Issues arise when utilizing the fill() function on a canvas due to its inconsistent and unpredictable behavior

I've been working on learning how to draw and fill different shapes using canvas and JavaScript. However, I've been having trouble getting my shapes to fill correctly. In my HTML document, all I have is a simple line:

<canvas id="canvas1" width="500" height="500"></canvas>

My JavaScript file consists of the following code:

function draw() {

    var canvas1 = document.getElementById('canvas1');
    
    if(canvas1.getContext) {
        var ctx = canvas1.getContext('2d');
        var gradient = ctx.createLinearGradient(0, 0, 50, 0);
        gradient.addColorStop(0, "blue");
        gradient.addColorStop(1, "white");
        
        ctx.beginPath();
        ctx.moveTo(25,25);
        ctx.lineTo(100, 25);
        ctx.stroke();
        
        ctx.moveTo(25, 50);
        ctx.bezierCurveTo(25, 50, 50, 80, 75, 60)
        ctx.fillStyle = "black";
        ctx.fill();
        
        ctx.beginPath();
        ctx.moveTo(75, 100);
        ctx.arc(50, 100, 25, 0, Math.PI*2, true);
        ctx.fillStyle = "black";
        ctx.fill();
                        
        ctx.beginPath();                        
        ctx.fillStyle = gradient;
        ctx.arc(75, 150, 25, 0, Math.PI*2, true);
        ctx.fill();
        
    }
}

However, despite following guides and trying various solutions, the result I'm seeing is not what I expected:

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

It's frustrating because when I change the color of my second circle, it fills perfectly fine. Furthermore, removing the last "ctx.beginPath();" results in my first circle being painted with a gradient. Yet, no matter what I try, I can't seem to replicate this bug for my second circle by rearranging the code or anything else.

Answer №1

Gradients are specified with a fixed position, so if you draw a circle outside the defined gradient area, it will appear transparent rather than filled.

You do not have to close the path manually as the fill() method will automatically close it for you. Just ensure that the gradient coordinates cover the desired fill area.

Instead of recalculating each time you need to fill an arc, you can create a generic function that takes in position and colors for filling:

Check out a demo here

/**
 * Fills a circle with a two-color gradient.
 * @param {Number} cx - center X
 * @param {Number} cy - center Y
 * @param {Number} radius - radius
 * @param {String} col1 - start color as CSS color string
 * @param {String} col2 - end color as CSS color string
 * @param {Boolean} [horiz=false] - Set true for horizontal gradient
*/
function fillCircle(cx, cy, radius, col1, col2, horiz) {

    var x = cx - radius,
        y = cy - radius,
        d = radius * 2,
        gradient;

    if (horiz) {
        gradient = ctx.createLinearGradient(x, 0, x+d, d);
    }
    else {
        gradient = ctx.createLinearGradient(0, y, 0, y+d);
    }

    gradient.addColorStop(0, col1);
    gradient.addColorStop(1, col2);

    ctx.fillStyle = gradient;
    ctx.beginPath();
    ctx.arc(cx, cy, radius, 0, 2*Math.PI);
    ctx.fill();
}

Simply use it like this:

fillCircle(200, 200, 70, 'yellow', 'red');

The optional flag at the end allows for a horizontal gradient if set to true.

Answer №2

Remember to use ctx.closePath(); after completing each individual shape or line.

    ctx.beginPath();
    ctx.moveTo(25, 50);
    ctx.bezierCurveTo(25, 50, 50, 80, 75, 60)
    ctx.strokeStyle = "black";
    ctx.stroke();
    ctx.closePath();

To ensure the gradient aligns correctly with your shape on the canvas, adjust the coordinates accordingly. In this case, start the gradient at 25,50 instead of 0,0.

 var gradient = ctx.createLinearGradient(25, 50, 75, 60);

For more details and an example, visit: http://jsfiddle.net/bC75t/1/

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