Develop Reference

How to create upload in each hexagon shape in canvas?

I create multi shape with canvas, but I want to upload a photo by clicking on each hexagon.
How to create with jquery?
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
const a = 2 * Math.PI / 6;
const r = 50;
// 1st
x = r;
y = r;
drawHexagon(x, y);
// 2nd
x = x + r + r * Math.cos(a);
y = y + r * Math.sin(a);
drawHexagon(x, y);
// 3rd
x = x + r + r * Math.cos(a);
y = y - r * Math.sin(a);
drawHexagon(x, y);
// 4th
x = x + r + r * Math.cos(a);
y = y + r * Math.sin(a);
drawHexagon(x, y);
function drawHexagon(x, y) {
ctx.beginPath();
for (var i = 0; i < 6; i++) {
ctx.lineTo(x + r * Math.cos(a * i), y + r * Math.sin(a * i));
}
ctx.closePath();
ctx.stroke();
}
<canvas id="canvas" width="800" height="500" />

I'm going to answer the first part of your problem...
The comment from ggorlen is spot on, you have a complex situation, you must break it down into multiple questions and tackle each individually.
So how do we detect a mouse event over a particular shape in a canvas?
My recommendation use Path2D:
https://developer.mozilla.org/en-US/docs/Web/API/Path2D
It comes with a handy function isPointInPath to detect if a point is in or path:
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/isPointInPath
Sample code below, I'm using the mouse move event but you can use any other:
const canvas = document.getElementById('canvas');
const ctx = canvas.getContext('2d');
class Hexagon {
constructor(x, y, r, a) {
this.path = new Path2D()
for (var i = a; i < a+Math.PI*2; i+=Math.PI/3) {
this.path.lineTo(x + r * Math.cos(i), y + r * Math.sin(i));
}
}
draw(evt) {
ctx.beginPath()
var rect = canvas.getBoundingClientRect()
var x = evt.clientX - rect.left
var y = evt.clientY - rect.top
ctx.fillStyle = ctx.isPointInPath(this.path, x, y) ? "red" : "green"
ctx.fill(this.path)
}
}
shapes = []
shapes.push(new Hexagon( 50, 50, 40, 0))
shapes.push(new Hexagon(125, 90, 45, 0.5))
shapes.push(new Hexagon(200, 50, 30, 0.8))
shapes.push(new Hexagon(275, 90, 53, 4.1))
canvas.addEventListener("mousemove", function(evt) {
ctx.clearRect(0, 0, canvas.width, canvas.height)
shapes.forEach((s) => s.draw(evt))
}
)
shapes.forEach((s) => s.draw({ clientX: 0, clientY: 0 }))
<canvas id="canvas" width="400" height="180" />
I hardcoded your constants to keep this example as small as possible
Also my class Hexagon takes the radius and angle as parameters that way we can have different Hexagons see: constructor(x, y, r, a)

