After much tinkering around and testing, I came to the following conclusions:
1.) Double Strobing Illusion: It is indeed visible on CRT when displaying 60fps animation at a 120Hz screen refresh. It is pretty subtle on most content, I didnt even notice it at first until I looked for it. Its most noticeable on fast moving sprites or backgrounds, and almost completely unnoticeable on slower animation. Its one of those things where once you see it, you cant "unsee" it, but the illusion (and I use that word on purpose, because it is indeed an illusion and is not able to be captured by video or photograph) , in my opinion is subtle enough even on fast moving content to not make it a dealbreaker. Opinions will vary on that, of course.
2.) Black Frame Insertion on 60fps 120Hz Content: When objections arose concerning the double strobing effect, I argued that black frame insertion could be used, as an alternative to motion interpolation, to alleviate the double strobing effect. Some folks disagreed that BFI would work on a CRT like it does with LCD or OLED.
After training myself to easily recognize the effect in specific areas of certain games, I used the BFI feature in Retroarch and carefully observed those same areas again in those same games and I can say with 100% confidence that the BFI completely eliminates the double strobing illusion. This comes at the cost of significant brightness loss vs the non-BFI 240p120 mode.Xer Xian wrote:The double-image effect of repeated frames on a CRT cannot be alleviated with BFI.
3.)480p60 with Full Artificial Scanlines: The question arose-
Xer Xian wrote:I think the question here is what's wrong with line-doubling 240p and adding full-intensity fake scanlines?
Fair enough, seems like an honest question and to be frank, I hadnt thought to compare it at that point. So, I did some in depth comparisons of 480p60 with full artificial scanlines vs true 240p120 and made several observations. First, at least on CRT, the true 240p120 mode is significantly brighter than 480p with scanlines. This manifests itself, as it was also discussed, in the appearance of thinner, more natural looking (in my opinion) scanlines than the 480p60 + artificial scanlines mode. So indeed there is a significant visual difference between the two modes, at least on CRT.
4.)240p120 w/ BFI vs 480p with Full Artificial Scanlines: So during the course of testing all these modes, I made an interesting observation-- 240p120 w/BFI and 480p with full artificial scanlines are virtually identical. In overall brightness, they appear to my eye as identical to each other. In terms of scanline thickness, again, identical. Per Fudohs comments in the OSSC Pro thread, and my own previous experiences with 480p modes, I was not expecting this, but I can not detect a noticeable difference. Finally, in terms of the double strobe illusion, it is completely absent from both modes.
In conclusion, I still stand by my original stance that a 240p120 mode is worth implementing on the new OSSC. It is far brighter and has more natural looking scanlines than the alternatives. After comparing 240p120 BFI and line doubled 480p with full scanlines on CRT, it would be tempting to say that a 240p120 BFI mode would not be worth the trouble to implement as its indistinguishable from line doubled 480p w/ full scanlines, BUT-- I believe that the 240p120 BFI mode would be merited on 120Hz LCDs, where it would not only alleviate the double strobing illusion, but also greatly reduce blurring that is prevalent on 60Hz content on LCD screens.
Finally, I took some photographs of each mode and carefully stitched them together. I used a shutter speed of 1/20 sec with an ISO of 1600 for each shot, and shot them from the identical location on a tripod. I was very pleased with how they came out as they represent the differences when viewed live very closely, even if the 240p120 default mode is a bit overexposed. The differences you see here are very , very close to the differences you see in person.
Full Size: https://i.imgur.com/RAvJmYV.jpg