How to shade the circle in canvas - html

Question

I am working with HTML5 with canvas. I already draw a 2D circle.Now i want to shade the circle with a color.but the shading look like a 3D circle.Is this possible with canvas?.Thank you.

Answer 1

Fake smoke and mirrors
To fake a light on a sphere. I am guessing it is a sphere as you say circle and you could mean a donut. This technique will work for a donut as well.
So to lighting.
Phong Shading
The most basic lighting model is Phong (from memory). It uses the angle between the incoming light ray and the surface normal (a line going out from the surface at 90 deg). The amount of reflected light is the cosine of that angle time the light intensity.
Spheres a easy
As the sphere is symmetrical this allows us to use a radial gradient to apply the value for each pixel on the sphere and for a sphere with the light directly overhead this produces a perfect phong shaded sphere with very little effort.
The code that does that. x,y are the center of the sphere and r is the radius. The angle between the light and the surface normal is easy to calculate as you move out from the center of the sphere. It starts at zero and ends at Math.PI/2 (90deg). So the reflected value is the cosine of that angle.
var grd = ctx.createRadialGradient(x,y,0,x,y,r);
var step = (Math.PI/2)/r;
for(var i = 0; i < (Math.PI/2); i += step){
var c = "" + Math.floor(Math.max(0,255 * Math.abs(Math.cos(i)));
grd.addColorStop(i/(Math.PI/2),"rgba("+c+","+c+","+c+","1)");
}
That code creates a gradient to fit the circle.
Mod for Homer food
To do for a donut you need to modify i. The donut has an inner and outer radius (r1, r2) so inside the for loop modify i
var ii = (i/(Math.PI/2)); // normalise i
ii *= r2; // scale to outer edge
ii = ((r1+r2)/2)-ii; // get distance from center line
ii = ii / ((r2-r1)/2); // normalise to half the width;
ii = ii * Math.PI * (1/2); // scale to get the surface norm on the donut.
// use ii as the surface normal to calculate refelected light
var c = "" + Math.floor(Math.max(0,255 * Math.abs(Math.cos(ii)));
Phong Shading Sucks
By phong shading sucks big time and will not do. This also does not allow for lights that are off center or even partly behind the sphere.
We need to add the ability for off centered light. Luck has it that the radial gradients can be offset
var grd = ctx.createRadialGradient(x,y,0,x,y,r);
The first 3 numbers are the start circle of the gradient and can be positioned anywhere. The problem is that when we move the start location the phong shading model falls apart. To fix that there is a little smoke and mirrors stuff that can make the eye believe what the brain wants.
We adjust the fall off, the brightness, the spread, and the angle for each colour stop on the radial gradient depending on how far the light is from the center.
Specular highlights
This improves it a bit but still not the best. Another important component of lighting is specular reflections (the highlight). This is dependent on the angle between the reflected light and the eye. As we do not want to do all that (javascript is slow) we will cludge it via a slight modification of the phong shading. We simply multiply the surface normal by a value greater than 1. Though not perfect it works well.
Surface properties and environment
Next light is coloured, the sphere has reflective qualities that depend on frequency and there is ambient light as well. We don't want to model all this stuff so we need a way to fake it.
This can be done via compositing (Used for almost all 3D movie production). We build up the lighting one layer at a time. The 2D API provides compositing operations for us so we can create several gradients and layer them.
There is a lot more math involved but I have tried to keep it as simple as possible.
A demo
The following demo does a real time shading of a sphere (will work on all radially symmetrical objects) Apart from some setup code for canvas and mouse the demo has two parts the main loop does the compositing by layering the lights and the function createGradient creates the gradient.
The lights used can be found in the object lights and have various properties to control the layer. The first layer should use comp = source-in and lum = 1 or you will end up with the background showing through. All other layer lights can be what every you want.
The flag spec tells the shader that the light is specular and must include the specPower > 1 as I do not vet its existence.
The colours of the light is in the array col and represent Red, green and blue. The values can be greater the 256 and less than 0 as light in the natural world has a huge dynamic range and some effect need you to ramp up the incoming light way above the 255 limit of the RGB pixel.
I add a final "multiply" to the layered result. This is the magic touch in the smoke and mirror method.
If you like the code play with the values and layers. Move the mouse to change the light source location.