Handle mouse hovering image inside of canvas isometric grid

I got a isometric grid in html canvas.
I am trying to handle the mouse hover the buildings.
Some buildings will have different heights.
As you can see in the image below I am hovering a tile, the mouse pointer is inside the blueish tile.
The problem is when the mouse pointer is off the ground tile, or in the middle of the building image, the highlighted tile goes off.
Need a way to click on each individual building, how can this be resolved?
Main basic functions:
let applied_map = ref([]); // tileMap
let tile_images = ref([]); // this will contain loaded IMAGES for canvas to consume from
let tile_height = ref(50);
let tile_width = ref(100);
const renderTiles = (x, y) => {
let tileWidth = tile_width.value;
let tileHeight = tile_height.value;
let tile_half_width = tileWidth / 2;
let tile_half_height = tileHeight / 2;
for (let tileX = 0; tileX < gridSize.value; ++tileX) {
for (let tileY = 0; tileY < gridSize.value; ++tileY) {
let renderX = x + (tileX - tileY) * tile_half_width;
let renderY = y + (tileX + tileY) * tile_half_height;
let tile = applied_map.value[tileY * gridSize.value + tileX];
renderTileBackground(renderX, renderY + 50, tileWidth, tileHeight);
if (tile !== -1) {
if (tile_images.value.length) {
renderTexturedTile(
tile_images.value[tile].img,
renderX,
renderY + 40,
tileHeight
);
}
}
}
}
if (
hoverTileX.value >= 0 &&
hoverTileY.value >= 0 &&
hoverTileX.value < gridSize.value &&
hoverTileY.value < gridSize.value
) {
let renderX = x + (hoverTileX.value - hoverTileY.value) * tile_half_width;
let renderY = y + (hoverTileX.value + hoverTileY.value) * tile_half_height;
renderTileHover(renderX, renderY + 50, tileWidth, tileHeight);
}
};
const renderTileBackground = (x, y, width, height) => {
ctx.value.beginPath();
ctx.value.setLineDash([5, 5]);
ctx.value.strokeStyle = "black";
ctx.value.fillStyle = "rgba(25,34, 44,0.2)";
ctx.value.lineWidth = 1;
ctx.value.moveTo(x, y);
ctx.value.lineTo(x + width / 2, y - height / 2);
ctx.value.lineTo(x + width, y);
ctx.value.lineTo(x + width / 2, y + height / 2);
ctx.value.lineTo(x, y);
ctx.value.stroke();
ctx.value.fill();
};
const renderTexturedTile = (imgSrc, x, y, tileHeight) => {
let offsetY = tileHeight - imgSrc.height;
ctx.value.drawImage(imgSrc, x, y + offsetY);
};
const renderTileHover = (x, y, width, height) => {
ctx.value.beginPath();
ctx.value.setLineDash([]);
ctx.value.strokeStyle = "rgba(161, 153, 255, 0.8)";
ctx.value.fillStyle = "rgba(161, 153, 255, 0.4)";
ctx.value.lineWidth = 2;
ctx.value.moveTo(x, y);
ctx.value.lineTo(x + width / 2, y - height / 2);
ctx.value.lineTo(x + width, y);
ctx.value.lineTo(x + width / 2, y + height / 2);
ctx.value.lineTo(x, y);
ctx.value.stroke();
ctx.value.fill();
};
Updates after answer below
Based on Helder Sepulveda answer I created a function drawCube.
And added to my click function and to the renderTiles. So on click and frame update it creates a cube with 3 faces,and its placed on same position as the building and stores the Path on a global variable, the cube follows the isometric position.
In the drawCube, there is a condition where i need to hide the right face from the cube. Hide if there's a building on the next tile. So if you hover the building it wont trigger the last building on.
//...some code click function
//...
if (tile_images.value[tileIndex] !== undefined) {
drawCube(
hoverTileX.value + tile_height.value,
hoverTileY.value +
Number(tile_images.value[tileIndex].img.height / 2) -
10,
tile_height.value, // grow X pos to left
tile_height.value, // grow X pos to right,
Number(tile_images.value[tileIndex].img.height / 2), // height,
ctx.value,
{
tile_index: tileIndex - 1 < 0 ? 0 : tileIndex - 1,
}
);
}
This is the drawCube
const drawCube = (x, y, wx, wy, h, the_ctx, options = {}) => {
// https://codepen.io/AshKyd/pen/JYXEpL
let path = new Path2D();
let hide_options = {
left_face: false,
right_face: false,
top_face: false,
};
if (options.hasOwnProperty("hide")) {
hide_options = Object.assign(hide_options, options.hide);
}
// left face
if (!hide_options.left_face) {
path.moveTo(x, y);
path.lineTo(x - wx, y - wx * 0.5);
path.lineTo(x - wx, y - h - wx * 0.5);
path.lineTo(x, y - h * 1);
}
// right;
if (
!hide_options.right_face &&
!coliders.value[options.tile_index].hide_right_face
) {
path.moveTo(x, y);
path.lineTo(x + wy, y - wy * 0.5);
path.lineTo(x + wy, y - h - wy * 0.5);
path.lineTo(x, y - h * 1);
}
//top
if (!hide_options.right_face) {
path.moveTo(x, y - h);
path.lineTo(x - wx, y - h - wx * 0.5);
path.lineTo(x - wx + wy, y - h - (wx * 0.5 + wy * 0.5));
path.lineTo(x + wy, y - h - wy * 0.5);
}
// the_ctx.beginPath();
let isONHover = the_ctx.isPointInPath(
path,
mousePosition.x - 10,
mousePosition.y - 10
);
the_ctx.fillStyle = null;
if (isONHover) {
// let indx = options.tile_pos.y * gridSize.value + options.tile_pos.x;
//this is the click on object event
if (isMouseDown.value) {
//Trigger
if (buildozer.value === true) {
coliders.value[options.tile_index] = -1;
applied_map.value[options.tile_index] = -1;
}
isMouseDown.value = false;
}
the_ctx.fillStyle = "green";
}
the_ctx.fill(path);
if (
coliders.value[options.tile_index] == -1 &&
applied_map.value[options.tile_index]
) {
coliders.value[options.tile_index] = path;
}
};

