Div isn't at right position

Question

I am using this color wheel picker, and I'm trying to add a div as the dragger instead of having it embedded in the canvas.

I created an outer div (a wrapper), and inserted a div (dragger), then the canvas. I made the dragger div's position to absolute. Then in the redraw(e) function, I set the left and top to the following:

dragger.style.left = currentX + 'px';
dragger.style.top = currentY + 'px';

This works i.e. the dragger moves when it should, but the it's at the wrong position.

How can I get the dragger to be at the same position as the cursor?

JSFiddle

var b = document.body;
var c = document.getElementsByTagName('canvas')[0];
var a = c.getContext('2d');


var wrapper = document.getElementById('wrapper');
var dragger = document.createElement('div');
dragger.id = 'dragger';
wrapper.appendChild(dragger);
wrapper.insertBefore(dragger, c);


document.body.clientWidth; // fix bug in webkit: http://qfox.nl/weblog/218

(function() {

  // Declare constants and variables to help with minification
  // Some of these are inlined (with comments to the side with the actual equation)
  var doc = document;
  doc.c = doc.createElement;
  b.a = b.appendChild;

  var width = c.width = c.height = 400,
    label = b.a(doc.c("p")),
    input = b.a(doc.c("input")),
    imageData = a.createImageData(width, width),
    pixels = imageData.data,
    oneHundred = input.value = input.max = 100,
    circleOffset = 10,
    diameter = 380, //width-circleOffset*2,
    radius = 190, //diameter / 2,
    radiusPlusOffset = 200, //radius + circleOffset
    radiusSquared = radius * radius,
    two55 = 255,
    currentY = oneHundred,
    currentX = -currentY,
    wheelPixel = 16040; // circleOffset*4*width+circleOffset*4;

  // Math helpers
  var math = Math,
    PI = math.PI,
    PI2 = PI * 2,
    sqrt = math.sqrt,
    atan2 = math.atan2;

  // Setup DOM properties
  b.style.textAlign = "center";
  label.style.font = "2em courier";
  input.type = "range";

  // Load color wheel data into memory.
  for (y = input.min = 0; y < width; y++) {
    for (x = 0; x < width; x++) {
      var rx = x - radius,
        ry = y - radius,
        d = rx * rx + ry * ry,
        rgb = hsvToRgb(
          (atan2(ry, rx) + PI) / PI2, // Hue
          sqrt(d) / radius, // Saturation
          1 // Value
        );

      // Print current color, but hide if outside the area of the circle
      pixels[wheelPixel++] = rgb[0];
      pixels[wheelPixel++] = rgb[1];
      pixels[wheelPixel++] = rgb[2];
      pixels[wheelPixel++] = d > radiusSquared ? 0 : two55;
    }
  }

  // Bind Event Handlers
  input.onchange = redraw;
  c.onmousedown = doc.onmouseup = function(e) {
    // Unbind mousemove if this is a mouseup event, or bind mousemove if this a mousedown event
    doc.onmousemove = /p/.test(e.type) ? 0 : (redraw(e), redraw);
  }

  // Handle manual calls + mousemove event handler + input change event handler all in one place.
  function redraw(e) {

    // Only process an actual change if it is triggered by the mousemove or mousedown event.
    // Otherwise e.pageX will be undefined, which will cause the result to be NaN, so it will fallback to the current value
    currentX = e.pageX - c.offsetLeft - radiusPlusOffset || currentX;
    currentY = e.pageY - c.offsetTop - radiusPlusOffset || currentY;

    // Scope these locally so the compiler will minify the names.  Will manually remove the 'var' keyword in the minified version.
    var theta = atan2(currentY, currentX),
      d = currentX * currentX + currentY * currentY;

    // If the x/y is not in the circle, find angle between center and mouse point:
    //   Draw a line at that angle from center with the distance of radius
    //   Use that point on the circumference as the draggable location
    if (d > radiusSquared) {
      currentX = radius * math.cos(theta);
      currentY = radius * math.sin(theta);
      theta = atan2(currentY, currentX);
      d = currentX * currentX + currentY * currentY;
    }

    label.textContent = b.style.background = hsvToRgb(
      (theta + PI) / PI2, // Current hue (how many degrees along the circle)
      sqrt(d) / radius, // Current saturation (how close to the middle)
      input.value / oneHundred // Current value (input type="range" slider value)
    )[3];

    dragger.style.left = currentX + 'px';
    dragger.style.top = currentY + 'px';
    // Reset to color wheel and draw a spot on the current location. 
    a.putImageData(imageData, 0, 0);

