Using CSS3 turn triangle into square with animation on hover

Question

I am trying to develop a CSS box hover effect using HTML5 & CSS3 but I cannot get this to work. I would like to make an effect like seen below:

when the user is not hovering

when user is hovering

i.e. how can I make a blue triangle and turn it into a blue square when the user hovers over it using HTML5 and CSS3? I need this only using HTML5 & CSS3 and not using canvas.

This element work with canvas perfectly like as below

var ctx = document.getElementById("c").getContext("2d"); 
ctx.fillStyle = "#0000ff"; 
function normal() { 
    ctx.clearRect(0,0,256,256);
    ctx.beginPath(); 
    ctx.moveTo(256,256);                  
    ctx.lineTo(256,0);
    ctx.lineTo(0,0);
    ctx.closePath();
    ctx.fill(); bars()
    ctx.fillStyle="#0000ff"; for (i=0;i

But I need only using HTML5 & CSS3 scripting languages

Answer 1

Using SVG: (the entire effect that you are looking for)

I know you've asked for HTML(5) + CSS(3) but you could also use a SVG path element to produce this effect like in the below snippet. (Note: This uses SVG animations and its browser support can be different compared to CSS animations.)

svg {
  height: 150px;
  width: 150px;
  stroke: black;
}
#blue {
  stroke: blue;
  stroke-width: 10;
}
svg polygon {
  fill: blue;
}
#white {
  stroke: white;
  stroke-width: 10;
}
#icon {
  fill: transparent;
}

<svg viewBox='0 0 100 100'>
  <defs>
    <clipPath id='clipper' clipPathUnits='objectBoundingBox'>
      <path d='M0,0 1,0 1,1 0,0z'>
        <animate attributeType="XML" attributeName="d" from="M0,0 1,0 1,1 0,0z" to="M0,0 1,0 1,1 0,1z" dur="1s" begin="icon.mouseover" fill="freeze" />
        <animate attributeType="XML" attributeName="d" from="M0,0 1,0 1,1 0,1z" to="M0,0 1,0 1,1 0,0z" dur="1s" begin="icon.mouseout" fill="freeze" />
      </path>
    </clipPath>
    <g id='lines'>
      <line x1='20' y1='30' x2='80' y2='30' />
      <line x1='20' y1='50' x2='80' y2='50' />
      <line x1='20' y1='70' x2='80' y2='70' />
    </g>
  </defs>
  <use xlink:href='#lines' id='blue' />
  <g clip-path='url(#clipper)'>
    <polygon points='0,0 0,100 100,100 100,0' />
    <use xlink:href='#lines' id='white' />
  </g>
  <g>
    <polygon points='0,0 0,100 100,100 100,0' id='icon' />
  </g>
</svg>

---

The below are answers to the question - how to turn triangle into square with animation.

Using Borders:

You could do it using border like in the below snippet. Initially only the right and top borders have the blue color but on hover we set the color to all border sides. This method is pretty simple and will work in all browsers (including IE8) but you cannot add content directly to this div (as doing so will affect the triangle shape) and so you'd have to place content on top of the shape using positioning or set the shape using a pseudo-element.

.shape{
  height: 0px;
  width: 0px;
  border: 50px solid transparent;
  border-color: blue blue transparent transparent;
  transition: all 1s;
}
.shape:hover{
  border-color: blue;
}

<div class='shape'></div>

---

Using Transforms:

You could add rotate transform on a pseudo-element, set overflow: hidden on parent to produce the triangle and then reverse/nullify the transform on hover.

.shape {
  position: relative;
  height: 100px;
  width: 100px;
  overflow: hidden;
}
.shape:after {
  position: absolute;
  content: '';
  height: calc(100% * 1.414); /* using Pythogras theorem */
  width: calc(100% * 1.414); /* using Pythogras theorem */
  transform: rotate(-45deg);
  transform-origin: left top;
  background: blue;
  transition: all 1s;
}
.shape:hover:after {
  transform: rotate(0deg);
}

<div class='shape'></div>

You could also use a skewX transform instead of a rotate transform if you wish to avoid calculating the height and width like in the previous snippet.

.shape {
  position: relative;
  height: 100px;
  width: 100px;
  overflow: hidden;
}
.shape:after {
  position: absolute;
  content: '';
  height: 100%;
  width: 100%;
  transform: skewX(45deg);
  transform-origin: left top;
  background: blue;
  transition: all 1s;
}
.shape:hover:after {
  transform: skewX(0deg);
}

<div class='shape'></div>

---

Using Gradients:

You could use linear-gradients to create a triangle and then turn it into a square on hover by doubling the background-size.

.shape{
  height: 100px;
  width: 100px;
  background: linear-gradient(to bottom left, blue 49.5%, transparent 50.5%);
  background-position: 100% 0%;
  background-size: 100% 100%;
  transition: all 1s;
}
.shape:hover{
  background-size: 200% 200%; /* just double the background size on hover */
}

<script src="https://cdnjs.cloudflare.com/ajax/libs/prefixfree/1.0.7/prefixfree.min.js"></script>
<div class='shape'></div>

Answer 2

In spite of the complete answer from Harry, couldn't resist to post an answer with another approach, suggested by the image in the OP.

Let's use blend modes, and see what can be achieved (but with more limited support)

.test {
    width: 200px;
    height: 200px;
    background-color: white;
    display: inline-block;
    background-image: linear-gradient(blue, blue), linear-gradient(blue, blue), linear-gradient(blue, blue);
    background-size: 100px 30px;
    background-repeaT: no-repeat;
    background-position: center 30px, center center, center 140px;
    border: solid 1px black;
    position: relative;
    overflow: hidden;
}

.test:after {
  content: "";
  position: absolute;
  left: 0px;
  top: 0px;
  width: 100%;
  height: 100%;
  background: linear-gradient(45deg, yellow 50%, transparent 50%);
  mix-blend-mode: difference;
  transition: all 1s;
}

.one:after {
  transform-origin: bottom right;
}

.one:hover:after {
  transform: rotate(-45deg);
}

.two:hover:after {
  opacity: 0;
}

.three:after {
  background: none;
  box-shadow: -1000px 1000px 0px 1000px yellow;
  transform-origin: top left;
  transform: rotate3d(1,1,0,87deg);
}

.three:hover:after {
  transform: rotate3d(1,1,0,0deg);
}

<div class="test one"></div>
<div class="test two"></div>
<div class="test three"></div>

the third one is a little bit tricky, and not quite perfect. But you get the idea.