I realize this is a simple Trigonometry question, but my high school is failing me right now.
Given an angle, that I have converted into radians to get the first point. How do I figure the next two points of the triangle to draw on the canvas, so as to make a small triangle always point outwards to the circle. So lets say Ive drawn a circle of a given radius already. Now I want a function to plot a triangle that sits on the edge of the circle inside of it, that points outwards no matter the angle. (follows the edge, so to speak)
function drawPointerTriangle(ctx, angle){
var radians = angle * (Math.PI/180)
var startX = this.radius + this.radius/1.34 * Math.cos(radians)
var startY = this.radius - this.radius/1.34 * Math.sin(radians)
// This gives me my starting point on the outer edge of the circle, plotted at the angle I need
ctx.moveTo(startX, startY);
// HOW DO I THEN CALCULATE x1,y1 and x2, y2. So that no matter what angle I enter into this function, the arrow/triangle always points outwards to the circle.
ctx.lineTo(x1, y1);
ctx.lineTo(x2, y2);
}
Example
You don't say what type of triangle you want to draw so I suppose that it is an equilateral triangle.
Take a look at this image (credit here)
I will call 3 points p1, p2, p3 from top right to bottom right, counterclockwise.
You can easily calculate the coordinate of three points of the triangle in the coordinate system with the origin is coincident with the triangle's centroid.
Given a point belongs to the edge of the circle and the point p1 that we just calculated, we can calculate parameters of the translation from our main coordinate system to the triangle's coordinate system. Then, we just have to translate the coordinate of two other points back to our main coordinate system. That is (x1,y1) and (x2,y2).
You can take a look at the demo below that is based on your code.
const w = 300;
const h = 300;
function calculateTrianglePoints(angle, width) {
let r = width / Math.sqrt(3);
let firstPoint = [
r * Math.cos(angle),
r * Math.sin(angle),
]
let secondPoint = [
r * Math.cos(angle + 2 * Math.PI / 3),
r * Math.sin(angle + 2 * Math.PI / 3),
]
let thirdPoint = [
r * Math.cos(angle + 4 * Math.PI / 3),
r * Math.sin(angle + 4 * Math.PI / 3),
]
return [firstPoint, secondPoint, thirdPoint]
}
const radius = 100
const triangleWidth = 20;
function drawPointerTriangle(ctx, angle) {
var radians = angle * (Math.PI / 180)
var startX = radius * Math.cos(radians)
var startY = radius * Math.sin(radians)
var [pt0, pt1, pt2] = calculateTrianglePoints(radians, triangleWidth);
var delta = [
startX - pt0[0],
startY - pt0[1],
]
pt1[0] = pt1[0] + delta[0]
pt1[1] = pt1[1] + delta[1]
pt2[0] = pt2[0] + delta[0]
pt2[1] = pt2[1] + delta[1]
ctx.beginPath();
// This gives me my starting point on the outer edge of the circle, plotted at the angle I need
ctx.moveTo(startX, startY);
[x1, y1] = pt1;
[x2, y2] = pt2;
// HOW DO I THEN CALCULATE x1,y1 and x2, y2. So that no matter what angle I enter into this function, the arrow/triangle always points outwards to the circle.
