The Scaleline is a CSS-Element with a fixed pixel width depending on the actual resolution/zoom. Since I didn't managed to "retrace" the math on all the DPI/PointResolution/Scale thing i just extract the width from the .ol-scale-line-inner CSS-Container with jQuery.
The Drawing will be included in the postcompose event. I'm not a Pro-Programmer - so feel free to enhance this basic example. An enhanced version of this can be found at the WEBGIS-App
<!DOCTYPE html>
<html>
<head>
<title>Export map example</title>
</head>
<body>
<script>
//[...] see export-map example
/** Add a Scale/Scaleline to the CANVAS itself **/
// we need the width of the scaleline-container (style="width:100px")
map.addControl(new ol.control.ScaleLine());
InsertToCanvas = (function() {
//get the canvas element
var canvas = $('canvas').get(0);
//get the Scaleline div container the style-width property
var olscale = $('.ol-scale-line-inner');
//Scaleline thicknes
var line1 = 6;
//Offset from the left
var x_offset = 10;
//offset from the bottom
var y_offset = 30;
var fontsize1 = 15;
var font1 = fontsize1 + 'px Arial';
// how big should the scale be (original css-width multiplied)
var multiplier = 2;
/* go for it */
function WriteScaletoCanvas(e) {
var ctx = e.context;
var scalewidth = parseInt(olscale.css('width'),10)*multiplier;
var scale = olscale.text();
var scalenumber = parseInt(scale,10)*multiplier;
var scaleunit = scale.match(/[Aa-zZ]{1,}/g);
//Scale Text
ctx.beginPath();
ctx.textAlign = "left";
ctx.strokeStyle = "#ffffff";
ctx.fillStyle = "#000000";
ctx.lineWidth = 5;
ctx.font = font1;
ctx.strokeText([scalenumber + ' ' + scaleunit], x_offset + fontsize1 / 2, canvas.height - y_offset - fontsize1 / 2);
ctx.fillText([scalenumber + ' ' + scaleunit], x_offset + fontsize1 / 2, canvas.height - y_offset - fontsize1 / 2);
//Scale Dimensions
var xzero = scalewidth + x_offset;
var yzero = canvas.height - y_offset;
var xfirst = x_offset + scalewidth * 1 / 4;
var xsecond = xfirst + scalewidth * 1 / 4;
var xthird = xsecond + scalewidth * 1 / 4;
var xfourth = xthird + scalewidth * 1 / 4;
// Stroke
ctx.beginPath();
ctx.lineWidth = line1 + 2;
ctx.strokeStyle = "#000000";
ctx.fillStyle = "#ffffff";
ctx.moveTo(x_offset, yzero);
ctx.lineTo(xzero + 1, yzero);
ctx.stroke();
//sections black/white
ctx.beginPath();
ctx.lineWidth = line1;
ctx.strokeStyle = "#000000";
ctx.moveTo(x_offset, yzero);
ctx.lineTo(xfirst, yzero);
ctx.stroke();
ctx.beginPath();
ctx.lineWidth = line1;
ctx.strokeStyle = "#FFFFFF";
ctx.moveTo(xfirst, yzero);
ctx.lineTo(xsecond, yzero);
ctx.stroke();
ctx.beginPath();
ctx.lineWidth = line1;
ctx.strokeStyle = "#000000";
ctx.moveTo(xsecond, yzero);
ctx.lineTo(xthird, yzero);
ctx.stroke();
ctx.beginPath();
ctx.lineWidth = line1;
ctx.strokeStyle = "#FFFFFF";
ctx.moveTo(xthird, yzero);
ctx.lineTo(xfourth, yzero);
ctx.stroke();
}
function postcompose() {
map.on('postcompose', function (evt) {
WriteScaletoCanvas(evt);
});
}
return {
postcompose : postcompose
};
})();
InsertToCanvas.postcompose();
//this is it
// rest of export-map example [...]
</script>
</body>
</html>