What is the best way to incorporate arrowheads into the lines that have been sketched using canvas

Question

What is the best way to incorporate arrowheads into the lines that have been sketched using canvas

I need assistance with incorporating arrowheads at the end of linear plots drawn on a coordinate grid using my custom function. You can view an example of the current setup in this JsFiddle: https://jsfiddle.net/zje14n92/1/

Below is the code snippet responsible for drawing the linear plots:

if (canvas1.getContext) {
canvas1.width  = x_axis * 2;
canvas1.height = y_axis * 2;
var ctx1 = canvas1.getContext("2d");

ctx1.font = "10px sans-serif";
ctx1.strokeText('   ', x_axis+50, 50);

ctx1.lineWidth = 1;

ctx1.beginPath();

ctx1.strokeStyle = 'black';

x = -x_max;
y = 4*x + 5; // Modify this line to change the equation
ctx1.moveTo(offsetX(x), offsetY(y));

while (x < x_max) { // Include broken line code here
    x += 0.1;
    y = 4*x+5; // Another line to modify equation
    ctx1.lineTo(offsetX(x), offsetY(y));
}
ctx1.stroke();

ctx1.closePath();

I've been trying to implement arrowheads at the endpoints similar to what I found in this JsFiddle (http://jsfiddle.net/m1erickson/Sg7EZ/), but I'm struggling to integrate it into my existing code. If anyone knows of a straightforward way to achieve this, I'd greatly appreciate your guidance!

var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
startRadians+=((this.x2>this.x1)?-90:90)*Math.PI/180;
        this.drawArrowhead(ctx,this.x1,this.y1,startRadians);
        // draw the ending arrowhead
        var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
        endRadians+=((this.x2>this.x1)?90:-90)*Math.PI/180;
        this.drawArrowhead(ctx,this.x2,this.y2,endRadians);
    }

    Line.prototype.drawArrowhead=function(ctx,x,y,radians){
        ctx.save();
        ctx.beginPath();
        ctx.translate(x, y);
        ctx.rotate(radians);
        ctx.moveTo(0,0);
        ctx.lineTo(5,20);
        ctx.lineTo(-5,20);
        ctx.closePath();
        ctx.restore();
        ctx.fill();
}

If there is a simple method to add arrowheads to the ends of lines, please advise. Thank you for any help provided!

javascript canvas

Answer 1

Answer №1

To identify any points where your infinite plot line exits the canvas (a rectangle).

The plot line may exit the canvas at 0, 1, or 2 points. If it exits at 0 or 1 point, arrowheads are not required. If it exits at 2 points, place arrowheads at both exit points using the arrowhead fiddle.

In order to check for exit points, consider the canvas rectangle as four lines forming the rectangle. Then check for intersections between the plot line and each of these four rectangle lines.

This function detects intersections between two lines:

// Credit: http://paulbourke.net/geometry/pointlineplane/
// p0 & p1 represent points on the first line
// p2 & p3 represent points on the second line
// returns the intersection point (null if no intersection)
function line2lineIntersection(p0,p1,p2,p3) {

    var unknownA = (p3.x-p2.x) * (p0.y-p2.y) - (p3.y-p2.y) * (p0.x-p2.x);
    var unknownB = (p1.x-p0.x) * (p0.y-p2.y) - (p1.y-p0.y) * (p0.x-p2.x);
    var denominator  = (p3.y-p2.y) * (p1.x-p0.x) - (p3.x-p2.x) * (p1.y-p0.y);        

    // Check for Coincident Lines
    if(unknownA==0 && unknownB==0 && denominator==0){return(null);}

    // Check for Parallel Lines
    if (denominator == 0) return null;

    // Criteria for Intersection of Line Segments
    unknownA /= denominator;
    unknownB /= denominator;

    var isIntersecting=(unknownA>=0 && unknownA<=1 && unknownB>=0 && unknownB<=1)

    if(!isIntersecting){return(null);}

    return({
        x: p0.x + unknownA * (p1.x-p0.x),
        y: p0.y + unknownA * (p1.y-p0.y)
    });
}

Identify intersections between the plot line and the 4 canvas rectangle lines:

var exitTop=line2lineIntersection(
    {x:0,y:0}, { x:canvas.width, y:0},
    yourLinePoint0, yourLinePoint1
);

var exitRight=line2lineIntersection(
    {x:canvas.width,y:0}, { x:canvas.width, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var exitBottom=line2lineIntersection(
    {x:0,y:canvas.height}, { x:canvas.width, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var exitLeft=line2lineIntersection(
    {x:0,y:0}, { x:0, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var intersections=[];
if(exitTop){ intersections.push(exitTop); }
if(exitRight){ intersections.push(exitRight); }
if(exitBottom){ intersections.push(exitBottom); }
if(exitLeft){ intersections.push(exitLeft); }

if(intersections.length==2){
    // feed your 2 exit points into your arrow drawing script 
}

Proceed by feeding the 2 exit points into your arrow drawing script:

// create a new line object
var line=new Line(
    intersections[0].x, intersections[0].y,
    intersections[1].x, intersections[1].y
);

// draw the line
line.drawWithArrowheads(context);

Example code and Demo available via provided link

Click <a href="https://i.sstatic.net/qzTPq.png" rel="nofollow noreferrer">here</a> to view addition

[Include answer code in initial script]

https://i.sstatic.net/qzTPq.png

///////////
//
function Line(x1,y1,x2,y2){
    this.x1=x1;
    this.y1=y1;
    this.x2=x2;
    this.y2=y2;
}
Line.prototype.drawWithArrowheads=function(color){
    ctx.strokeStyle=color || "black";
    ctx.fillStyle=color || "black";
    ctx.lineWidth=1;   
    ctx.beginPath();
    ctx.moveTo(this.x1,this.y1);
    ctx.lineTo(this.x2,this.y2);
    ctx.stroke();
    var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
    startRadians+=((this.x2>this.x1)?-90:90)*Math.PI/180;
    this.drawArrowhead(ctx,this.x1,this.y1,startRadians);
    var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
    endRadians+=((this.x2>this.x1)?90:-90)*Math.PI/180;
    this.drawArrowhead(ctx,this.x2,this.y2,endRadians);
}
//
Line.prototype.drawArrowhead=function(ctx,x,y,radians){
    ctx.save();
    ctx.beginPath();
    ctx.translate(x,y);
    ctx.rotate(radians);
    ctx.moveTo(0,0);
    ctx.lineTo(5,20);
    ctx.lineTo(-5,20);
    ctx.closePath();
    ctx.restore();
    ctx.fill();
}
////////////////////

Specific styling properties mentioned in the script

<div class="relative">
  <canvas id="axes"></canvas>
  <canvas id="plot"></canvas>

Answer 2

To identify any points where your infinite plot line exits the canvas (a rectangle).

The plot line may exit the canvas at 0, 1, or 2 points. If it exits at 0 or 1 point, arrowheads are not required. If it exits at 2 points, place arrowheads at both exit points using the arrowhead fiddle.

In order to check for exit points, consider the canvas rectangle as four lines forming the rectangle. Then check for intersections between the plot line and each of these four rectangle lines.

This function detects intersections between two lines:

// Credit: http://paulbourke.net/geometry/pointlineplane/
// p0 & p1 represent points on the first line
// p2 & p3 represent points on the second line
// returns the intersection point (null if no intersection)
function line2lineIntersection(p0,p1,p2,p3) {

    var unknownA = (p3.x-p2.x) * (p0.y-p2.y) - (p3.y-p2.y) * (p0.x-p2.x);
    var unknownB = (p1.x-p0.x) * (p0.y-p2.y) - (p1.y-p0.y) * (p0.x-p2.x);
    var denominator  = (p3.y-p2.y) * (p1.x-p0.x) - (p3.x-p2.x) * (p1.y-p0.y);        

    // Check for Coincident Lines
    if(unknownA==0 && unknownB==0 && denominator==0){return(null);}

    // Check for Parallel Lines
    if (denominator == 0) return null;

    // Criteria for Intersection of Line Segments
    unknownA /= denominator;
    unknownB /= denominator;

    var isIntersecting=(unknownA>=0 && unknownA<=1 && unknownB>=0 && unknownB<=1)

    if(!isIntersecting){return(null);}

    return({
        x: p0.x + unknownA * (p1.x-p0.x),
        y: p0.y + unknownA * (p1.y-p0.y)
    });
}

Identify intersections between the plot line and the 4 canvas rectangle lines:

var exitTop=line2lineIntersection(
    {x:0,y:0}, { x:canvas.width, y:0},
    yourLinePoint0, yourLinePoint1
);

var exitRight=line2lineIntersection(
    {x:canvas.width,y:0}, { x:canvas.width, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var exitBottom=line2lineIntersection(
    {x:0,y:canvas.height}, { x:canvas.width, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var exitLeft=line2lineIntersection(
    {x:0,y:0}, { x:0, y:canvas.height},
    yourLinePoint0, yourLinePoint1
);

var intersections=[];
if(exitTop){ intersections.push(exitTop); }
if(exitRight){ intersections.push(exitRight); }
if(exitBottom){ intersections.push(exitBottom); }
if(exitLeft){ intersections.push(exitLeft); }

if(intersections.length==2){
    // feed your 2 exit points into your arrow drawing script 
}

Proceed by feeding the 2 exit points into your arrow drawing script:

// create a new line object
var line=new Line(
    intersections[0].x, intersections[0].y,
    intersections[1].x, intersections[1].y
);

// draw the line
line.drawWithArrowheads(context);

Example code and Demo available via provided link

Click <a href="https://i.sstatic.net/qzTPq.png" rel="nofollow noreferrer">here</a> to view addition

[Include answer code in initial script]

https://i.sstatic.net/qzTPq.png

///////////
//
function Line(x1,y1,x2,y2){
    this.x1=x1;
    this.y1=y1;
    this.x2=x2;
    this.y2=y2;
}
Line.prototype.drawWithArrowheads=function(color){
    ctx.strokeStyle=color || "black";
    ctx.fillStyle=color || "black";
    ctx.lineWidth=1;   
    ctx.beginPath();
    ctx.moveTo(this.x1,this.y1);
    ctx.lineTo(this.x2,this.y2);
    ctx.stroke();
    var startRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
    startRadians+=((this.x2>this.x1)?-90:90)*Math.PI/180;
    this.drawArrowhead(ctx,this.x1,this.y1,startRadians);
    var endRadians=Math.atan((this.y2-this.y1)/(this.x2-this.x1));
    endRadians+=((this.x2>this.x1)?90:-90)*Math.PI/180;
    this.drawArrowhead(ctx,this.x2,this.y2,endRadians);
}
//
Line.prototype.drawArrowhead=function(ctx,x,y,radians){
    ctx.save();
    ctx.beginPath();
    ctx.translate(x,y);
    ctx.rotate(radians);
    ctx.moveTo(0,0);
    ctx.lineTo(5,20);
    ctx.lineTo(-5,20);
    ctx.closePath();
    ctx.restore();
    ctx.fill();
}
////////////////////

Specific styling properties mentioned in the script

<div class="relative">
  <canvas id="axes"></canvas>
  <canvas id="plot"></canvas>

Answer 3

Answer №2

The final 4 lines in the jsFiddle demonstrate how to use the code:

    // initialize a new line object
    var line = new Line(50, 50, 250, 275);
    // draw the line with arrowheads
    line.drawWithArrowheads(context);

(Remember to refer back to the top of the jsFiddle for the definition of Line).

In essence, this code calculates the angle at which the line is moving and then renders a triangle at that angle.

It saves the current state,
translates to the endpoint of the line,
rotates to align the arrowhead correctly,
creates the arrowhead shape,
fills in the shape, and finally,
restores the initial state.

The most challenging part is determining the angle needed to orient the arrowhead. For a multi-point line, you would likely use the last two points for this calculation.

Answer 4

The final 4 lines in the jsFiddle demonstrate how to use the code:

    // initialize a new line object
    var line = new Line(50, 50, 250, 275);
    // draw the line with arrowheads
    line.drawWithArrowheads(context);

(Remember to refer back to the top of the jsFiddle for the definition of Line).

In essence, this code calculates the angle at which the line is moving and then renders a triangle at that angle.

It saves the current state,
translates to the endpoint of the line,
rotates to align the arrowhead correctly,
creates the arrowhead shape,
fills in the shape, and finally,
restores the initial state.

The most challenging part is determining the angle needed to orient the arrowhead. For a multi-point line, you would likely use the last two points for this calculation.

What is the best way to incorporate arrowheads into the lines that have been sketched using canvas

Answer №1

Answer №2

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