Under Defeat explosion slowdowns not properly emulated

[oldmadmax]

I want to replicate on flycast this cool dramatic slowdowns in Under Defeat on real dreamcast when boss dies , using bomb ,during big explosions (video example below):

https://youtu.be/4bfyqYDMoQg?feature=shared&t=152

I was able to replicated this effect by underclocking flycast to 100 mhz but im not sure maybe there better way to replicate it, maybe its emulation flaw.

So i have few questions:

1. how Under Defeat on real dreamcast doing this
2. is game also underclock on real hardware and to what value
3. on the old nulldc there is a check option "underclock fpu" that would emulate this slowdowns (not sure how that work)

Pls help me run UD on flycast as close to real hardware as possible

[flyinghead]

1. I haven't played this game on real hardware but I suspect it's the GPU that can't keep up the 60 FPS frame rate when these large explosions happen. This might be caused by the large amount of transparent geometry being drawn at these times: from 500-1000 to 3000-4000 polygons. If you look at the video frame by frame you can see some frames being skipped when explosions happen. Given that the video is at 30 FPS, it means that 2+ frames are skipped, leading to this slow motion effect.
2. There's no way to underclock the SH4 on real hardware so it's not what the game is doing. However underclocking produces a similar effect because the emulated CPU can't keep up with the 60 FPS frame rate.
3. nullDC (and reicast) use a hack where the FPU is (infinitely!) overclocked. This isn't the case in Flycast anymore so there's no need for such a setting.

So I believe this effect isn't properly emulated on Flycast because the emulated GPU is much faster than on original hardware. The emulated render time should be increased when lots of geometry is being drawn, particularly transparent ones but it's hard to get right since it's not documented anywhere.

[oldmadmax]

Thank you so much for looking at this seams like emulation flaw.

I think i found pretty good test for slowdowns for you to try (considering that player dont do anything there):

notice slowdown on 4 boss explosion in this video - it start from tower explode , then keep slowdown when tower burns and totally explode
https://youtu.be/T8T7YiO7fSU?feature=shared&t=1038
exactly same happen on 2 loop:
https://youtu.be/T8T7YiO7fSU?feature=shared&t=2536

i was unable to trigger (keep) similar slowdown during tower burns (after first explodsion) even on cpu underclock to 100mhz or nulldc with FPU underclocked

here the memory card save with replay so you dont even need to play , to use it chose practice from game menu , then load replay , and pick nr 10 replay . Not sure if flycast memory card file is compatible with real dreamcast to repeat the test on real thing

https://www.mediafire.com/file/6ls27le3mkkyqb8/vmu_save_A1.bin/file

hope that helps