This is not real lighting it is fake, but who cares as long as it looks OK. lol
UPDATE
Found a bug so fixed it and while I was here, changed the code to randomize the lights when you click the left mouse button. This is so you can see the range of lighting that can be achieved when using the ctx.globalCompositeOperation in combination with gradients.
var demo = function(){
/** fullScreenCanvas.js begin **/
var canvas = (function(){
var canvas = document.getElementById("canv");
if(canvas !== null){
document.body.removeChild(canvas);
}
// creates a blank image with 2d context
canvas = document.createElement("canvas");
canvas.id = "canv";
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
canvas.style.position = "absolute";
canvas.style.top = "0px";
canvas.style.left = "0px";
canvas.style.zIndex = 1000;
canvas.ctx = canvas.getContext("2d");
document.body.appendChild(canvas);
return canvas;
})();
var ctx = canvas.ctx;
/** fullScreenCanvas.js end **/
/** MouseFull.js begin **/
if(typeof mouse !== "undefined"){ // if the mouse exists
if( mouse.removeMouse !== undefined){
mouse.removeMouse(); // remove prviouse events
}
}else{
var mouse;
}
var canvasMouseCallBack = undefined; // if needed
mouse = (function(){
var mouse = {
x : 0, y : 0, w : 0, alt : false, shift : false, ctrl : false,
interfaceId : 0, buttonLastRaw : 0, buttonRaw : 0,
over : false, // mouse is over the element
bm : [1, 2, 4, 6, 5, 3], // masks for setting and clearing button raw bits;
getInterfaceId : function () { return this.interfaceId++; }, // For UI functions
startMouse:undefined,
mouseEvents : "mousemove,mousedown,mouseup,mouseout,mouseover,mousewheel,DOMMouseScroll".split(",")
};
function mouseMove(e) {
var t = e.type, m = mouse;
m.x = e.offsetX; m.y = e.offsetY;
if (m.x === undefined) { m.x = e.clientX; m.y = e.clientY; }
m.alt = e.altKey;m.shift = e.shiftKey;m.ctrl = e.ctrlKey;
if (t === "mousedown") { m.buttonRaw |= m.bm[e.which-1];
} else if (t === "mouseup") { m.buttonRaw &= m.bm[e.which + 2];
} else if (t === "mouseout") { m.buttonRaw = 0; m.over = false;
} else if (t === "mouseover") { m.over = true;
} else if (t === "mousewheel") { m.w = e.wheelDelta;
} else if (t === "DOMMouseScroll") { m.w = -e.detail;}
if (canvasMouseCallBack) { canvasMouseCallBack(mouse); }
e.preventDefault();
}
function startMouse(element){
if(element === undefined){
element = document;
}
mouse.element = element;
mouse.mouseEvents.forEach(
function(n){
element.addEventListener(n, mouseMove);
}
);
element.addEventListener("contextmenu", function (e) {e.preventDefault();}, false);
}
mouse.removeMouse = function(){
if(mouse.element !== undefined){
mouse.mouseEvents.forEach(
function(n){
mouse.element.removeEventListener(n, mouseMove);
}
);
canvasMouseCallBack = undefined;
}
}
mouse.mouseStart = startMouse;
return mouse;
})();
if(typeof canvas !== "undefined"){
mouse.mouseStart(canvas);
}else{
mouse.mouseStart();
}
/** MouseFull.js end **/
// draws the circle
function drawCircle(c){
ctx.beginPath();
ctx.arc(c.x,c.y,c.r,0,Math.PI*2);
ctx.fill();
}
function drawCircle1(c){
ctx.beginPath();
var x = c.x;
var y = c.y;
var r = c.r * 0.95;
ctx.moveTo(x,y - r);
ctx.quadraticCurveTo(x + r * 0.8, y - r , x + r *1, y - r / 10);
ctx.quadraticCurveTo(x + r , y + r/3 , x , y + r/3);
ctx.quadraticCurveTo(x - r , y + r/3 , x - r , y - r /10 );
ctx.quadraticCurveTo(x - r * 0.8, y - r , x , y- r );
ctx.fill();
}
function drawShadowShadow(circle,light){
var x = light.x; // get the light position as we will modify it
var y = light.y;
var r = circle.r * 1.1;
var vX = x - circle.x; // get the vector to the light source
var vY = y - circle.y;
var dist = -Math.sqrt(vX*vX+vY*vY)*0.3;
var dir = Math.atan2(vY,vX);
lx = Math.cos(dir) * dist + circle.x; // light canb not go past radius
ly = Math.sin(dir) * dist + circle.y;
var grd = ctx.createRadialGradient(lx,ly,r * 1/4 ,lx,ly,r);
grd.addColorStop(0,"rgba(0,0,0,1)");
grd.addColorStop(1,"rgba(0,0,0,0)");
ctx.fillStyle = grd;
drawCircle({x:lx,y:ly,r:r})
}
// 2D light simulation. This is just an approximation and does not match real world stuff
// based on Phong shading.
// x,y,r descript the imagined sphere
// light is the light source
// ambient is the ambient lighting
// amount is the amount of this layers effect has on the finnal result
function createGradient(circle,light,ambient,amount){
var r,g,b; // colour channels
var x = circle.x; // get lazy coder values
var y = circle.y;
var r = circle.r;
var lx = light.x; // get the light position as we will modify it
var ly = light.y;
var vX = light.x - x; // get the vector to the light source
var vY = light.y - y;
// get the distance to the light source
var dist = Math.sqrt(vX*vX+vY*vY);
// id the light is a specular source then move it to half its position away
dist *= light.spec ? 0.5 : 1;
// get the direction of the light source.
var dir = Math.atan2(vY,vX);
// fix light position
lx = Math.cos(dir)*dist+x; // light canb not go past radius
ly = Math.sin(dir)*dist+y;
// add some dimming so that the light does not wash out.
dim = 1 - Math.min(1,(dist / (r*4)));
