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How does a screen work?

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573 points chkhd | 2 comments | 13 Jul 25 14:09 UTC | HN request time: 0.425s | source
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perching_aix ◴[13 Jul 25 23:46 UTC] No.44554866[source]▶
There are some sentences in this that are technically vague enough to pass, but I don't think are strictly speaking correct, and I believe will likely lead to a mistaken understanding:

> modern displays don't paint the image line-by-line (...) They light up each pixel simultaneously, refreshing the entire display at once.

The entire screen area is lit all the time now, yes, but refresh still typically happens line by line, top to bottom [0], left to right [0], for both LCDs and OLEDs. It's a scanning refresh, not a global refresh (sadly).

You can experimentally confirm this using a typical smartphone. Assuming a 60 Hz screen refresh, recording in slow motion will give you enough extra frames that the smartphone camera also likely operating in a scanning fashion (rolling shutter) won't impact the experiment. On the recording, you should see your screen refreshing in the aforementioned fashion.

[0] actual refresh direction depends on the display, this is for a typical desktop monitor

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1. krackers ◴[14 Jul 25 05:49 UTC] No.44556858[source]▶
Assuming a 60Hz refresh rate, does it take ~16ms (± the vblank inteval) for a complete cycle from top-left to bottom-right? Or does the scan happen faster than that (with something else being the limiting factor on overall refresh rate)?
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2. perching_aix ◴[14 Jul 25 07:02 UTC] No.44557202[source]▶
Yes, the refresh cycle takes ~16.6 ms. There's another "point" chasing behind the refresh "point", that will be where the panel's response time has finished catching up with the refresh. In between these points, the pixels are slowly morphing from one color to the next. On LCDs, the area between these two points is quite sizeable, definitely more than a few lines, sometimes even hundreds of lines. On a 1080p 60 Hz display, just 1 ms of response time corresponds to 64.8 lines (6% of the screen) being constantly in flux, for example.

The difference between LCDs and CRTs in this regard then, is that on a CRT you only ever got light during that chase section. The initial state is full darkness, and the final state is full darkness too. It's a pulse.