javascript - Placing text label along a line on a canvas - Stack Overflow

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I managed to draw a line on a canvas using html5:

ctx.moveTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.stroke();

This works. I now want to "annotate" the line with text. So basically, I want there to be custom (e.g. whatever I pass in) text appearing along the length of the line. The difficulty is that the line can appear in any orientation (e.g. have any slope) so the text needs to be oriented accordingly. Any ideas how to start?

I managed to draw a line on a canvas using html5:

ctx.moveTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.stroke();

This works. I now want to "annotate" the line with text. So basically, I want there to be custom (e.g. whatever I pass in) text appearing along the length of the line. The difficulty is that the line can appear in any orientation (e.g. have any slope) so the text needs to be oriented accordingly. Any ideas how to start?

• javascript
• html
• canvas

Share Improve this question Follow edited Feb 21, 2011 at 20:52 Phrogz 304k113113 gold badges667667 silver badges757757 bronze badges asked Feb 21, 2011 at 16:06 johnjohn 21711 gold badge44 silver badges66 bronze badges

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2 Answers 2

Sorted by: Reset to default 15

I have created an example of this on my website. In general, you want to:

1. translate the context to the anchor point of the text, then
2. rotate the context by the amount (in radians) you desire, and then
3. fillText as normal.

I have included the relevant portion of my example below; I leave it as an exercise to the reader to detect when the text is upside down and handle it as desired. Edit: view the source on my site for additional code that keeps the text upright and also auto-truncates it.

function drawLabel( ctx, text, p1, p2, alignment, padding ){
  if (!alignment) alignment = 'center';
  if (!padding) padding = 0;

  var dx = p2.x - p1.x;
  var dy = p2.y - p1.y;   
  var p, pad;
  if (alignment=='center'){
    p = p1;
    pad = 1/2;
  } else {
    var left = alignment=='left';
    p = left ? p1 : p2;
    pad = padding / Math.sqrt(dx*dx+dy*dy) * (left ? 1 : -1);
  }

  ctx.save();
  ctx.textAlign = alignment;
  ctx.translate(p.x+dx*pad,p.y+dy*pad);
  ctx.rotate(Math.atan2(dy,dx));
  ctx.fillText(text,0,0);
  ctx.restore();
}

For Firefox only you also have the option of using mozTextAlongPath. _{^(Deprecated)}

I used it and it worked =) I just changed something so that when I make the node spin, the label is always in a good position to be read:

In my redraw function I put something like this:

particleSystem.eachEdge(function(edge, pt1, pt2){
      // edge: {source:Node, target:Node, length:#, data:{}}
      // pt1:  {x:#, y:#}  source position in screen coords
      // pt2:  {x:#, y:#}  target position in screen coords
      // draw a line from pt1 to pt2

     var dx = (pt2.x - pt1.x);
     var dy = (pt2.y - pt1.y);
     var p, pad;
     var alignment = "center";


      //ctx.label(edge.data.role,dx,dy,5,90,14);
      ctx.strokeStyle = "rgba(0,0,0, .333)";
      ctx.lineWidth = 1;
      ctx.beginPath();
      ctx.moveTo(pt1.x, pt1.y);
      ctx.lineTo(pt2.x, pt2.y);
      ctx.stroke();


     p = pt1;
     pad = 1/2;
     ctx.save();
 ctx.textAlign = alignment;
 ctx.translate(p.x+dx*pad,p.y+dy*pad);
     if(dx < 0)
     {
     ctx.rotate(Math.atan2(dy,dx) - Math.PI);  //to avoid label upside down
     }
     else
     {
     ctx.rotate(Math.atan2(dy,dx));

 }
     ctx.fillStyle = "black"
     ctx.fillText(edge.data.role,0,0);

 ctx.restore();
    })          

Thanks,

Dámaris.

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