ctx.lineTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.closePath();
ctx.fillStyle = '#FF0000';
ctx.fill();
}
function drawCircle(ctx, radius) {
ctx.beginPath();
ctx.arc(0, 0, radius, 0, 2 * Math.PI);
ctx.closePath();
ctx.fillStyle = '#000';
ctx.fill();
}
function clear(ctx) {
ctx.fillStyle = '#fff';
ctx.fillRect(-w / 2, -h / 2, w, h);
}
function normalizeAngle(pointCoordinate, angle) {
const [x, y] = pointCoordinate;
if (x > 0 && y > 0) return angle;
else if (x > 0 && y < 0) return 360 + angle;
else if (x < 0 && y < 0) return 180 - angle;
else if (x < 0 && y > 0) return 180 - angle;
}
function getAngleFromPoint(point) {
const [x, y] = point;
if (x == 0 && y == 0) return 0;
else if (x == 0) return 90 * (y > 0 ? 1 : -1);
else if (y == 0) return 180 * (x >= 0 ? 0: 1);
const radians = Math.asin(y / Math.sqrt(
x ** 2 + y ** 2
))
return normalizeAngle(point, radians / (Math.PI / 180))
}
document.addEventListener('DOMContentLoaded', function() {
const canvas = document.querySelector('canvas');
const angleText = document.querySelector('.angle');
const ctx = canvas.getContext('2d');
ctx.translate(w / 2, h / 2);
drawCircle(ctx, radius);
drawPointerTriangle(ctx, 0);
canvas.addEventListener('mousemove', _.throttle(function(ev) {
let mouseCoordinate = [
ev.clientX - w / 2,
ev.clientY - h / 2
]
let degAngle = getAngleFromPoint(mouseCoordinate)
clear(ctx);
drawCircle(ctx, radius);
drawPointerTriangle(ctx, degAngle)
angleText.innerText = Math.floor((360 - degAngle)*100)/100;
}, 15))
})
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.9.1/underscore-min.js"></script>
<canvas width=300 height=300></canvas>
<div class="angle">0</div>
reduce the radius, change the angle and call again cos/sin:
function drawPointerTriangle(ctx, angle)
{
var radians = angle * (Math.PI/180);
var radius = this.radius/1.34;
var startX = this.center.x + radius * Math.cos(radians);
var startY = this.center.y + radius * Math.sin(radians);
ctx.moveTo(startX, startY);
radius *= 0.9;
radians += 0.1;
var x1 = this.center.x + radius * Math.cos(radians);
var y1 = this.center.y + radius * Math.sin(radians);
radians -= 0.2;
var x1 = this.center.x + radius * Math.cos(radians);
var y1 = this.center.y + radius * Math.sin(radians);
ctx.lineTo(x1, y1);
ctx.lineTo(x2, y2);
ctx.lineTo(startX, startY);
}
the resulting triangle's size is proportional to the size of the circle.
in case you need an equilateral, fixed size triangle, use this:
//get h by pythagoras
h = sqrt( a^2 - (a/2)^2 );)
//get phi using arcustangens:
phi = atan( a/2, radius-h );
//reduced radius h by pythagoras:
radius = sqrt( (radius-h)^2 + (a/2)^2 );
radians += phi;
...
radians -= 2*phi;
...
Related
I want to make a gradient that covers the whole canvas whatever the angle of it.
So I used a method found on a Stack Overflow post which is finally incorrect. The solution is almost right but, in fact, the canvas is not totally covered by the gradient.
It is this answer: https://stackoverflow.com/a/45628098/5594331
(You have to look at the last point named "Example of best fit.")
In my code example below, the yellow part should not be visible because it should be covered by the black and white gradient. This is mostly the code written in Blindman67's answer with some adjustments to highlight the problem.
I have drawn in green the control points of the gradient. With the right calculations, these should be stretched to the edges of the canvas at any angle.
var ctx = canvas.getContext("2d");
var w = canvas.width;
var h = canvas.height;
function bestFitGradient(angle){
var dist = Math.sqrt(w * w + h * h) / 2; // get the diagonal length
var diagAngle = Math.asin((h / 2) / dist); // get the diagonal angle
// Do the symmetry on the angle (move to first quad
var a1 = ((angle % (Math.PI *2))+ Math.PI*4) % (Math.PI * 2);
if(a1 > Math.PI){ a1 -= Math.PI }
if(a1 > Math.PI / 2 && a1 <= Math.PI){ a1 = (Math.PI / 2) - (a1 - (Math.PI / 2)) }
// get angles from center to edges for along and right of gradient
var ang1 = Math.PI/2 - diagAngle - Math.abs(a1);
var ang2 = Math.abs(diagAngle - Math.abs(a1));
// get distance from center to horizontal and vertical edges
var dist1 = Math.cos(ang1) * h;
var dist2 = Math.cos(ang2) * w;
// get the max distance
var scale = Math.max(dist2, dist1) / 2;
// get the vector to the start and end of gradient
var dx = Math.cos(angle) * scale;
var dy = Math.sin(angle) * scale;
var x0 = w / 2 + dx;
var y0 = h / 2 + dy;
var x1 = w / 2 - dx;
var y1 = h / 2 - dy;
// create the gradient
const g = ctx.createLinearGradient(x0, y0, x1, y1);
// add colours
g.addColorStop(0, "yellow");
g.addColorStop(0, "white");
g.addColorStop(.5, "black");
g.addColorStop(1, "white");
g.addColorStop(1, "yellow");
return {
g: g,
x0: x0,
y0: y0,
x1: x1,
y1: y1
};
}
function update(timer){
var r = bestFitGradient(timer / 1000);
// draw gradient
ctx.fillStyle = r.g;
ctx.fillRect(0,0,w,h);
// draw points
ctx.lineWidth = 3;
ctx.fillStyle = '#00FF00';
ctx.strokeStyle = '#FF0000';
ctx.beginPath();
ctx.arc(r.x0, r.y0, 5, 0, 2 * Math.PI, false);
ctx.stroke();
ctx.fill();
ctx.beginPath();
ctx.arc(r.x1, r.y1, 5, 0, 2 * Math.PI, false);
ctx.stroke();
ctx.fill();
requestAnimationFrame(update);
}
requestAnimationFrame(update);
canvas {
border : 2px solid red;
}
<canvas id="canvas" width="300" height="200"></canvas>
In this fiddle there is a function that calculates the distance between a rotated line and a point:
function distanceToPoint(px, py, angle) {
const cx = width / 2;
const cy = height / 2;
return Math.abs((Math.cos(angle) * (px - cx)) - (Math.sin(angle) * (py - cy)));
}
Which is then used to find the maximum distance between the line and the corner points (only two points are considered, because the distances to the other two points are mirrored):
const dist = Math.max(
distanceToPoint(0, 0, angle),
distanceToPoint(0, height, angle)
);
Which can be used to calculate offset points for the end of the gradient:
const ox = Math.cos(angle) * dist;
const oy = Math.sin(angle) * dist;
const gradient = context.createLinearGradient(
width / 2 + ox,
height / 2 + oy,
width / 2 - ox,
height / 2 - oy
)
Implemented a canvas, Drawing a square there and get the calculated coordinates,
Can see on the following pic the drawing:
I'm calculating and getting the upleft point X and Y coordinates,
And for the down right coordinates that i need, I'm adding the height and width, as follows:
{ upLeft: { x: position.x, y: position.y }, downRight: { x: position.x + position.width, y: position.y + position.height } },
Now i want to get the same dimensions when i'm rotating the canvas clockwise or anti-clockwise.
So i have the angle, And i try to calculate via the following function:
function getRotatedCoordinates(cx, cy, x, y, angle) {
let radians = (Math.PI / 180) * angle,
cos = Math.cos(radians),
sin = Math.sin(radians),
nx = (cos * (x - cx)) - (sin * (y - cy)) + cx,
ny = (cos * (y - cy)) + (sin * (x - cx)) + cy;
return [nx, ny];
}
And i'm calling the function via the following args and using it.
let newCoords = getRotatedCoordinates(0, 0, position.x, position.y, angle);
position.x = newCoords[0];
position.y = newCoords[1];
So firstly, I'm not sure that the cx and cy points are correct, I'm always entering 0 for both of them.
Secondly, I'm not getting the desired results, They are getting changed but i'm pretty sure that something is wrong with the x and y, So i guess that the function is wrong.
Thanks.
