Utilize the following CSS as a starting point (ensure .sample-box is a class applied to the image or its containing DIV):
.sample-box
{
background-color: rgba(255, 0, 0, 0.5);
height: 200px;
width: 400px;
}
.sample-box::before
{
position: absolute;
display: block;
width: 0;
height: 0;
margin-top: 150px; /* adjust based on ::before item height */
margin-left: 0px; /* position relative to main element padding */
content: "";
border-width: 0px;
border-style: solid;
border-color: transparent;
border-left-width: 400px; /* total width + padding of main element */
border-bottom-width: 50px; /* height of "mask" */
border-bottom-color: rgba(255, 255, 255, 1.0); /* color of "mask" */
}
.sample-box::after
{
position: absolute;
display: block;
width: 0px;
height: 0px;
margin-top: 132px; /* modify for optimal positioning */
margin-left: 0px; /* horizontal position for ::after item */
content: "";
border-width: 0px;
border-style: solid;
border-color: transparent;
border-right-width: 400px; /* total width + padding of main element */
border-bottom-width: 50px; /* height of "mask" */
border-bottom-color: rgba(255, 255, 255, 1.0); /* color of "mask" */
}