Metta Metta - 5 months ago 16
jQuery Question

Ring-shaped process spinner with fading gradient effect around the ring

I want to create a ring-shaped process spinner with CSS3 or JavaScript, similar to the loading progress spinner in Android.

The spinner should rotate continuously and be filled with a solid colour that fades out along the rim (i.e. a conical gradient) as in this picture:

A thin ring on a white background with the colour fading counter-clockwise from fully opaque cyan to fully transparent.

How can I achieve this?

Answer

This would be trivially easy if only CSS or SVG had conical gradients! Instead, we can approximate the effect by slicing up the gradient and covering the seams somehow.

Below you will find two solutions. The first solution uses an embedded SVG image; the second uses multiple CSS gradients and pseudo-elements.

Both start with a single div with a keyframe animation applied to make it rotate:

HTML:

<div class="spinner"></div>

CSS:

@keyframes rotate {
    from { transform: rotate(0deg);   }
    to   { transform: rotate(360deg); }
}

.spinner {
    animation: rotate 1s linear infinite;
    height: 200px;
    width: 200px;
}

You can use a progress element if you prefer, but you will find it a pain to style. Also note that unless you're using something like prefixfree.js, you'll need to add the vendor-prefixed versions of the @keyframes at-rule and the transform and animation properties.


The SVG solution

See the SVG demo on Dabblet.

@keyframes rotate {
    from { transform: rotate(0deg);   }
    to   { transform: rotate(360deg); }
}

.spinner {
    animation: rotate 1s linear infinite;
    background: url('data:image/svg+xml;base64,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') no-repeat;
    height: 200px;
    width: 200px;
}

Tested and working in IE 10, Chrome and Firefox.

Caveats

Changing the inner or outer radius of the ring is more painful than you might imagine, as it would require editing the clip path values. It's outside the scope of this answer to explain how to calculate it, but suffice to say it took a bit of geometry. I'll try to put a generator on GitHub if I get time.

How the SVG version works

That big blob of gibberish is just a Base64 encoded SVG image. Run it through a Base64 decoder and you'll see the original SVG image.

Here's the full image nicely indented and commented so you can see exactly how it works:

<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0,0 200,200">
    <defs>

        <!-- Ring shape centred on 100, 100 with inner radius 90px, outer
             radius 100px and a 12 degree gap at 348. -->
        <clipPath id="ring">
            <path d="M 200, 100
                     A 100, 100, 0, 1, 1, 197.81, 79.21
                     L 188.03, 81.29
                     A 90, 90, 0, 1, 0, 190, 100 z"/>
        </clipPath>

        <!-- Very simple Gaussian blur, used to visually merge sectors. -->
        <filter id="blur" x="0" y="0">
            <feGaussianBlur in="SourceGraphic" stdDeviation="3" />
        </filter>

        <!-- A 12 degree sector extending to 150px. -->
        <path id="p" d="M 250, 100
                        A 150, 150, 0, 0, 1, 246.72, 131.19
                        L 100, 100
                        A 0, 0, 0, 0, 0, 100, 100 z" fill="cyan"/>
    </defs>

    <!-- Clip the blurred sectors to the ring shape. -->
    <g clip-path="url(#ring)">

        <!-- Blur the sectors together to make a smooth shape and rotate
             them anti-clockwise by 6 degrees to hide the seam where the
             fully opaque sector blurs with the fully transparent one. -->
        <g filter="url(#blur)" transform="rotate(-6 100 100)">

            <!-- Each successive sector increases in opacity and is rotated
                 by a further 12 degrees. -->
            <use xlink:href="#p" fill-opacity="0"    transform="rotate(  0 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.03" transform="rotate( 12 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.07" transform="rotate( 24 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.1"  transform="rotate( 36 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.14" transform="rotate( 48 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.17" transform="rotate( 60 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.2"  transform="rotate( 72 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.24" transform="rotate( 84 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.28" transform="rotate( 96 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.31" transform="rotate(108 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.34" transform="rotate(120 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.38" transform="rotate(132 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.41" transform="rotate(144 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.45" transform="rotate(156 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.48" transform="rotate(168 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.52" transform="rotate(180 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.55" transform="rotate(192 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.59" transform="rotate(204 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.62" transform="rotate(216 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.66" transform="rotate(228 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.69" transform="rotate(240 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.7"  transform="rotate(252 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.72" transform="rotate(264 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.76" transform="rotate(276 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.79" transform="rotate(288 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.83" transform="rotate(300 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.86" transform="rotate(312 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.93" transform="rotate(324 100 100)"/>
            <use xlink:href="#p" fill-opacity="0.97" transform="rotate(336 100 100)"/>
            <use xlink:href="#p" fill-opacity="1"    transform="rotate(348 100 100)"/>
        </g>
    </g>
</svg>

This is minified, Base64 encoded and used as an inline CSS background image. You may also serve it as a separate file if you prefer. Technically, it should be possible to embed the image without the Base64 encoding, but right now that only works in Chrome.


The pure CSS solution

This solution uses separate linear gradients in each quadrant and relies on visual similarity to cover up the seams. The ring shape is formed using pseudo-elements.

See the pure CSS demo on Dabblet.

@keyframes rotate {
    from { transform: rotate(0deg);   }
    to   { transform: rotate(360deg); }
}

.spinner {
    animation: rotate 1s linear infinite;
    background: cyan;
    border-radius: 50%;
    height: 200px;
    width: 200px;
    position: relative;
}

.spinner:before,
.spinner:after {
    content: '';
    position: absolute;
}

.spinner:before {
    border-radius: 50%;
    background:
        linear-gradient(0deg,   hsla(0, 0%, 100%, 1  ) 50%, hsla(0, 0%, 100%, 0.9) 100%)   0%   0%,
        linear-gradient(90deg,  hsla(0, 0%, 100%, 0.9)  0%, hsla(0, 0%, 100%, 0.6) 100%) 100%   0%,
        linear-gradient(180deg, hsla(0, 0%, 100%, 0.6)  0%, hsla(0, 0%, 100%, 0.3) 100%) 100% 100%,
        linear-gradient(360deg, hsla(0, 0%, 100%, 0.3)  0%, hsla(0, 0%, 100%, 0  ) 100%)   0% 100%
    ;
    background-repeat: no-repeat;
    background-size: 50% 50%;
    top: -1px;
    bottom: -1px;
    left: -1px;
    right: -1px;
}

.spinner:after {
    background: white;
    border-radius: 50%;
    top: 3%;
    bottom: 3%;
    left: 3%;
    right: 3%;
}

Tested and working in IE 10, Chrome and Firefox.

Caveats

Unlike the SVG solution, this only works against a solid background colour. It also requires modification in several places if you want to change that colour, which is a pain.

How the pure CSS version works

  1. To start with, the spinner is styled as a circle with a uniform background colour. This will be the colour of the spinning gradient.

    .spinner {
        background: cyan;
        border-radius: 50%;
        /* ... */
    }
    
  2. Set things up so that we can overlay the pseudo-elements on top of the spinner:

    .spinner {
        /* ... */
        position: relative;
    }
    
    .spinner:before,
    .spinner:after {
        content: '';
        position: absolute;
    }
    
  3. This is the tricky bit. Each quadrant of the :before pseudo-element is set to a different linear gradient starting with opaque white and progressively becoming more and more transparent. Towards the centre it's easy to see where the gradients join up, but notice how around the outside the colours are close enough together that they appear to join up smoothly.

    .spinner:before {
        border-radius: 50%;
        background:
            linear-gradient(0deg,   hsla(0, 0%, 100%, 1  ) 50%, hsla(0, 0%, 100%, 0.9) 100%)   0%   0%,
            linear-gradient(90deg,  hsla(0, 0%, 100%, 0.9)  0%, hsla(0, 0%, 100%, 0.6) 100%) 100%   0%,
            linear-gradient(180deg, hsla(0, 0%, 100%, 0.6)  0%, hsla(0, 0%, 100%, 0.3) 100%) 100% 100%,
            linear-gradient(360deg, hsla(0, 0%, 100%, 0.3)  0%, hsla(0, 0%, 100%, 0  ) 100%)   0% 100%
        ;
        background-repeat: no-repeat;
        background-size: 50% 50%;
        top: -1px;
        bottom: -1px;
        left: -1px;
        right: -1px;
    }
    

    This is positioned so that it goes slightly over the edge of the spinner because if we position it right to the edge a faint rim of the background colour is visible.

  4. Finally, hide the middle bit using the ::after pseudo-element to make a ring shape:

    .spinner:after {
        background: white;
        border-radius: 50%;
        top: 3%;
        bottom: 3%;
        left: 3%;
        right: 3%;
    }
    

Et voilá!

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