Here is how I would do it:
function getRectangeCoordinates(x, y, width, height, angle) {
let points = [ [x, y] ]
let radians = (Math.PI / 180) * angle;
for (let i = 0; i < 3; i++) {
x += Math.cos(radians) * ((i == 1) ? height : width);
y += Math.sin(radians) * ((i == 1) ? height : width);
points.push([x, y])
radians += Math.PI / 2
}
return points
}
let canvas = document.createElement("canvas");
canvas.width = canvas.height = 140
let ctx = canvas.getContext('2d');
document.body.appendChild(canvas);
function draw(coords, radius) {
for (let i = 0; i < 4; i++) {
ctx.beginPath();
ctx.arc(coords[i][0], coords[i][1], radius, 0, 8);
ctx.moveTo(coords[i][0], coords[i][1]);
let next = (i + 1) % 4
ctx.lineTo(coords[next][0], coords[next][1]);
ctx.stroke();
}
}
let coords = getRectangeCoordinates(20, 10, 120, 40, 15)
console.log(JSON.stringify(coords))
draw(coords, 3)
ctx.strokeStyle = "red";
coords = getRectangeCoordinates(60, 40, 40, 50, 65)
draw(coords, 5)
ctx.strokeStyle = "blue";
coords = getRectangeCoordinates(120, 3, 20, 20, 45)
draw(coords, 2)
In the getRectangeCoordinates I'm returning all corners of a rectangle and the paraments of the function are the top left corner (x, y) the height and width of the rectangle and last the angle.
I'm drawing a few rectangles with different shapes and angles to show how it looks like
The calculations in the function are simple trigonometry here is a visual representation that could help you remember it the next time you need it:
I have program where I click three times, each click creating a point on canvas. I then calculate angle between those three points like this:
function find_angle(A, B, C) {
var AB = Math.sqrt(Math.pow(B.x - A.x, 2) + Math.pow(B.y - A.y, 2));
var BC = Math.sqrt(Math.pow(B.x - C.x, 2) + Math.pow(B.y - C.y, 2));
var AC = Math.sqrt(Math.pow(C.x - A.x, 2) + Math.pow(C.y - A.y, 2));
return Math.acos((BC * BC + AB * AB - AC * AC) / (2 * BC * AB));
}
In example picture above, the calculated angle is 93°. I need to move point 3 by -3° so the points make exactly 90°. I have this function for it:
var angleToCorrect = alpha * (Math.PI / 180) - 90 * (Math.PI / 180);
correct_angle(point2, point3, angleToCorrect)
...
function correct_angle(p2, p3, angle) {
var x = p2.x - p3.x;
var y = p2.y - p3.y;
var r = Math.sqrt(x * x + y * y); //circle radius. origin of the circle is point 2
return {
X: p2.x + (Math.cos(angle) * r),
Y: p2.y + (Math.sin(angle) * r)
};
}
Now, this function should return new x and y for point 3 with corrected angle to 90°. Yet the coordinates don't agree with what I expect. Can someone point out what I'm doing wrong?
To calculate the new position it isn't enough to provide just two of the points since the angle is measured between the three.
So inside this function you have to figure out what the current angle of the vector between point 1 and point 2 is. Javascript offers a nifty built-in function for this Math.atan2()
Now that we know the angle (in radians) we need to add the new angle to it. This makes sure we can place point 3 correctly.
function correct_angle(p1, p2, p3, angle)
{
var currentAngle=Math.atan2(p1.y-p2.y, p1.x-p2.x);
currentAngle+=angle;
var x = p2.x - p3.x;
var y = p2.y - p3.y;
var r = Math.sqrt(x * x + y * y);
return {
X: p2.x + (Math.cos(currentAngle) * r),
Y: p2.y + (Math.sin(currentAngle) * r)
};
}
The angle parameter of the function should be the target angle in radians (90 or 1.5707963267949 in your case)
Here's an interactive example:
Point = function(x, y) {
this.x = x;
this.y = y;
}
var pointA = new Point(162, 39);
var pointB = new Point(105, 161);
var pointC = new Point(211, 242);
var context = document.getElementById("canvas").getContext("2d");
function correct() {
var newPoint = correct_angle(pointA, pointB, pointC, 1.5707963267949);
pointC.x = newPoint.X;
pointC.y = newPoint.Y;
draw();
}
function correct_angle(p1, p2, p3, angle) {
var currentAngle = Math.atan2(p1.y - p2.y, p1.x - p2.x);
currentAngle += angle;
var x = p2.x - p3.x;
var y = p2.y - p3.y;
var r = Math.sqrt(x * x + y * y);
return {
X: p2.x + (Math.cos(currentAngle) * r),
Y: p2.y + (Math.sin(currentAngle) * r)
};
}
function draw() {
context.clearRect(0, 0, 400, 300);
context.fillStyle = "red";
context.beginPath();
context.arc(pointA.x, pointA.y, 10, 0, 2 * Math.PI);
context.fill();
context.beginPath();
context.arc(pointB.x, pointB.y, 10, 0, 2 * Math.PI);
context.fill();
context.beginPath();
context.arc(pointC.x, pointC.y, 10, 0, 2 * Math.PI);
context.fill();
}
draw();
<canvas id="canvas" width="400" height="300" style="background-color:#dddddd;"></canvas>
<button onclick="correct()" style="float:left">
correct me
</button>
I'm trying to draw a spiral starting from a "start" point up to an "end point. The spiral also has a given center point so it can draw the spirals around that center point.
