Here is a demo of what we are building. Hover the image to see the animation

Cool right? If you check the HTML tab you won’t see a lengthy code structure. A single image element is all that we will be using to build that complex-looking effect. Don’t look at the CSS for now and let’s build this together.

Note: At the time of writing this, Firefox doesn’t support the animation take a look in a Chrome or Safari-based browser to read the article.

Creating The Shape

It’s highly recommended that you read the previous article because I will be building on top of it. I will be reusing many tricks and the starting point of this article will be the last demo of the previous one.

And here is a figure to remind you the mask composition used to create the shape.

As you can see, a set of small circles is used in the “exclude” composition to create the inner curves, and another set of small circles is used in the “add” composition to create the outer curves. The idea is to move those circles in opposite directions to create and control our wavy circle.

Here is another figure to illustrate the trick

The idea is to move the circles and at the same time adjust their radius so they are always touching each other. Here is an interactive demo to better understand the movement of the circles. Use the slider to control the position of the circles.

Let’s write some code

Now that we have the geometry of shape in place, let’s translate this into code. It wasn’t an easy task at all. Finding the right formulas and translating everything into a CSS code was a bit tricky.

The first challenge is to find one variable that allows me to control the position of the small circles and at the same time their radius. I could have used multiple variables but having only one variable will make the shape easy to control as we only have to update one value and everything else will follow.

Initially, I thought about using a length variable which is logical since the radius can be expressed as a length and to move the circles I need a distance which is also a length. I wasn’t able to follow that root because finding the formulas and expressing them using CSS was almost impossible. Instead of a length I had to rely on an angle variable. It may sound wrong but it was indeed the right way to do it as I was able to find most of the formulas and write them using CSS.

Here is a figure to illustrate the angle I am referring to.

Let’s take two adjacent circles and draw a line between their center (illustrated in white). The [1] shows the initial shape where all the circles are perfectly aligned around the big circle. This will be the initial state so let’s consider we have an angle equal to 0deg. When we move the circles and get the shape in [2] the line will rotate a little and the angle of rotation will be our variable.

Don’t worry, I won’t start a boring geometry course. I just wanted to highlight the variable you need to adjust so you can visualize why it’s an angle and the angle of what. By the way, in the last interactive demo, you are adjusting that angle using the range slider.

Now, let’s take the shape of the previous article and try to introduce the new variable and adjust the different formulas. If you didn’t do it make sure to read that article to better understand what’s coming next. I might skip a few concepts I already explained in the previous article.

$n: 10; /* the number of circles/petals */

.flower {
  width: 400px;
  aspect-ratio: 1;
  --r: calc(50%/(1 + 1/sin(90deg/#{$n})));
  --g:/calc(2*var(--r)) calc(2*var(--r)) radial-gradient(50% 50%,#000 100%,#0000) no-repeat;
  $m1: ();
  $m2: ();
  @for $i from 1 through $n { 
    $m1: append($m1, 
     calc(50% + 50%*cos(360deg*#{$i/$n})) 
     calc(50% + 50%*sin(360deg*#{$i/$n}))
     var(--g), 
    comma);
    $m2: append($m2,
      calc(50% + 50%*cos((360deg*#{$i} + 180deg)/#{$n})) 
      calc(50% + 50%*sin((360deg*#{$i} + 180deg)/#{$n}))
      var(--g), 
    comma);
  }
  mask:
    #{$m1},
    radial-gradient(100% 100%,#000 calc(var(--r)/tan(90deg/#{$n})),#0000 0) intersect,
    radial-gradient(#000 0 0) exclude,
    #{$m2};
}

The Sass loop will generate two sets of circles: $m1 (the blue circles used with the add composition) and $m2 (the red circles used with the exclude composition).

First of all, we have to add the new angle variable

.flower {
   --a: 10deg;
}

Then we update the position and the radius of the circles:

$n: 10; /* the number of circles/petals */

.flower {
  --a: 10deg; 
  --s: 400px; /* shape size */

  width: var(--s);
  aspect-ratio: 1;
  --r: calc(var(--s)/(2 + 2/sin(90deg/#{$n})));
  --R: f(--r,--a);
  --g:/var(--R) var(--R) radial-gradient(50% 50%,#000 100%,#0000) no-repeat;
  $m1: ();
  $m2: ();
  @for $i from 1 through $n { 
    $m1: append($m1, 
     calc(50% + (50% - f1(--r,--a))*cos(360deg*#{$i/$n})) 
     calc(50% + (50% - f1(--r,--a))*sin(360deg*#{$i/$n}))
     var(--g), 
    comma);
    $m2: append($m2,
      calc(50% + (50% + f2(--r,--a))*cos((360deg*#{$i} + 180deg)/#{$n})) 
      calc(50% + (50% + f2(--r,--a))*sin((360deg*#{$i} + 180deg)/#{$n}))
      var(--g), 
    comma);
  }
  mask:
    #{$m1},
    radial-gradient(100% 100%,#000 calc(var(--r)/tan(90deg/#{$n})),#0000 0) intersect,
    radial-gradient(#000 0 0) exclude,
    #{$m2};
}

You might ask what’s going on with the f(), f1(), and f2(). It’s not a new CSS feature so don’t try to google it. I am using them as placeholders to make the code look easy and to better understand the logic.