    // Draw the current spot.
    // I have tried a rectangle, circle, and heart shape.
    /*
    // Rectangle:
    a.fillStyle = '#000';
    a.fillRect(currentX+radiusPlusOffset,currentY+radiusPlusOffset, 6, 6);
    */
    /*
    // Circle:
    a.beginPath();  
    a.strokeStyle = '#000';
    a.arc(~~currentX+radiusPlusOffset,~~currentY+radiusPlusOffset, 4, 0, PI2);
    a.stroke();
    */

    // Heart:
    a.font = "1em arial";
    a.fillText("♥", currentX + radiusPlusOffset - 4, currentY + radiusPlusOffset + 4);

  }

  // Created a shorter version of the HSV to RGB conversion function in TinyColor
  // https://github.com/bgrins/TinyColor/blob/master/tinycolor.js
  function hsvToRgb(h, s, v) {
    h *= 6;
    var i = ~~h,
      f = h - i,
      p = v * (1 - s),
      q = v * (1 - f * s),
      t = v * (1 - (1 - f) * s),
      mod = i % 6,
      r = [v, q, p, p, t, v][mod] * two55,
      g = [t, v, v, q, p, p][mod] * two55,
      b = [p, p, t, v, v, q][mod] * two55;

    return [r, g, b, "rgb(" + ~~r + "," + ~~g + "," + ~~b + ")"];
  }

  // Kick everything off
  redraw(0);

  /*
  // Just an idea I had to kick everything off with some changing colors…
  // Probably no way to squeeze this into 1k, but it could probably be a lot smaller than this:
  currentX = currentY = 1;
  var interval = setInterval(function() {
      currentX--;
      currentY*=1.05;
      redraw(0)
  }, 7);
    
  setTimeout(function() {
      clearInterval(interval)
  }, 700)
  */

})();

#wrapper {
  width: 400px;
  height: 400px;
  position: relative;
}
#dragger {
  background-color: orange;
  width: 15px;
  height: 15px;
  border-radius: 50%;
  position: relative;
}

<div id='wrapper'>
  <canvas id="c"></canvas>
</div>

Answer 1

Just add radiusPlusOffset here:

dragger.style.left = currentX + radiusPlusOffset + 'px';
dragger.style.top = currentY + radiusPlusOffset + 'px';

https://jsfiddle.net/qsvmyh3z/1/

Also adjust/subtract the height & width of the the dragger to pin point at the right location.

Answer 2

I got it pretty close by manipulating the currentX and currentY coordinates. The commenter above is close to the solution; ~50% of the height and width must be added to the relative position. I also recommend using the z-index property on the dragger, if you want the heart to be on top and the dragger between it and the color wheel.

Final and correct version with proper offset: Fiddle

If the dragger is reduced in size from 15px to 5px, add 7 and 4px respectively: https://jsfiddle.net/6n9zwahL/ (fixed amounts) or https://jsfiddle.net/mak3Lace/ (non-fixed, programmatic solution).

        dragger.style.left = (currentX + radiusPlusOffset + (radiusPlusOffset/30)) + 'px';
        dragger.style.top = (currentY + radiusPlusOffset+(radiusPlusOffset/40)) + 'px';

Preserved historical responses until I have time to edit:

Forked Fiddle

code - here it is being done manually in pixels but I'd create a variable and set it to a more accurate value by querying the browser for width:

        dragger.style.left = currentX + 210 + 'px';
        dragger.style.top = currentY +195 + 'px';

New Fiddle, with dragger exactly aligned to heart

New Positions:

dragger.style.left = currentX + 204 + 'px';
dragger.style.top = currentY +199 + 'px';

Added a pointer for improved ux: https://jsfiddle.net/4cLpvu2m/

To elaborate further, use the code you have for heart position:

currentX + radiusPlusOffset - 4, currentY + radiusPlusOffset + 4

and query browser for position of your dragger div. Then simply subtract x from x-dragger and y from y-dragger to get the difference. Add the difference to your dragger.style.left and so on (it happens that those numbers are ~204 and 199).

Another fork with alerts indicating x/y position values

Fork of Jessica's update, removing +/-4 to align elements: https://jsfiddle.net/hf5k2ecg/