// add a bit of pretend rotation on the z axis. This will bring in a little backlighting
var lightRotate = (1-dim) * (Math.PI/2);
// spread the light a bit when near the edges. Reduce a bit for spec light
var spread = Math.sin(lightRotate) * r * (light.spec ? 0.5 : 1);
// create a gradient
var grd = ctx.createRadialGradient(lx,ly,spread,x,y,r + dist);
// use the radius to workout what step will cover a pixel (approx)
var step = (Math.PI/2)/r;
// for each pixel going out on the radius add the caclualte light value
for(var i = 0; i < (Math.PI/2); i += step){
if(light.spec){
// fake spec light reduces dim fall off
// light reflected has sharper falloff
// do not include back light via Math.abs
r = Math.max(0,light.col[0] * Math.cos((i + lightRotate)*light.specPower) * 1-(dim * (1/3)) );
g = Math.max(0,light.col[1] * Math.cos((i + lightRotate)*light.specPower) * 1-(dim * (1/3)) );
b = Math.max(0,light.col[2] * Math.cos((i + lightRotate)*light.specPower) * 1-(dim * (1/3)) );
}else{
// light value is the source lum * the cos of the angle to the light
// Using the abs value of the refelected light to give fake back light.
// add a bit of rotation with (lightRotate)
// dim to stop washing out
// then clamp so does not go below zero
r = Math.max(0,light.col[0] * Math.abs(Math.cos(i + lightRotate)) * dim );
g = Math.max(0,light.col[1] * Math.abs(Math.cos(i + lightRotate)) * dim );
b = Math.max(0,light.col[2] * Math.abs(Math.cos(i + lightRotate)) * dim );
}
// add ambient light
if(light.useAmbient){
r += ambient[0];
g += ambient[1];
b += ambient[2];
}
// add the colour stop with the amount of the effect we want.
grd.addColorStop(i/(Math.PI/2),"rgba("+Math.floor(r)+","+Math.floor(g)+","+Math.floor(b)+","+amount+")");
}
//return the gradient;
return grd;
}
// define the circles
var circles = [
{
x: canvas.width * (1/2),
y: canvas.height * (1/2),
r: canvas.width * (1/8),
}
]
function R(val){
return val * Math.random();
}
var lights;
function getLights(){
return {
ambient : [10,30,50],
sources : [
{
x: 0, // position of light
y: 0,
col : [R(255),R(255),R(255)], // RGB intensities can be any value
lum : 1, // total lumanance for this light
comp : "source-over", // composite opperation
spec : false, // if true then use a pretend specular falloff
draw : drawCircle,
useAmbient : true,
},{ // this light is for a little accent and is at 180 degree from the light
x: 0,
y: 0,
col : [R(255),R(255),R(255)],
lum : R(1),
comp : "lighter",
spec : true, // if true then you MUST inclue spec power
specPower : R(3.2),
draw : drawCircle,
useAmbient : false,
},{
x: canvas.width,
y: canvas.height,
col : [R(1255),R(1255),R(1255)],
lum : R(0.5),
comp : "lighter",
spec : false,
draw : drawCircle,
useAmbient : false,
},{
x: canvas.width/2,
y: canvas.height/2 + canvas.width /4,
col : [R(155),R(155),R(155)],
lum : R(1),
comp : "lighter",
spec : true, // if true then you MUST inclue spec power
specPower : 2.32,
draw : drawCircle,
useAmbient : false,
},{
x: canvas.width/3,
y: canvas.height/3,
col : [R(1255),R(1255),R(1255)],
lum : R(0.2),
comp : "multiply",
spec : false,
draw : drawCircle,
useAmbient : false,
},{
x: canvas.width/2,
y: -100,
col : [R(2255),R(2555),R(2255)],
lum : R(0.3),
comp : "lighter",
spec : false,
draw : drawCircle1,
useAmbient : false,
}
]
}
}
lights = getLights();
/** FrameUpdate.js begin **/
var w = canvas.width;
var h = canvas.height;
var cw = w / 2;
var ch = h / 2;
ctx.font = "20px Arial";
ctx.textAlign = "center";
function update(){
ctx.setTransform(1,0,0,1,0,0);
ctx.fillStyle = "#A74"
ctx.fillRect(0,0,w,h);
ctx.fillStyle = "black";
ctx.fillText("Left click to change lights", canvas.width / 2, 20)
// set the moving light source to that of the mouse
if(mouse.buttonRaw === 1){
mouse.buttonRaw = 0;
lights = getLights();
}
lights.sources[0].x = mouse.x;
lights.sources[0].y = mouse.y;
if(lights.sources.length > 1){
lights.sources[1].x = mouse.x;
lights.sources[1].y = mouse.y;
}
drawShadowShadow(circles[0],lights.sources[0])
//do each sphere
for(var i = 0; i < circles.length; i ++){
// for each sphere do the each light
var cir = circles[i];
for(var j = 0; j < lights.sources.length; j ++){
var light = lights.sources[j];
ctx.fillStyle = createGradient(cir,light,lights.ambient,light.lum);
ctx.globalCompositeOperation = light.comp;
light.draw(circles[i]);
}
}
ctx.globalCompositeOperation = "source-over";
if(!STOP && (mouse.buttonRaw & 4)!== 4){
requestAnimationFrame(update);
}else{
if(typeof log === "function" ){
log("DONE!")
}
STOP = false;
var can = document.getElementById("canv");
if(can !== null){
document.body.removeChild(can);
}
}
}
if(typeof clearLog === "function" ){
clearLog();
}
update();
}
var STOP = false; // flag to tell demo app to stop
function resizeEvent(){
var waitForStopped = function(){
if(!STOP){ // wait for stop to return to false
demo();
return;
}
setTimeout(waitForStopped,200);
}
STOP = true;
setTimeout(waitForStopped,100);
}
window.addEventListener("resize",resizeEvent);
demo();
/** FrameUpdate.js end **/