I can't make it work, somehow the math is totally wrong.
Any advice on how to solve this?
The jsfiddle of the code I tried is here.
<!DOCTYPE HTML>
<html>
<body>
<canvas id="myCanvas" width="800" height="600" style="border:1px solid #c3c3c3;"></canvas>
<script type="text/javascript">
var c = document.getElementById("myCanvas");
var cxt = c.getContext("2d");
//center of the spiral coords:
var centerX = 400;
var centerY = 300;
//draw the center of spiral point:
drawCirc(centerX, centerY, 10, '#6f0c4f');
var gap = 8;
var STEPS_PER_ROTATION = 50;
var rotations = 4;
var increment = rotations * Math.PI / STEPS_PER_ROTATION;
var theta = increment;
//start point:
var startX = 500;
var startY = 380;
//end point:
var endX = 600
var endY = 300;
//draw the start and end points as small circles:
drawCirc(startX, startY, 6, '#FF0000');
drawCirc(endX, endY, 6, '#00FF00');
//trying to calculate theta start position:
theta = Math.abs(((centerX - startX) / Math.cos(theta)) / gap);
var ind = 0;
while (theta < rotations * Math.PI * 2) {
var newX = centerX + theta * Math.cos(theta) * gap;
var newY = centerY + theta * Math.sin(theta) * gap;
var ukwObj = { x: newX, y: newY };
if (ind == 0) {
//draw start point with differnt color to differentiate
drawCirc(newX, newY, 2, 'orange');
} else {
drawCirc(newX, newY);
}
ind++;
theta = theta + increment;
}
function drawCirc(x, y, radius = 2, stroke = '#000000') {
cxt.beginPath();
cxt.arc(x, y, radius, 0, 2 * Math.PI);
cxt.strokeStyle = stroke;
cxt.stroke();
cxt.fillStyle = stroke;
cxt.fill();
}
cxt.stroke(); // draw the spiral
</script>
</body>
</html>
The principle of plotting a circle is:
Given a center x,y, a radius r, we can plot points belonging to the circle by calcultating their coordinates as follow: px = x + r * Math.cos(angle) and py = y + r * Math.sin(angle) when angle varies from 0 to 2* Math.PI.
If r grows when angle varies, the we get an outward spiral (from angle 0 to angle 2*PI which is equivalent to 0).
For the problem at hand we need to calculate a start and end position of the spiral, in polar coordinates (distance, angle).
So we need to compute the start angle, start distance, end angle and end distance, and plot each point by gradually incrementing both the angle and the distance.
The initial theta calculation was wrong, I've changed it.
Then I needed to calculate the start distance between center and start point, and the end distance between center and end point.
During rotation, you progressivley goes from start distance to end distance.
The total angle distance should be totalTheta = numberOfRotation * 2 * Math.PI + (endAngle - startAngle), I replaced rotations * Math.PI * 2 by totalTheta)
Just for fun and to demonstrate it works for any intial condition, I've randomized start position, end position and number of rotations slightly.
I've also decremented the angle increment at each iteration to make the distance between each point look more even, but you can comment to keep a constant angular speed.
