At work last week, we started building a cool new project and part of the design revolves around angles. A particularly tricky part is angular-cropped images. The images need to have a 8 degree angle top and bottom to give a skewed look–however the image cannot be distorted.

Here's an example of what we want:

Bill Murray, but with the top and bottom of the photo with wedges sliced out

Some other points worth mentioning are that the images can be any size and in any width container.

After quickly abandoning my initial attempt with transform skew, I reached for the clip-path property. Clip path accepts a polygon value, which you can create with some 4 lots of 2 percentage coordinates. I like to use a web app called Clippy to quickly test out polygons so I knocked up a quick example:

Screen grab of Clippy web app showign a rough angled masked image

This uses the follwing CSS which kind of makes sense but it actually doesn't work.

clip-path: polygon(0 8%, 100% 0%, 100% 92%, 0% 100%);

The issue is, if the image proprtions change, the angle is going to change. Basically we need to work out the sides of a triangle when we only know 2 angles. Like this:

Photo showing the unknown values and known 8° angle of the triangle slice we need to 'clip' from our image'

One thing we can know though, is the aspect ratio of the image. For example the image we've been using is 16:9. Therefore we could add those measurements in and see if it helps us. As the clip-path will be in percent, we can scale it all up after.

Photo showing the 16 x value, 9 y value and known 8° angle of the triangle slice we need to 'clip' from our image'

Now we have a triangle where we know 1 side (16 width) and one angle (8°). Digging out my old maths textbooks, I can confirm that we need to use some GCSE trigonometry. To be specific, the phrase SOH CAH TOA.

Effectively, this is mnemonic to remember the 3 ways of working out the sides of a triangle, based on the information you have. We have the Opposite angle (8°) or O, the Adjacent side (16 wide) or A. O and A means we need to use T (Tan). Using the formula Tangent = Opposite / Adjacent (TOA). Algebra time...

Photo showing the formula Tangent = Opposite / Adjacent with values applied and then the equasion being simplified to its result revealing the 3rd missing side of the triangle as 2.248653355238256

That shows the other unknown side of our triangle is 2.248653355238256 in length. This doesn't solve our clip-path issue by itself. First we need to get this as a percentage of the whole length of the vertical side (9).

So, to work out the top angle, we need to divide this by 9 and multiply it by 100%:

tan(8deg) * 16 / 9 * 100%

For the bottom angle, we need to reverse it by taking it away from 100%:

100% - (tan(8deg) * 16 / 9 * 100%)

Unfortunately CSS (or any of it's preprocessors?) don't have native trigonometry functions. However, someone already made them! Take a look at the post Trigonometry in Sass. By adding these functions into our code, we can then use the functions in our SCSS. Here's our final clip-path for our 16x9 image:

clip-path: polygon(0 #{tan(8deg) * 16 / 9 * 100%}, 100% 0, 100% #{100% - (tan(8deg) * 16 / 9 * 100%)}, 0 100%);

And here is a Codepen showing the same applied to 16:9, 4:3 and 3:4 aspect ratio images (I'm using Intrinsic Ratios/Uncle Dave's Ole Padded Box to maintain the images' aspect ratio):

See the Pen Trigonometry Image Skew by matthew jackson (@matthewbeta) on CodePen.

Hope that didn't fry your brain like it did mine.