In a nutshell you need to be able to detect mouseover on more complex shapes ...
I recommend you to use Path2d:
https://developer.mozilla.org/en-US/docs/Web/API/Path2D
That way you can build any shape you like and then we have access to isPointInPath to detect if the mouse is over our shape.
https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/isPointInPath
Here is a small example:
class Shape {
constructor(x, y, width, height) {
this.path = new Path2D()
this.path.arc(x, y, 12, 0, 2 * Math.PI)
this.path.arc(x, y - 9, 8, 0, 1.5 * Math.PI)
this.path.lineTo(x + width / 2, y)
this.path.lineTo(x, y + height / 2)
this.path.lineTo(x - width / 2, y)
this.path.lineTo(x, y - height / 2)
this.path.lineTo(x + width / 2, y)
}
draw(ctx, pos) {
ctx.beginPath()
ctx.fillStyle = ctx.isPointInPath(this.path, pos.x, pos.y) ? "red" : "green"
ctx.fill(this.path)
}
}
function getMousePos(canvas, evt) {
var rect = canvas.getBoundingClientRect()
return {
x: evt.clientX - rect.left,
y: evt.clientY - rect.top
}
}
var canvas = document.getElementById("canvas")
var ctx = canvas.getContext("2d")
shapes = []
for (var i = 0; i < 4; i++) {
for (var j = 0; j < 4; j++) {
shapes.push(new Shape(50 + i * 40, 40 + j * 40, 40, 20))
}
}
canvas.addEventListener("mousemove", function(evt) {
ctx.clearRect(0, 0, canvas.width, canvas.height)
var mousePos = getMousePos(canvas, evt)
shapes.forEach((s) => {s.draw(ctx, mousePos)})
},
false
)
shapes.forEach((s) => {
s.draw(ctx, {x: 0, y: 0})
})
<canvas id="canvas" width="200" height="200"></canvas>
This example draws a "complex" shape (two arcs and a few lines) and the shape changes color to red when the mouse is hovering the shape

How to get an angle by using tangent in javascript?

The red circle is at a known angle of 130°, then I want to draw the navy line from the center to 130° using x and y of the red circle but it looks like I missed the calculation.
Currently, the angle of the Navy line is a reflection to the angle of the red line and if I add minus sign ➖ to *diffX * at line13, it'll work as expected but Why do I need to do that by myself, why can't the Calculations at line 10 and 13 figured out if x should be minus ➖ or plus.
I couldn't figure out where I was wrong..any help/suggestions are appreciated!
let ctx, W = innerWidth,
H = innerHeight;
// params for the red circle
let hypothenus = 100;
let knownAngle = (-130 * Math.PI) / 180;
let x = (W / 2) + Math.cos(knownAngle) * hypothenus;
let y = (H / 2) + Math.sin(knownAngle) * hypothenus;
// params for navy line
let diffX = x - (W / 2);
let diffY = (H / 2) - y;
let dist = Math.hypot(diffX, diffY); // pythagoras
let unknownAngle = -Math.atan2(diffY, diffX);
let newX = (W / 2) + Math.cos(unknownAngle) * dist;
let newY = (H / 2) + Math.sin(unknownAngle) * dist;
let angInDegree1 = ~~Math.abs(knownAngle * 180 / Math.PI);
let angInDegree2 = ~~Math.abs(unknownAngle * 180 / Math.PI) | 0;
const msg = document.getElementById("msg")
msg.innerHTML = `Hypothenus1: ${hypothenus}, angle: ${angInDegree1}<br>`;
msg.innerHTML +=`Hypothenus2: ${dist}, angle: ${angInDegree2}`;
// everything to be rendered to the screen
const update = () => {
if (ctx == null) return;
// drawing the red line
draw.line([W / 2, 0], [W / 2, H], 6, "red");
draw.line([0, H / 2], [W, H / 2], 6, "red");
// the red circle
draw.circle([x, y], 10, "red");
// draw line
draw.line([W / 2, H / 2], [newX, newY], 4, "navy");
}
// utility object for drawing
const draw = {
line(from, to, width, color) {
with(ctx) {
beginPath();
lineWidth = width;
strokeStyle = color;
moveTo(...from);
lineTo(...to);
stroke();
closePath();
}
},
circle(pos, radius, color) {
ctx.beginPath();
ctx.fillStyle = color;
ctx.arc(...pos, radius, 0, 2 * Math.PI);
ctx.fill();
ctx.closePath();
}
}
// init function
const init = () => {
ctx = document.querySelector("#cvs").getContext("2d");
W = ctx.canvas.width = innerWidth;
H = ctx.canvas.height = innerHeight;
update();
}
window.addEventListener("load", init);
<div id="msg"></div>
<canvas id="cvs"></canvas>

