I want to tell you about something that might seem oddly specific and perhaps too technical, but a) at the end of it you will have a useful phrase somewhere in your brain that will pay off one day, and b) I swear I will make it worth your while.

Have you ever seen this problem?

The screenshot on the left is fine. But there is something wrong with the one on the right. In light mode, the shadow is wispy and weird. In dark mode, things are even stranger, and the shadow is almost… a glow?

I stumbled upon this problem occasionally for years now – there are a few screenshots on the blog with this weird problem, even – but it was never feeling like a deal breaker. However, I finally sat down to figure it out today.

Turns out, there are two kinds of approaches to alpha channel/​transparency. The normal one we all know well is called “straight alpha.” But on the right, we were looking at “premultiplied alpha” – something entirely more complicated, where the background is baked into transparency for… reasons. Premultiplied alpha is conceptually – and often literally – dirtier, but it also has benefits: more flexibility, better filtering, sometimes better performance. As far as I understand, premultiplied alpha exists primarily in the world of video and vfx, but occasionally it rears its unconventionally attractive head in our boring static 2D world of screenshots, too.

In my case, I finally figured out this was happening whenever I’ve pasted the screenshot from the clipboard to Photoshop instead of Preview – for some reason, a screenshot then got an alpha channel premultiplied against white background. But I wouldn’t be surprised if it happens to some of you under other conditions, too.

So, “premultiplied alpha.” That’s the useful phrase. What was the other thing?

This is an absolutely hilarious 7-minute video by Captain Disillusion that talks about various challenges with the alpha channel:

Captain Disillusion (or, Alan Melikdjanian) is one of my favourite YouTube educators. His work is mostly debunking fake videos – his most well-known one is about the Cricet bracelet, although my personal fav is one about laminar flow – and they’re just constantly interesting and hilarious at the same time.

Disillusion also occasionally does a more straightforward “let’s talk about some technical aspects of video production” episode which he bundles under a “CD/” umbrella. Here’s a handy list of all of them:

• Intro to the whole series
• Aspect ratio
• Frame rate
• Resolution
• Interlacing (and addendum)
• Color
• Alpha channel
• Shutter speed
• Blur

I am sharing this list because you should watch them all. Most are <10minutes, they are consistently entertaining, and even though none of them are about UX design, there is enough overlap between the two universes that you will come out of it all a lot smarter.

Pragmatically, in my case, I searched for [premultiplied alpha] + [Photoshop] and quickly learned of a new-to-me menu option: Layer > Matting > Remove White Matte. It turns premultiplied alpha back to straight alpha, and fixes the screenshot.

Non-pragmatically? If you want to really understand premultiplied alpha, the last thing I can do is suggest another great internet educator, Bartosz Ciechanowski, who has a more comprehensive interactive web explainer. There will be math. There will also be sliders. You decide.