CSS 3 Shape: "Inverse Circle" or "Cut Out Circle" - css

Question

I want to create a shape, which i would describe as "inverse circle":
The image is somehow inaccurate, because the black line should continue along the outer border of the div element.
Here is a demo of what i have at the moment: http://jsfiddle.net/n9fTF/
Is that even possible with CSS without images?

Answer 1

Update: CSS3 Radial Background Gradient Option
(For those browsers supporting it--tested in FF and Chrome--IE10, Safari should work too).
One "problem" with my original answer is those situations where one does not have a solid background that they are working against. This update creates the same effect allowing for a transparent "gap" between the circle and it's inverse cutout.
See example fiddle.
CSS
.inversePair {
border: 1px solid black;
display: inline-block;
position: relative;
height: 100px;
text-align: center;
line-height: 100px;
vertical-align: middle;
}
#a {
width: 100px;
border-radius: 50px;
background: grey;
z-index: 1;
}
#b {
width: 200px;
/* need to play with margin/padding adjustment
based on your desired "gap" */
padding-left: 30px;
margin-left: -30px;
/* real borders */
border-left: none;
-webkit-border-top-right-radius: 20px;
-webkit-border-bottom-right-radius: 20px;
-moz-border-radius-topright: 20px;
-moz-border-radius-bottomright: 20px;
border-top-right-radius: 20px;
border-bottom-right-radius: 20px;
/* the inverse circle "cut" */
background-image: -moz-radial-gradient(
-23px 50%, /* the -23px left position varies by your "gap" */
circle closest-corner, /* keep radius to half height */
transparent 0, /* transparent at center */
transparent 55px, /*transparent at edge of gap */
black 56px, /* start circle "border" */
grey 57px /* end circle border and begin color of rest of background */
);
background-image: -webkit-radial-gradient(-23px 50%, circle closest-corner, rgba(0, 0, 0, 0) 0, rgba(0, 0, 0, 0) 55px, black 56px, grey 57px);
background-image: -ms-radial-gradient(-23px 50%, circle closest-corner, rgba(0, 0, 0, 0) 0, rgba(0, 0, 0, 0) 55px, black 56px, grey 57px);
background-image: -o-radial-gradient(-23px 50%, circle closest-corner, rgba(0, 0, 0, 0) 0, rgba(0, 0, 0, 0) 55px, black 56px, grey 57px);
background-image: radial-gradient(-23px 50%, circle closest-corner, rgba(0, 0, 0, 0) 0, rgba(0, 0, 0, 0) 55px, black 56px, grey 57px);
}
Original Answer
Took more effort than I expected to get the z-indexing to work (this seems to ignore the negative z-index), however, this gives a nice clean look (tested in IE9, FF, Chrome):
HTML
<div id="a" class="inversePair">A</div>
<div id="b" class="inversePair">B</div>
CSS
.inversePair {
border: 1px solid black;
background: grey;
display: inline-block;
position: relative;
height: 100px;
text-align: center;
line-height: 100px;
vertical-align: middle;
}
#a {
width: 100px;
border-radius: 50px;
}
#a:before {
content:' ';
left: -6px;
top: -6px;
position: absolute;
z-index: -1;
width: 112px; /* 5px gap */
height: 112px;
border-radius: 56px;
background-color: white;
}
#b {
width: 200px;
z-index: -2;
padding-left: 50px;
margin-left: -55px;
overflow: hidden;
-webkit-border-top-right-radius: 20px;
-webkit-border-bottom-right-radius: 20px;
-moz-border-radius-topright: 20px;
-moz-border-radius-bottomright: 20px;
border-top-right-radius: 20px;
border-bottom-right-radius: 20px;
}
#b:before {
content:' ';
left: -58px;
top: -7px;
position: absolute;
width: 114px; /* 5px gap, 1px border */
height: 114px;
border-radius: 57px;
background-color: black;
}

Answer 2

I can't really tell from your drawing how rounded you want the points, but here's one possibility:
http://jsfiddle.net/n9fTF/6/
If the points need to be more rounded, you'll need to put some circles on the ends so they blend with the big scoop.

Answer 3

Different approach : Box-shadows
This approach uses CSS box shadows which are supported by IE9+ (canIuse)
DEMO
Output :
HTML :
<div id="a">
<div id="b"></div>
</div>
CSS :
#a{
overflow:hidden;
border-radius:20px;
position:relative;
display:inline-block;
}
#a:before, #a:after{
content:'';
width: 100px;
border-radius: 50%;
}
#a:before {
height: 100px;
float:left;
border: 1px solid black;
background: grey;
}
#a:after {
position:absolute;
left:14px; top:-6px;
height:114px;
box-shadow: 1px 0px 0px 0px #000, 110px 0px 0px 68px #808080;
background:none;
z-index:-1;
}
#b {
width: 200px;
height: 100px;
background:none;
margin-left:-15px;
border: 1px solid black;
border-left:none;
float:left;
border-top-right-radius: 20px;
border-bottom-right-radius: 20px;
}

Answer 4

This is a very interesting question. I've recently posted a tutorial on how to make Inverse Border Radius in CSS (here) and I think this could easily be adapted for your case.
The trick is to create a span that generates the inverse border using a very simple concept - very thick borders. And use the inside section by hiding them. What you would have to do in addition to my script provided is add another border-radius to the top-left corner as I am only using the top-right one. Make the span aligned to the left of the item you want by absolute positioning, and increase the height/width of span accordingly and voila you have your inverse border-radius.

Answer 5

Using clip path this can be done .
let precision = 64;
let radius = 50;
let c = [...Array(precision)].map((_, i) => {
let a = -i/(precision-1)*Math.PI*2;
let x = Math.cos(a)*radius + 100;
let y = Math.sin(a)*radius + 50;
return `${x}% ${y}%`
})
document.querySelector('.circleContainer').style.clipPath =
`polygon(100% 50%, 100% 100%, 0 100%, 0 0, 100% 0, 100% 50%, ${c.join(',')})`;
.container{
display: flex;
position: relative;
width: 200px;
}
.left{
background: blue;
width: 100px;
height: 100px;
border-top-left-radius: 50%;
border-bottom-left-radius: 50%;
}
.circleContainer {
background: blue;
width: 100px;
height: 100px;
}
.innerCircle{
width: 80px;
height: 80px;
border-radius: 50%;
background: orange;
position: absolute;
top: 10px;
right: -40px;
}
<div class='container'>
<div class='left'></div>
<div class='circleContainer'></div>
<div class='innerCircle'></div>
</div>
Using an approah which I found here

Answer 6

Someone else done it somewhere from what I found...
JSFiddle: http://jsfiddle.net/ajeN7/
and the question: CSS3 Inverted Rounded Corner
Hopefully that helps!

Answer 7

Introduce an absolutely positioned borderless white circle which sits behind the gray circle at an offset. You will need to set the z-index of the dark circle to ensure that it sits above the white circle:
#c {
position: absolute;
border: 0;
left: 30px;
width: 100px;
height: 100px;
border-radius: 50px;
background: white;
}
Demo.
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