Answer 2

As #danday74 says, you can use a gradient to add depth to your circle.
You can also use shadowing to add depth to your circle.
Here's a proof-of-concept illustrating a 3d donut:
I leave it to you to design your desired circle
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var PI=Math.PI;
drawShadow(150,150,120,50);
function drawShadow(cx,cy,r,strokewidth){
ctx.save();
ctx.strokeStyle='white';
ctx.lineWidth=5;
ctx.shadowColor='black';
ctx.shadowBlur=15;
//
ctx.beginPath();
ctx.arc(cx,cy,r-5,0,PI*2);
ctx.clip();
//
ctx.beginPath();
ctx.arc(cx,cy,r,0,PI*2);
ctx.stroke();
//
ctx.beginPath();
ctx.arc(cx,cy,r-strokewidth,0,PI*2);
ctx.stroke();
ctx.shadowColor='rgba(0,0,0,0)';
//
ctx.beginPath();
ctx.arc(cx,cy,r-strokewidth,0,PI*2);
ctx.fillStyle='white'
ctx.fill();
//
ctx.restore();
}
body{ background-color: white; }
canvas{border:1px solid red; margin:0 auto; }
<canvas id="canvas" width=300 height=300></canvas>

Answer 3

Various thoughts which you can investigate ...
1 use an image as the texture for the circle
2 use a gradient to fill the circle, probably a radial gradient
3 consider using an image mask, a black / white mask which defines transparency ( prob not the right solution here )