Transparent half circle cut out of a div

Question

I would like to make a transparent cut out half circle shape using only CSS3. The only requirement is that all the elements that form the shape must

Answer 1

I needed rounded corners only on the bottom of a responsive image. I started from @sandeep fiddle and improved it for my needs:

            .rect
            {
                height: 85vh;
                    position: relative;
                background-color: red;
                width: 60vw;
            }

            .circle-helper{
                display: block;
                width: 100%;
                padding-bottom: 50%;
                bottom: 0;
                left: 0;
                overflow: hidden;
                position: absolute;
                background-color: transparent;

                    }
            .circle{
                display: block;
                width: 100%;
                padding-bottom: 100%;
                // height: 500px;
                bottom: 0;
                left: 0;
                overflow: hidden;
                position: absolute;
                background-color: transparent;
            }

            .circle:after{
                box-sizing: content-box;
                content: '';
                width: 100%;
                height: 100%;
                -moz-border-radius: 50%;
                -webkit-border-radius: 50%;
                border-radius: 50%;
                background: rgba(0,0,0,0);
                position: absolute;
                transform: translate(-50%, -50%);
                top: 50%;
                left: 50%;
                border: 300px solid blue;
                }
                top: 50%
                left: 50%
                border: 300px solid blue

https://jsfiddle.net/mop00j22/

Answer 2

Try this.

body{
  
  background-color:#333;
  passing:0px;
  height:0px;
  
}
#app{
  background:#333 url('https://source.unsplash.com/random') no-repeat;
  background-size:cover;
  width:360px;
  height:560px;
  position:relative;
  overflow:hidden;
}
.app-bar{
  width:100%;
  height:50px;
  position:absolute;
  bottom:0px;
  left:0;
  
  
    
}

.app-bar .bar{
  line-height:50px;
  
  position:relative;
  width:100%;
  height:50px;
  background-image: radial-gradient(circle 35px at 315px 0, transparent 700px, #f44336 50px);
    
}


.app-bar .bar i{
  color:#FFF;
  display:block;
  line-height:50px;
  float:left;
  width:50px;
  text-align:center;
  cursor:pointer;
  margin-top:0px;
}
.app-bar .bar i:hover{
  background-color:rgba(0,0,0,.1);
}
.app-bar .bar button{
  padding:0px;
  box-sizing:border;
  text-align:center;
  margin:0px;
  bordeR:0px;
  outline:0px;
  width:60px;
  height:60px;
  line-height:60px;
  cursor:pointer;
  color:#FFFFFF;
  display:block;
  border-radius:50%;
  position:absolute;
  top:-30px;
  left:100%;
  margin-left:-75px;
  background-color:#f44336;
  transition: all .2s ease;
  
}
.app-bar .bar button span{
  line-height:60px;
  font-size:30px;
  
}
.app-bar .bar button:hover{
  transform:rotate(45deg);
  transition: all .2s ease;
}

<div id="app">
  <div class="app-bar">
    
    <div class="bar">
      <i class="material-icons">menu</i>
      <i class="material-icons">search</i>
      <button class="button">
        <span class="material-icons">add</span>
      </button>
    </div>
  </div>
<link rel="stylesheet" type="text/css" href="https://cdnjs.cloudflare.com/ajax/libs/material-design-icons/3.0.1/iconfont/material-icons.min.css" >