Seems you are using too much minuses.
At first, you define angle -130 degrees, close to -3Pi/4. Cosine and sine values for this angle are about -0.7, using hypothenus = 100, we get x =W/2-70, y = H/2-70
diffX = x - W/2 = -70
diffY = y - H/2 = -70
atan2(-70, -70) gives -2.3561 radians = -3/4*Pi = -135 degrees
When you change sign of diffY (note - diffY formula is wrong, not difX one!), you make reflection against OX axis, and change angle sign - that is why another minus before Math.atan2 is required
Corrected code:
let diffX = x - (W / 2);
let diffY = y - (H / 2);
let dist = Math.hypot(diffX, diffY); // pythagoras
let unknownAngle = Math.atan2(diffY, diffX);

given 2 center coordinates, how to find all Rectangle axes?

for a game I'm building, I need to draw a rectangle on two sides of a line made from two coordinates.
I have an image illustrating this "hard to ask" question.
given coordinates (-4,3) and (3, -4)
given that the width of the rectangle will be 4 (for example)
I need to find all (x1, y1), (x2, y2), (x3, y3), (x4, y4)
** I need to write this in Javascript eventually.
your help is much appreciated.

I've tried to solve this using javascript & canvas. The problem is that the coordinates in canvas are upside down, I suppose you already know this. Also since your rect would be extremely small, I've multiplied your numbers by 10.
const canvas = document.querySelector("canvas");
const ctx = canvas.getContext("2d");
let cw = canvas.width = 300,
cx = cw / 2;
let ch = canvas.height = 300,
cy = ch / 2;
const rad = Math.PI / 180;
ctx.translate(cx,cy)
//axis
ctx.strokeStyle = "#d9d9d9";
ctx.beginPath();
ctx.moveTo(-cx,0);
ctx.lineTo(cx,0);
ctx.moveTo(0,-cy);
ctx.lineTo(0,cy);
ctx.stroke();
// your data
let p1={x:-40,y:30};
let p2={x:30,y:-40};
// the angle of the initial line
let angle = Math.atan2(p2.y-p1.y, p2.x-p1.x);
// the center of the line
let c =
{ x: p1.x + (p2.x - p1.x)/2,
y: p1.y + (p2.y - p1.y)/2
}
let w = dist(p1, p2);//the width of the rect
let h = 60;//the height of the rect
// draw the initial line
line(p1,p2);
// draw the center as a red point
marker(c);
// calculate the opoints of the rect
function rectPoints(w,h){
let p1 = {
x : c.x -w/2,
y : c.y -h/2
}
let p2 = {
x : c.x + w/2,
y : c.y -h/2
}
let p3 = {
x : c.x + w/2,
y : c.y +h/2
}
let p4 = {
x : c.x -w/2,
y : c.y +h/2
}
// this rotate all the points relative to the center c
return [
rotate(p1,c, angle),
rotate(p2,c, angle),
rotate(p3,c, angle),
rotate(p4,c, angle)
]
}
// draw the rect
ctx.strokeStyle = "blue";
drawRect(rectPoints(w,h));
// some helpful functions
function line(p1,p2){
ctx.beginPath();
ctx.moveTo(p1.x,p1.y);
ctx.lineTo(p2.x,p2.y);
ctx.stroke();
}
function dist(p1, p2) {
let dx = p2.x - p1.x;
let dy = p2.y - p1.y;
return Math.sqrt(dx * dx + dy * dy);
}
function marker(p,color){
ctx.beginPath();
ctx.fillStyle = color || "red";
ctx.arc(p.x,p.y,4,0,2*Math.PI);
ctx.fill();
}
function rotate(p,c, angle){
let cos = Math.cos(angle);
let sin = Math.sin(angle);
return {
x: c.x + (p.x - c.x) * cos - (p.y - c.y) * sin,
y: c.y + (p.x - c.x) * sin + (p.y - c.y) * cos
}
}
function drawRect(points){
ctx.beginPath();
ctx.moveTo(points[0].x,points[0].y);
ctx.lineTo(points[1].x,points[1].y);
ctx.lineTo(points[2].x,points[2].y);
ctx.lineTo(points[3].x,points[3].y);
ctx.lineTo(points[0].x,points[0].y);
ctx.closePath();
ctx.stroke();
}
canvas{border:1px solid #d9d9d9}
<canvas></canvas>

