Image artifacts for BFI frequency != content frequency

[Baron of Sun]

Hi!

I know about the double image effect when running 120 Hz BFI over 60 Hz content. What is if the BFI runs at any other frequency other than exactly 2 times the content frequency, like for example 62 Hz or 75 Hz where the division of both frequencies does not result in a natural number?

What kind of artifacts can be expected for these scenarios? How would it look like in comparison to the double test ufos from the 120/60 case?

I'm wondering because there are games that not always run at exactly 60 fps. Lets say they run at 55 fps and I activate 60 Hz BFI. Does that still result in a double image, but the duplicate is just less bright, or is the duplicate less shifted to the direction of the movement?

I remember a diagram from blurbusters showing the artifacts for natural number divisors for BFI vs content frequency, but I cannot imagine how the effect looks for non-natural number divisors.

[purplemelon1]

With the amount of questions you have had, i would have assumed you already had a bfi monitor by now. Well 55fps is technically a 1.09 change. So it should be a ufo with a 2 pixels apart ghost or something like that.
Personally i wouldn't expect most people to really visually notice it. Or rather distracted. Maybe even 70% of users on this forum today. Rather they'll know they're at 55fps because it isn't 60fps. I forgot what discorz called it There is motion clarity and motion ___. Strobing is good but you can't beat a real 1000fps for fixed gazed vision.

[Chief Blur Buster]

I know about the double image effect when running 120 Hz BFI over 60 Hz content. What is if the BFI runs at any other frequency other than exactly 2 times the content frequency, like for example 62 Hz or 75 Hz where the division of both frequencies does not result in a natural number?

Here's the common guide at Blur Busters:

So if stroberate=240 and framerate=80, you will get 3 duplicate images.

For in-betweens, you will get a uncomfortable jitter/stutter between X and X+1 duplicate images. So if you have framerate=24 and stroberate=60, you will have the 3:2 pulldown equivalent (triple strobe, then double strobe, then triple strobe, then double strobe). Those are more uncomfortable than integer divisors.

That's why I advocate reducing your refresh rate to make sure your stroberate matches framerate, if you decide to use strobe-based motion blur reduction. That's one of the main disadvantages of strobe-based motion blur reduction.

I'm wondering because there are games that not always run at exactly 60 fps. Lets say they run at 55 fps and I activate 60 Hz BFI. Does that still result in a double image, but the duplicate is just less bright, or is the duplicate less shifted to the direction of the movement?

For 55fps at 60Hz strobed, you'll get 5 stutters per second, and those stutters will be a brief (1/60sec+1/60sec) flash of double image. Which is a more amplified stutterfeel than not using strobing.

YMMV on whether the cons outweigh the pros.

I remember a diagram from blurbusters showing the artifacts for natural number divisors for BFI vs content frequency, but I cannot imagine how the effect looks for non-natural number divisors.

It just means that each occurance of each 1 stutter includes a surge of an extra image. And that can happen several times a second. So if you have 24fps at 60Hz strobe, you have an average of 2.5 images. So you have one sequence of two double images, then another sequence of three images, then another sequence of two images, and so on.

Although I haven't added an intentional stutter-adder, you can still play with software-based double images at www.testufo.com/blackframes#multistrobe=2 (2 image) and www.testufo.com/blackframes#multistrobe=3 (3 image).

Since it's simulating an impulsed display via software-based black frame insertion, you'll need 2 refresh cycles per impulsed refresh cycle (1 for visible frame and 1 for black frame), so doing triple image requires 3 sequences of that, so your framerate during this demo will be 1/6th for the triple-strobe demo (e.g. 20fps+triples during 120Hz), and 1/4th for the double-strobe demo (e.g. 30fps+doubles during 120Hz).

The in-betweens (of 2 and 3) are simply a stuttery mess in between the two, whereupon each occurance of one stutter = one sudden extra image (especially when closer to 2-2.5 average) or one sudden fewer image (especially near 2.5-3 average). I may create a custom TestUFO demo to show what it looks like, based on this inquiry.

[purplemelon1]

I was a bit wrong. It must be worse than I thought with the stutters the chief mentions.
Kyube corroborated a strobed frame rate will feel like its frame rate. posting.php?mode=quote&p=119330

[Baron of Sun]

Thanks for the replies!

Question for Chief: Do I understand you correctly that this non-natural divisor is represented in a time-averaged manner (one time 2 double images, another time 3 double images for your 24/60 example, so on average it's 2.5 double images) and not in a spatial manner because this artifact is globally visible on the display? I ask because I'm thinking of another case and wondering if that behaves the same: I'm experimenting with optical choppers, so the strobing is more like a rolling scan and the movement is independent on the display refresh. Let's say the TV refreshes with 60 Hz @ 60 fps and the chopper strobes with 61 Hz. In this case the rolling shutter is 1 frame faster per second than the display, or 1/60th frame faster per frame. Here we would have a double image on 1/60th part of the display and this part is moving with such a speed that it covers the display height every second. Or in other words: This artifact is not only time dependent, but also spatially dependent.

Am I correct with my thoughts? I'd really love to understand these different behaviors to make progress with my experiment. I'll share more of that experiment in the another thread later.