events.
  var rect = canvas.getBoundingClientRect();
  var mouseX = (e.clientX - rect.left - translateX) / scaleFillNative,
    mouseY = (e.clientY - rect.top - translateY) / scaleFillNative;

  ctx.fillRect(mouseX - 5, mouseY - 5, 10, 10);
}

// an initial call
resize();

<canvas id="canvas"></canvas>

Yep ... debouncing on `resize` would be a plus. – markE Nov 15 '16 at 16:50 — markE, Nov 15 '16 at 16:50

How to get Mouse Position on Transformed HTML5 Canvas

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