Points A, B form vector
M.X = B.X - A.X
M.Y = B.Y - A.Y
Perpendicular vector
P.X = -M.Y
P.Y = M.X
Length of P:
Len = Math.sqrt(P.X*P.X + P.Y*P.Y)
Normalized (unit) perpendicular:
uP.X = P.X / Len
uP.Y = P.Y / Len
Points
X1 = A.X + uP.X * HalfWidth
Y1 = A.Y + uP.Y * HalfWidth
(X4, Y4) = (A.X - uP.X * HalfWidth, A.Y - uP.Y * HalfWidth)
and similar for points 2 and 3 around B

Position div at exact cursors location

I am using this color wheel picker, and I'm trying to add a div as the dragger instead of having it embedded in the canvas. I got it working thanks to these answers.
The problem is, the dragger is a bit off from the cursor. The obvious solution would be to just subtract from the draggers left and top position. Like this:
dragger.style.left = (currentX + radiusPlusOffset - 13) + 'px';
dragger.style.top = (currentY + radiusPlusOffset - 13) + 'px';
Another problem comes up when I subtract 13. If you drag the dragger all the way to the right or bottom, it doesn't go all the way. If you drag it all the way to the left or top, it goes passed the canvas's border.
Basically what I'm trying to achieve, is to have the dragger at the cursor pointers exact location, and the draggable shouldn't go passed the canvas's border. How can I achieve that?
JSFiddle
var b = document.body;
var c = document.getElementsByTagName('canvas')[0];
var a = c.getContext('2d');
var wrapper = document.getElementById('wrapper');
var dragger = document.createElement('div');
dragger.id = 'dragger';
wrapper.appendChild(dragger);
wrapper.insertBefore(dragger, c);
document.body.clientWidth; // fix bug in webkit: http://qfox.nl/weblog/218
(function() {
// Declare constants and variables to help with minification
// Some of these are inlined (with comments to the side with the actual equation)
var doc = document;
doc.c = doc.createElement;
b.a = b.appendChild;
var width = c.width = c.height = 400,
label = b.a(doc.c("p")),
input = b.a(doc.c("input")),
imageData = a.createImageData(width, width),
pixels = imageData.data,
oneHundred = input.value = input.max = 100,
circleOffset = 0,
diameter = width - circleOffset * 2,
radius = diameter / 2,
radiusPlusOffset = radius + circleOffset,
radiusSquared = radius * radius,
two55 = 255,
currentY = oneHundred,
currentX = -currentY,
wheelPixel = circleOffset * 4 * width + circleOffset * 4;
// Math helpers
var math = Math,
PI = math.PI,
PI2 = PI * 2,
sqrt = math.sqrt,
atan2 = math.atan2;
// Setup DOM properties
b.style.textAlign = "center";
label.style.font = "2em courier";
input.type = "range";
// Load color wheel data into memory.
for (y = input.min = 0; y < width; y++) {
for (x = 0; x < width; x++) {
var rx = x - radius,
ry = y - radius,
d = rx * rx + ry * ry,
rgb = hsvToRgb(
(atan2(ry, rx) + PI) / PI2, // Hue
sqrt(d) / radius, // Saturation
1 // Value
);
// Print current color, but hide if outside the area of the circle
pixels[wheelPixel++] = rgb[0];
pixels[wheelPixel++] = rgb[1];
pixels[wheelPixel++] = rgb[2];
pixels[wheelPixel++] = d > radiusSquared ? 0 : two55;
}
}
a.putImageData(imageData, 0, 0);
// Bind Event Handlers
input.onchange = redraw;
dragger.onmousedown = c.onmousedown = doc.onmouseup = function(e) {
// Unbind mousemove if this is a mouseup event, or bind mousemove if this a mousedown event
doc.onmousemove = /p/.test(e.type) ? 0 : (redraw(e), redraw);
}
// Handle manual calls + mousemove event handler + input change event handler all in one place.
function redraw(e) {
// Only process an actual change if it is triggered by the mousemove or mousedown event.
// Otherwise e.pageX will be undefined, which will cause the result to be NaN, so it will fallback to the current value