The circles need a new radius so I am defining --R where its value is a function of the old radius --r and the new angle variable --a. As for the positions, the first set of circles needs to get closer to the center so I am reducing their distance 50% - f1() while the second set needs to get far so I am increasing their distance 50% + f2().

I won’t get into the math details of each function otherwise this will turn into a boring geometry course. All you have to do is to understand the main logic and at the end, you can easily adjust the shape using CSS variables.

Also, note the use of the --s variable to control the size. With this new configuration, I am not able to rely on percentage so I need to have the size as a variable and use it to calculate the radius --r.

Here is the full demo where you can play with the different values to control the shape.

Introducing The Image Element

Let’s now consider an <img> element instead of a <div>.

It works fine but our goal is to have the image within the shape like the first example. To do this, we add some padding (or border) to leave space for the background and also add border-radius to round the image.

Now it’s perfect! I am using a padding value equal to 2.2*var(--r) but there is no particular logic behind it. It’s what gives me something that looks good to me. Feel free to update it if you want to increase or decrease the space around the image.

Adding The Animation

Let’s move to the interesting part which is the animation. We will first adjust the shape on hover and this is the easiest part because we already did the hard job by finding one variable to control the shape. All we have to do is to update that variable on hover.

img {
  --a: 28deg;
}
img:hover {
  --a: 10deg;
}

The above will simply change the shape but will not give us a smooth animation it is because by default we cannot animate CSS variables (custom properties). To do this we need to register them using @property.

@property --a {
  syntax: "<angle>";
  inherits: true;
  initial-value: 0deg;
}

Then add a transition like below

img {
  transition: --a .3s
}

Now we have a perfect hover effect!

Let’s tackle the rotation. It’s clear that we cannot rotate the whole element as the image needs to remain straight and only the shape needs to rotate. To do this, we will introduce a new angle variable and use it within the formulas that define the position of the circles.

If you look at the Sass loop that generates the code of the circles, you will notice that the increment $i is used to define an angle, and this angle is used to correctly place each circle. If we update that angle, we update the position. The idea is to update the angle of all the circles with the same value so they all move the same way to simulate a rotation.

In the demo above, you will see I am registering a new variable and applying an animation to it

@property --o {
  syntax: "<angle>";
  inherits: true;
  initial-value: 0deg;
}
img {
  animation: rotate 20s infinite linear;
}
@keyframes rotate {
  to { --o: 360deg; }
}

Then I am using that variable within the Sass loop to update the angle of the circles. Instead of 360deg*#{$i/$n}, we use 360deg*#{$i/$n} + var(--o) and instead of (360deg*#{$i} + 180deg)/#{$n} we use (360deg*#{$i} + 180deg)/#{$n} + var(--o).

The final touch is to increase the speed of the rotation on hover. For this, I am going to introduce the animation-composition property. It’s a pretty new propery so you may have not heard about it, but it’s a powerful property that I invite you to explore. Plus the browser support is pretty good.

I will update the code of the animation like below:

img {
  animation: 
    rotate 20s infinite linear,
    rotate 20s infinite linear paused;
  animation-composition: add
}
img:hover {
  animation-play-state: running;
}

I am applying the same animation twice while making the second one paused. On hover, both animations are running. Let’s have a look at the definition of animation-composition: add

The animation-composition CSS property specifies the composite operation to use when multiple animations affect the same property simultaneously.

Then:

add
The effect value builds on the underlying value of the property. This operation produces an additive effect.

We are using the same animation so we are affecting the same property (the variable --o) and the use of add will create an additive effect. It means that the element will rotate faster when both animations are running.

Try it!

The concept of animation-composition is not easy to grasp at first glance, but imagine that you add an animation on the top of another one. The first animation is rotating the element then we add another animation that will also rotate the element. If you rotate an element that is already rotating then you get a faster rotation. We can also decrease the rotation speed using the same technique. This time, the second animation needs to apply an opposite rotation using the reverse keywords.

Conclusion

We are done! We created a complex-looking effect using only CSS and a single HTML element. It was a good opportunity to explore modern CSS features such as mask, trigonometric functions, @property, etc

You are probably thinking it’s a bit too much, right? What’s the point of introducing all this complexity for a fancy effect that you will probably never use? The goal is not really to build the effect and use it but to push the limit of CSS and explore new features. In the end, you have a bunch of CSS tricks that you can use elsewhere.

We learned how to use mask-composite. We learned how to animate CSS variables using @property. We played with gradients. We discovered the animation-composition property. etc. One day, you will for sure need to use those CSS tricks!

.fade-out-text {
  -webkit-mask-image: linear-gradient(to bottom, white, transparent);
  mask-image: linear-gradient(to bottom, white, transparent);
}

The gradient technique is such a classic though. I remember playing with that over a decade ago. Here’s a proper demo of masking though: