Placing a letter randomly in a canvas offset by a % from border

Question

I have this canvas where I place a letter randomly within the canvas.

var w = Math.random() * canvas.width;
var h = Math.random() * canvas.height;

drawRandomCircle(canvas,w,h);

function drawRandomCircle(canvas,w,h) 
{
    
    var fontSize = '35';            
    var ctx = canvas.getContext("2d");
    
    var color = 'rgba(245, 66, 66,1.0)';
    ctx.fillStyle = color;
    ctx.font = fontSize + 'pt Arial';
    ctx.fillText('O', w, h);
    
} 

The results:

I would like to improve further on the function to include an offset in % from the canvas boundary to limit where the letter will appear.

The results would be similar to something similar to this.

Any ideas?

Answer 1

You need to take into account the 10% on the borders.

Try the following which uses this principle... but also remember that the co-ordinates for the canvas are top-left based... but when you do the font it will go UP (not down) so you have to take that into account as well.

var canvas = document.getElementsByTagName("canvas")[0];
var fontSizePx = 35;

// Get 10% of the width/height
var cw = (canvas.width / 10);
var ch = (canvas.height / 10);

// Get 80% of the width/height but minus the size of the font
var cw80 = (cw * 8) - fontSizePx;
var ch80 = (ch * 8) - fontSizePx;

for(var i = 0; i < 10; i++) {
  // Get random value within center 80%
  var w = (Math.random() * cw80) + cw;
  // Add on the size of the font to move it down
  var h = (Math.random() * ch80) + ch + fontSizePx;
  drawRandomCircle(canvas,w,h);
}

function drawRandomCircle(canvas,w,h) {
  var ctx = canvas.getContext("2d");
  var color = 'rgba(245, 66, 66,1.0)';
  ctx.fillStyle = color;
  ctx.font = fontSizePx.toString() + 'px Arial';
  ctx.fillText('O', w, h);
}

canvas {
  border:1px solid black;
}

<canvas></canvas>