The solution below will randomly choose an orange dot, a green dot, a number of turns to complete the spiral, and will plot the spiral around the fixed purple dot.
var c = document.getElementById("myCanvas");
var cxt = c.getContext("2d");
//center of the spiral coords:
var centerX = 200;
var centerY = 150;
//draw the center of spiral point:
drawCirc(centerX, centerY, 10, '#6f0c4f');
var gap = 8;
var STEPS_PER_ROTATION = 50;
var rotations = 1 + parseInt(Math.random() * 5, 10);
var increment = rotations * Math.PI / STEPS_PER_ROTATION;
var theta = increment;
var dist = 0;
//start point:
var startX = centerX + (Math.random() * 150 - 75);
var startY = centerY + (Math.random() * 150 - 75);
var startAngleOffset = startX > centerX ? (startY > centerY ? 0 : 0) : (startY > centerY ? Math.PI : Math.PI);
var startAngleSign = startX > centerX ? (startY > centerY ? 1 : -1) : (startY > centerY ? -1 : 1);
//end point:
var endX = centerX + (Math.random() * 150 - 75);
var endY = centerY + (Math.random() * 150 - 75);
var endAngleOffset = endX > centerX ? (endY > centerY ? 0 : 0) : (endY > centerY ? Math.PI : Math.PI);
var endAngleSign = endX > centerX ? (endY > centerY ? 1 : -1) : (endY > centerY ? -1 : 1);
//draw the start and end points as small circles:
drawCirc(startX, startY, 6, '#FF0000');
drawCirc(endX, endY, 6, '#00FF00');
var startTheta = theta = startAngleOffset + startAngleSign * Math.atan(Math.abs(startY - centerY)/Math.abs(startX - centerX));
var endTheta = endAngleOffset + endAngleSign * Math.atan(Math.abs(endY - centerY)/Math.abs(endX - centerX));
var totalTheta = rotations * 2 * Math.PI + (endTheta - startTheta)
dist = Math.sqrt(Math.pow(startY - centerY, 2) + Math.pow(startX - centerX, 2));
finalDist = Math.sqrt(Math.pow(endY - centerY, 2) + Math.pow(endX - centerX, 2));
var ind = 0;
while (theta -startTheta < totalTheta) {
var currentDist = (dist + ((finalDist - dist)* ((theta - startTheta) / (totalTheta))));
var newX = centerX + currentDist * Math.cos(theta);
var newY = centerY + currentDist * Math.sin(theta);
var ukwObj = { x: newX, y: newY };
if (ind == 0) {
//draw start point with differnt color to differentiate
drawCirc(newX, newY, 2, 'orange');
} else {
drawCirc(newX, newY);
}
ind++;
theta = theta + increment;
// decrement increment to make the space between points look more regular
increment = Math.max(0.01, increment - 0.00096);
}
function drawCirc(x, y, radius = 2, stroke = '#000000') {
cxt.beginPath();
cxt.arc(x, y, radius, 0, 2 * Math.PI);
cxt.strokeStyle = stroke;
cxt.stroke();
cxt.fillStyle = stroke;
cxt.fill();
}
cxt.stroke(); // draw the spiral
<!DOCTYPE HTML>
<html>
<body>
<canvas id="myCanvas" width="800" height="600" style="border:1px solid #c3c3c3;"></canvas>
</body>
</html>
I'm working on a flow-network visualization with Javascript.
Vertices are represented as circles and edges are represented as arrows.
