select and change color of a line in html5 canvas?

Question

I wrote this code to draw random graphs. I have been trying in vain to find how I can select a line in the graph so that I can apply the prim's algorithm as one selects lines and see if they have found the minimum tree.

 function draw(n,rep){
        var cvs=document.getElementsByTagName('canvas')[0];   
        /** 
         * @type CanvasRenderingContext2D 
         **/
        var ctx=cvs.getContext('2d');
        ctx.beginPath();
        var randomX=[];
        var randomY=[];
        ctx.lineWidth=2;
        ctx.font  = '3'+' Arial';
        var weights=[];
        var lastRandomx=Math.random()*200;
        var lastRandomy=Math.random()*200;
        for (var i = 0; i <n ; i++) {
            var cwidth = cvs.width;
    var cheight = cvs.height;                
            randomX[i] = Math.random()*cwidth*2/3;
    randomY[i] = Math.random()*cheight*2/3;
            weights[i]=Math.round(Math.random()*20);                        
            ctx.fillRect(randomX[i],randomY[i],5,5);        
    ctx.moveTo(lastRandomx,lastRandomy);
    ctx.lineTo(randomX[i],randomY[i]);               
            lastRandomx=randomX[i];
            lastRandomy=randomY[i];
        }
        for (var i = 0; i < rep; i++) {
            var rand=Math.round(rep*Math.random());
            ctx.lineTo(randomX[rand],randomY[rand]);
        } 
        ctx.closePath();
        ctx.stroke();
};  

I found this in stackoverflow and it doesn't help much. How to select lines that are drawn on a HTML5 Canvas?. I was wondering if there's a prewritten code so that I need not write it from scratch.

I was thinking if I could find the location of the mouse as it moves and each time check to see if the location of mouse is on the line as in here Finding if a point is on a line. Please help and suggest if there's any prewritten code because I'm restricted by time. Thank you in advance.

Answer 1

You have to loop through your line array(s) and for each line segment do:

Core principle is to add a line to the path and then test if (x,y) is on that line:

ctx.beginPath();
ctx.moveTo(x1, y1);  // start of line
ctx.lineTo(x2, y2);  // end of line

// this will test the point against the line (lineWidth matters)
if (ctx.isPointInStroke(x, y)) {
    // draw line segment in f.ex. different color here
    ctx.strokeStyle = "red";
    ctx.stroke();    // we already have a line segment on the path
}

There is no need to actually stroke the line, just rebuild the path. Adopt as needed.

Here is a full example:

var ctx = canvas.getContext("2d"),
    lines = [],  // store line segments for demo
    count = 10,  // max 10 lines for demo
    i = 0;

for(; i < count; i++) {
  var x = Math.random() * canvas.width;  // random point for end points
  var y = Math.random() * canvas.height;
  
  if (i) ctx.lineTo(x, y);  // if not first line, add lineTo
  else ctx.moveTo(x, y);    // start point
  
  lines.push({              // store point to create a poly-line
    x: x,
    y: y
  });
}

ctx.lineWidth = 5;
ctx.lineJoin = "round";
ctx.strokeStyle = "blue";
ctx.stroke();              // ..and draw line

// here we use the principle
canvas.onclick = function(e) {

  var r = canvas.getBoundingClientRect(), // adjust to proper mouse position
      x = e.clientX - r.left,
      y = e.clientY - r.top,
      i = 0
  
  // for each line segment, build segment to path and check
  for(; i < count - 1; i++) {
    ctx.beginPath();                        // new segment
    ctx.moveTo(lines[i].x, lines[i].y);     // start is current point
    ctx.lineTo(lines[i+1].x, lines[i+1].y); // end point is next
    if (ctx.isPointInStroke(x, y)) {        // x,y is on line?
      ctx.strokeStyle = "red";              // stroke red for demo
      ctx.stroke();
      break;
    }
  }
}

<canvas id=canvas width=500 height=500></canvas>

To increase sensitivity you can adjust lineWidth to a larger value (there is no need to redraw).

Answer 2

You can use math to determine which line is closest to the mouse.

Here's example code and a demo:

// canvas related variables
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var cw=canvas.width;
var ch=canvas.height;
var $canvas=$("#canvas");
var canvasOffset=$canvas.offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
ctx.lineWidth=2;

// linear interpolation -- needed in setClosestLine()
var lerp=function(a,b,x){ return(a+x*(b-a)); };

// vars to track which line is closest to the mouse
var closestLineIndex=-1;
var closestX,closestY;

// make some random lines and save them in lines[]
var n=5;
var lines=[];
var randomX=function(){return(Math.random()*cw*.67);}
var randomY=function(){return(Math.random()*ch*.67);}
var lastX=randomX();
var lastY=randomY();
for(var i=0;i<n;i++){
  var x=Math.random()*cw*.67;
  var y=Math.random()*ch*.67;
  var dx=x-lastX;
  var dy=y-lastY;
  var line={
    x0:lastX,
    y0:lastY,
    x1:x,
    y1:y,
    weight:Math.round(Math.random()*20),
    // precalc often used values
    dx:dx,
    dy:dy,
    dx2dy2:dx*dx+dy*dy,
  };
  lines.push(line);
  lastX=x;
  lastY=y;
}


redraw();

$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});


//////////////////////////////


function setClosestLine(mx,my) {

  closestLineIndex=-1;
  var minDistanceSquared=100000000;

  // examine each line & 
  // determine which line is closest to the mouse (mx,my)
  for(var i=0;i<lines.length;i++){
    var line=lines[i];

    var dx=line.x1-line.x0;
    var dy=line.y1-line.y0;
    var t=((mx-line.x0)*line.dx+(my-line.y0)*line.dy)/line.dx2dy2;
    var x=lerp(line.x0, line.x1, t);
    var y=lerp(line.y0, line.y1, t);
    var dx1=mx-x;
    var dy1=my-y;
    var distSquared=dx1*dx1+dy1*dy1;
    if(distSquared<minDistanceSquared){
      minDistanceSquared=distSquared;
      closestLineIndex=i;
      closestX=x;
      closestY=y;
    }
  }

};


function redraw(){

  // clear the canvas
  ctx.clearRect(0,0,cw,ch);

  // draw all lines
  ctx.strokeStyle='black';
  for(var i=0;i<lines.length;i++){   
    var line=lines[i];
    ctx.beginPath();
    ctx.moveTo(line.x0,line.y0);
    ctx.lineTo(line.x1,line.y1);
    ctx.stroke();
  }

  // draw the line closest to the mouse in red
  if(closestLineIndex<0){return;}
  var line=lines[closestLineIndex];
  ctx.strokeStyle='red';
  ctx.beginPath();
  ctx.moveTo(line.x0,line.y0);
  ctx.lineTo(line.x1,line.y1);
  ctx.stroke();
  ctx.fillText("Index:"+closestLineIndex+", weight:"+line.weight,10,15);
}

function handleMouseMove(e){
  e.preventDefault();
  e.stopPropagation();

  mouseX=parseInt(e.clientX-offsetX);
  mouseY=parseInt(e.clientY-offsetY);

  setClosestLine(mouseX,mouseY);

  redraw();

}

body{ background-color: ivory; }
#canvas{border:1px solid red;}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
<h4>Closest line is drawn in red<br>Closest line's weight is reported top-left</h4>
<canvas id="canvas" width=300 height=300></canvas>