Author Topic: Rolling Shutter 1080p vs 720p modes  (Read 2253 times)

johannsebastianbach

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Rolling Shutter 1080p vs 720p modes
« on: December 25, 2017, 10:11:30 PM »
Hi guys,

I was wondering, if there is a difference between rolling shutter in the squeezed mode?

In 1920x818 25p Magic Lantern shows me a rolling shutter of 16,5 μs/line but in the squeezed 1920x490 25p I have 19 μs/line.

If I calculate 818*16,5/1000 = 13,5 ms for complete readout.
But what about the slowmo, do I count with again 818*19/1000 cause the sensor is read out entirely or does the sensor read ONLY 490 lines? Can't imagine, since the rolling shutter is still visible on first sight. Are the 16,5 and 19 μs/line the factors I should consider regarding rolling shutter?

Did anybody manage to drastically speed up the readout process so far?

ItsMeLenny

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #1 on: December 31, 2017, 12:40:52 AM »
I wonder if the A and B timings are different for 1080p and 720p.

Also note that 1080p is 25fps/30fps where as 720p is 50fps/60fps.

johannsebastianbach

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #2 on: January 03, 2018, 11:53:20 PM »
I wonder if the A and B timings are different for 1080p and 720p.
It shows me so in the ML settings.

Also note that 1080p is 25fps/30fps where as 720p is 50fps/60fps.
I used FPS Override to 25 fps in both modes.

a1ex

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #3 on: January 07, 2018, 06:47:24 PM »
Answers can be found in ML menu and this thread - the FPS override submenu displays both the time required for one line (microseconds), and the time required to read one frame (milliseconds and percentage of frame duration, which is 1/fps).

Of course, 50 fps overridden to 25 can have lower rolling shutter (depending of options from menu), but image quality will suffer, since there will be fewer lines read out from the sensor (that's why you'll have to stretch the image).

To minimize the rolling shutter, reduce the timer A value manually until other numbers are no longer changing (todo: I should probably include a preset for that).

This gives:
25 fps, Canon default: timer A 480: 20 µs/line
     => 25.8ms at 1290 lines, or 21.6ms at 1080 lines
25 fps, with FPS override enabled and timer A reduced to 398: 16.6 µs/line
     => 21.4ms at 1290 lines, 17.9ms at 1080 lines
25 fps overridden from 50, timer A reduced to 410 (to try lower values, one has to edit the source): 17.1 µs/line
     => 11.5ms at 672 lines, or 1120 after desqueezing (5:3)
     => 11.08 ms at 1080 lines after desqueezing
     => 13.24 ms at a hypothetical 1290 lines (normalized so you can compare)
25 fps overridden from 50 with 3x3 crop_rec, timer A reduced to 410
     => nothing to gain (exercise for the reader).

So, you are able to cut the rolling shutter in half (actually a 48.7% decrease from Canon default, at the expense of some image quality). Whether that actually makes a difference in practice, I don't know.

(BTW, noticed a roundoff error in the menu - 16.5 should be really 16.6, 20.0 is correct and so on; the numbers above are corrected)

To get even lower rolling shutter, one may experiment with adtg_gui - there may be other registers to tweak. You can start by understanding the registers tweaked by crop_rec (crop_rec_4k branch) and fiddling from there - one might be able to reduce the horizontal resolution to get a lower rolling shutter.

TODO: I'm going to document the crop_rec registers better, maybe that's helpful.

johannsebastianbach

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #4 on: January 10, 2018, 01:40:27 AM »
Thank you so much for the explanation a1ex!

I remember reading about A and B timers and Rolling shutter in other threads, but didn't come accross this one from 2014! I was struggeling with rolling shutter for a long time and (also actually not sure if it is also have a impact on how "filmic" the footage looks? Just a feeling) but with the slowmo-mode and overcranking timer A the effect is huge!

Tested some fast pans side by side and it helps a lot. Personally I never cared to stretch it in post and nobody has ever noticed the vertical resolution in my clips.  :D

johannsebastianbach

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #5 on: January 15, 2018, 10:40:18 PM »
Getting a hard time to set a shutter speed to 180° in 1280-mode at 25fps. I can fine-tune shutter to 1/50, but once I get out of the ML menu, it almost always instantly switches back to 1/64.

a1ex

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #6 on: January 15, 2018, 11:36:15 PM »
Set FPS override mode to Low light (that would scale the shutter speed, keeping the shutter angle constant), then adjust the timers to match the values from Exact FPS (to get as close as you can to 25.0 FPS).

BTW, tried decreasing timer A below 410 in 720p50, but at 408 the image goes bad.

johannsebastianbach

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Re: Rolling Shutter 1080p vs 720p modes
« Reply #7 on: January 16, 2018, 12:05:41 PM »
Set FPS override mode to Low light (that would scale the shutter speed, keeping the shutter angle constant), then adjust the timers to match the values from Exact FPS (to get as close as you can to 25.0 FPS).
Thank you for the quick tip. :)

BTW, tried decreasing timer A below 410 in 720p50, but at 408 the image goes bad.
Too bad… Thanks for trying!