Here is my Edge class:
function Edge(u, v) {
this.u = u; // start vertex
this.v = v; // end vertex
this.draw = function() {
var x1 = u.x;
var y1 = u.y;
var x2 = v.x;
var y2 = v.y;
context.beginPath();
context.moveTo(x1, y1);
context.lineTo(x2, y2);
context.stroke();
var dx = x1 - x2;
var dy = y1 - y2;
var length = Math.sqrt(dx * dx + dy * dy);
x1 = x1 - Math.round(dx / ((length / (radius))));
y1 = y1 - Math.round(dy / ((length / (radius))));
x2 = x2 + Math.round(dx / ((length / (radius))));
y2 = y2 + Math.round(dy / ((length / (radius))));
// calculate the angle of the edge
var deg = (Math.atan(dy / dx)) * 180.0 / Math.PI;
if (dx < 0) {
deg += 180.0;
}
if (deg < 0) {
deg += 360.0;
}
// calculate the angle for the two triangle points
var deg1 = ((deg + 25 + 90) % 360) * Math.PI * 2 / 360.0;
var deg2 = ((deg + 335 + 90) % 360) * Math.PI * 2 / 360.0;
// calculate the triangle points
var arrowx = [];
var arrowy = [];
arrowx[0] = x2;
arrowy[0] = y2;
arrowx[1] = Math.round(x2 + 12 * Math.sin(deg1));
arrowy[1] = Math.round(y2 - 12 * Math.cos(deg1));
arrowx[2] = Math.round(x2 + 12 * Math.sin(deg2));
arrowy[2] = Math.round(y2 - 12 * Math.cos(deg2));
context.beginPath();
context.moveTo(arrowx[0], arrowy[0]);
context.lineTo(arrowx[1], arrowy[1]);
context.lineTo(arrowx[2], arrowy[2]);
context.closePath();
context.stroke();
context.fillStyle = "black";
context.fill();
};
}
Given the code
var canvas = document.getElementById('canvas'); // canvas element
var context = canvas.getContext("2d");
context.lineWidth = 1;
context.strokeStyle = "black";
var radius = 20; // vertex radius
var u = {
x: 50,
y: 80
};
var v = {
x: 150,
y: 200
};
var e = new Edge(u, v);
e.draw();
The draw() function will draw an edge between two vertices like this:
If we add the code
var k = new Edge(v, u);
k.draw();
We will get:
but I want to draw edges both directions as following:
(sorry for my bad paint skills)
Of course the vertices and the edge directions are not fixed.
A working example (with drawing vertex fucntion) on JSFiddle:
https://jsfiddle.net/Romansko/0fu01oec/18/
Aligning axis to a line.
It can make everything a little easier if you rotate the rendering to align with the line. Once you do that it is then easy to draw above or below the line as that is just in the y direction and along the line is the x direction.
Thus if you have a line
const line = {
p1 : { x : ? , y : ? },
p2 : { x : ? , y : ? },
};
Convert it to a vector and normalise that vector
// as vector from p1 to p2
var nx = line.p2.x - line.p1.x;
var ny = line.p2.y - line.p1.y;
// then get length
const len = Math.sqrt(nx * nx + ny * ny);
// use the length to normalise the vector
nx /= len;
ny /= len;
The normalised vector represents the new x axis we want to render along, and the y axis is at 90 deg to that. We can use setTransform to set both axis and the origin (0,0) point at the start of the line.
ctx.setTransform(
nx, ny, // the x axis
-ny, nx, // the y axis at 90 deg to the x axis
line.p1.x, line.p1.y // the origin (0,0)
)
Now rendering the line and arrow heads is easy as they are axis aligned
ctx.beginPath();
ctx.lineTo(0,0); // start of line
ctx.lineTo(len,0); // end of line
ctx.stroke();
// add the arrow head
ctx.beginPath();
ctx.lineTo(len,0); // tip of arrow
ctx.lineTo(len - 10, 10);
ctx.lineTo(len - 10, -10);
ctx.fill();
To render two lines offset from the center
var offset = 10;
ctx.beginPath();
ctx.lineTo(0,offset); // start of line
ctx.lineTo(len,offset); // end of line
ctx.moveTo(0,-offset); // start of second line
ctx.lineTo(len,-offset); // end of second line
ctx.stroke();
// add the arrow head
ctx.beginPath();
ctx.lineTo(len,offset); // tip of arrow
ctx.lineTo(len - 10, offset+10);
ctx.lineTo(len - 10, offset-10);
ctx.fill();
offset = -10;
// add second arrow head
ctx.beginPath();
ctx.lineTo(0,offset); // tip of arrow
ctx.lineTo(10, offset+10);
ctx.lineTo(10, offset-10);
ctx.fill();
And you can reset the transform with
ctx.setTransform(1,0,0,1,0,0); // restore default transform