currentX = e.pageX - c.offsetLeft - radiusPlusOffset || currentX;
currentY = e.pageY - c.offsetTop - radiusPlusOffset || currentY;
// Scope these locally so the compiler will minify the names. Will manually remove the 'var' keyword in the minified version.
var theta = atan2(currentY, currentX),
d = currentX * currentX + currentY * currentY;
// If the x/y is not in the circle, find angle between center and mouse point:
// Draw a line at that angle from center with the distance of radius
// Use that point on the circumference as the draggable location
if (d > radiusSquared) {
currentX = radius * math.cos(theta);
currentY = radius * math.sin(theta);
theta = atan2(currentY, currentX);
d = currentX * currentX + currentY * currentY;
}
label.textContent = b.style.background = hsvToRgb(
(theta + PI) / PI2, // Current hue (how many degrees along the circle)
sqrt(d) / radius, // Current saturation (how close to the middle)
input.value / oneHundred // Current value (input type="range" slider value)
)[3];
dragger.style.left = (~~currentX + radiusPlusOffset - 13) + 'px';
dragger.style.top = (~~currentY + radiusPlusOffset - 13) + 'px';
// Reset to color wheel and draw a spot on the current location.
// Draw the current spot.
// I have tried a rectangle, circle, and heart shape.
/*
// Rectangle:
a.fillStyle = '#000';
a.fillRect(currentX+radiusPlusOffset,currentY+radiusPlusOffset, 6, 6);
*/
// Circle:
/*a.beginPath();
a.strokeStyle = 'white';
a.arc(~~currentX+radiusPlusOffset,~~currentY+radiusPlusOffset, 4, 0, PI2);
a.stroke();*/
// Heart:
//a.font = "1em arial";
//a.fillText("♥", currentX + radiusPlusOffset - 4, currentY + radiusPlusOffset + 4);
}
// Created a shorter version of the HSV to RGB conversion function in TinyColor
// https://github.com/bgrins/TinyColor/blob/master/tinycolor.js
function hsvToRgb(h, s, v) {
h *= 6;
var i = ~~h,
f = h - i,
p = v * (1 - s),
q = v * (1 - f * s),
t = v * (1 - (1 - f) * s),
mod = i % 6,
r = [v, q, p, p, t, v][mod] * two55,
g = [t, v, v, q, p, p][mod] * two55,
b = [p, p, t, v, v, q][mod] * two55;
return [r, g, b, "rgb(" + ~~r + "," + ~~g + "," + ~~b + ")"];
}
// Kick everything off
redraw(0);
/*
// Just an idea I had to kick everything off with some changing colors…
// Probably no way to squeeze this into 1k, but it could probably be a lot smaller than this:
currentX = currentY = 1;
var interval = setInterval(function() {
currentX--;
currentY*=1.05;
redraw(0)
}, 7);
setTimeout(function() {
clearInterval(interval)
}, 700)
*/
})();
#c {
border: 7px solid black;
border-radius: 50%;
}
#wrapper {
width: 400px;
height: 400px;
position: relative;
cursor: pointer;
}
#wrapper:active {
//cursor: none;
}
#dragger {
width: 8px;
height: 8px;
border-radius: 50%;
display: block;
position: absolute;
border: 2px solid black;
}
<div id='wrapper'>
<canvas id="c"></canvas>
</div>

You're just subtracting in the wrong place.
Instead of subtracting from the elements position, subtract directly from the mouse pointers position.
This code actually moves the element, offsetting it relative to the pointer, and making it appear to be outside the borders of the color picker
dragger.style.left = (~~currentX + radiusPlusOffset - 13) + 'px';
dragger.style.top = (~~currentY + radiusPlusOffset - 13) + 'px';
... which is not what you really want, you want the calculated numbers for the pointer to be exactly center of the dragger element, so you should extract from the pointers position instead, that way the limits of the dragger isn't affected, and it stays within the borders of the color picker
currentX = e.pageX - c.offsetLeft - radiusPlusOffset -13 || currentX;
currentY = e.pageY - c.offsetTop - radiusPlusOffset -13 || currentY;
FIDDLE