Black Light Pro 1.1 on the App Store

The short version of this post is that Black Light Pro 1.1 is out, and is now available on the Mac App Store. If you’re interested in the story behind this, read below.

Last year when I was developing Black Light Pro, I intended it to be available on the App Store as well as directly from my website, like all my other apps. Things don’t always work as intended though. The App Store is famous for its strict sandboxing requirements, disallowing access to many functions of the OS in the name of security. One of these is for changing the current color profile of the screen.

There’s two ways really the operating system lets us affect how colors are rendered on a screen:

1. You can directly change the display transfer table and it’ll have an instantaneous effect. It’s a bit fragile too because many things can reset this transfer table unexpectedly, like the display waking from sleep, plugging a secondary screen, other apps quitting, etc.

2. You can change the color profile of a screen. The color profile tells many things about a screen, one of them is the display transfer table the operating system should use. When the transfer table applied in method #1 gets reset, it becomes whatever is in the screen’s color profile. This makes the profile the ideal place to apply a long-lasting effect.

In Black Light I always used method #1. For Black Light Pro I wanted to use #2 to make the effect more robust by changing the screen’s profile while the app is running. This is better because it avoids the flashes of “normal” colors that happen whenever the transfer table is reset.

So, the sandbox got in the way. Since you can’t change the color profile of a screen from a sandboxed app, that feature was out for the Mac App Store version. The app would still work inside the sandbox, but changing the profile wouldn’t. I didn’t want to remove the feature (because it truly makes things better), and I did not want to make one version sandboxed and the other not for various reasons. In the end, I released it only on my website.

A few months ago, it came to my attention that Black Light Pro would sometimes crash unexpectedly. This would manifest by the app disappearing from the menu bar and scheduled effects no longer triggering. From the crash logs it looked related to the code for changing the profile.

I couldn’t figure out exactly how to reproduce it, but it seems like a reference counting problem with a value returned from one of the function calls needed to change the profile. Not sure if I could do something about it (the bug could be on Apple’s side), I decided to move the profile-management code to a separate process: if that separate process crashes, then a new one could be re-spawned and continue where the other left. It’s fault-tolerant now.

This complicated bug fix has an interesting effect: it makes it possible to sandbox the main process. If the main process is sandboxed, that means it can go on the Mac App Store… as long as the profile management process is not included. And that’s what I’ve done for 1.1.

I’m a bit proud now to announce Black Light Pro is now available on the App Store. Also the color profile functionality is more robust now in the direct version.