HTML Canvas to WPF XAML Canvas - html

Question

I have an ASP.NET application that allows users to click or tap on a Canvas to indicate pain locations on a body image. A body image is displayed on the Canvas and is the same size as the Canvas.
function drawBodyMap() {
var c = document.getElementById('myCanvas');
var ctx = c.getContext('2d');
var imageObj = new Image();
imageObj.src = 'https://.../body.jpg';
imageObj.onload = function () {
ctx.drawImage(imageObj, 0, 0, 600, 367);
};
}
<canvas id="myCanvas" width="600" height="367"></canvas>
<script>
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect();
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
};
}
var canvas = document.getElementById('myCanvas');
var ctx = canvas.getContext('2d');
canvas.addEventListener('mouseup', function (evt) {
if (ixPos > 9)
return;
var mousePos = getMousePos(canvas, evt);
bodyX[ixPos] = mousePos.x;
bodyY[ixPos] = mousePos.y;
painType[ixPos] = pain_type;
ixPos++;
ctx.beginPath();
ctx.arc(mousePos.x, mousePos.y, 8, 0, 2 * Math.PI);
if (pain_type == 1)
ctx.fillStyle = "#DC143C";
else if (pain_type == 2)
ctx.fillStyle = "#EA728A";
else if (pain_type == 3)
ctx.fillStyle = "#DAA520";
else if (pain_type == 4)
ctx.fillStyle = "#008000";
else if (pain_type == 5)
ctx.fillStyle = "#4169E1";
ctx.fill();
}, false);
</script>
The X,Y points added to the Canvas on the body image are saved to a database. These points are then loaded into a WPF application that displays the same body image on an XAML Canvas. C# code then adds the points over the image.
WPF CODE:
private void DisplayBodyPain()
{
List<BodyPain> pain = gFunc.sws.GetBodyPain(MemberID);
foreach (BodyPain bp in pain)
{
Border b = new Border();
b.Tag = bp.PainType.ToString();
b.Cursor = Cursors.Hand;
b.Width = 16;
b.Height = 16;
b.CornerRadius = new CornerRadius(8);
b.Background = GetPainBrush((byte)bp.PainType);
cvsBody.Children.Add(b);
Canvas.SetTop(b, bp.YPos);
Canvas.SetLeft(b, bp.XPos);
}
}
The problem I have is that the points drawn on the XAML Canvas are all slightly different from the points that were drawn on the HTML Canvas. Each point is not in exactly the same location.
Is there a way I can fix this? Should I be doing it differently?
HTML Canvas
WPF Canvas

Answer 1

I think you need to subtract the size of the marker from the coordinate where you want to place it. For the last two lines, try this instead:
Canvas.SetTop(b, bp.YPos - (b.Height / 2));
Canvas.SetLeft(b, bp.XPos - (b.Width / 2));
By subtracting half the marker's height and width, the center of the marker is placed on the desired coordinates.