Made it look worse
I was excited about the idea of using an iPhone to calibrate my external monitor, but the ICC profile generated by the app introduced severe color banding/posterization that wasn't present before. Smooth gradients became visibly separated into bands, especially in darker areas.
I tried reverting to my original color profile and the problem immediately disappeared, so the issue appears to be related to the generated calibration profile.
The concept is great, but at least with my display, the resulting profile was unusable. I wouldn't recommend relying on this for display calibration without carefully checking gradients and shadow detail afterward.
Response from developer
Thank you for this. You found a real bug, and the detail about darker areas is what let us track it down. The tone curve was being drawn exactly through every measured point. Those points come from two different kinds of patch, and on a display whose channels do not add up cleanly to white the two disagree by several percent. The curve zigzagged between them, and inverting a zigzag into a correction table is what dropped neighbouring levels onto one another. The curve is now fitted between the measurements instead. Replaying our recorded runs, the ones that banded went from four in thirty-three to two, and both that remain are runs where the monitor was still warming up. Two things changed besides the fix. The report now shows how many of the 256 levels a profile costs, and the app says so before you install one that would band, including when the monitor adjusted its own brightness mid-measurement. If you give it another try, the gradients should hold. If they do not, the report will now tell you why.
