Micro Machines (Master System) : Graphical glitches

[tuzepoito]

I have noticed various graphical glitches with the game Micro Machines for the Master System, with what appears to be misaligned sprites.

Strangely enough, I tested on the latest SDL binary of Genesis-Plus-GX compiled on Windows, and with the SDL binary, the bug doesn't appear.

ROM: Micro Machines (Europe) (Master System) SHA-256 hash: CFBC86DECCDCD31B64F886B1C344CC017E39C665928820BDFA16AE65D281057A
OS: Windows 10
RetroArch version: 1.20.0
Core: Genesis-Plus-GX 1.7.4 99678a0

[ekeeke]

I can confirm it does not occur upstream (see screenshots below) so it might be caused by some libretro core specific settings or modifications

[tuzepoito]

Thanks for the comment. On the other hand, I've tried with a fresh (portable) install of RetroArch and the issue still happens.

OS: Windows 10
RetroArch version: 1.21.0
Core: Genesis Plus GX v1.7.4 f52a3c1

[KamiLordus]

Same problem here, on Windows 11.
It seems that the only solution is to go into core options and change some options (e.g., aspect ratio).
Then the graphical errors are fixed (at least until the next time you run the game).

RA 1.21 latest core
Genesis Plus GX latest core

[ekeeke]

Just a thought: this game makes use of Master System 2 extra video mode (256×224) and above graphical errors look a lot like errors you would get when running this game on Master System 1 (which is limited to 256×192 video mode) so maybe your libretro core is forcing Master System 1 model emulation for some reason ? This can be changed through System core setting (although Master System 2 model should be picked by default for this game when set to AUTO).

Also check VDP mode is set to AUTO or 50hz for this game, as i think it also glitches when running in 60hz mode.

[KamiLordus]

@ekeeke: No matter if I set all core options to default or change the Master System Model II emulation, the game still has the same graphic bugs as in post 1.

As I mentioned earlier, changing the aspect ratio to a different one (e.g., 4:3) and returning it to the default setting will cause the game to display correctly (without errors). But this only works once, until the next time you launch the game.

P.S. Don't look at my fps counter. It's only 120 in the RA menu when I took screenshots, normal in-game 50hz, 50 fps.

[ekeeke]

Looking at libretro.c in this repository, i think i found the cause of this bug.

The following lines force the value of bitmap.viewport.changed, clearing any value set by the emulator (during emulation of previous frame)

Genesis-Plus-GX/libretro/libretro.c

Lines 3968 to 3970 in a69c81e

	if ( ((system_hw == SYSTEM_MARKIII) || (system_hw & SYSTEM_SMS) || (system_hw == SYSTEM_PBC))
	&& reg[0] != reg0_prev)
	bitmap.viewport.changed = 9;

The problem with that code is that, when a SMS game activates extra height mode, the emulator sets bit 1 of bitmap.viewport.changed (using 0x2 as OR mask) so it is taken in account at the start of next frame (bit 1 is checked at the start of system_frame_sms) but the above code in libretro.c will simply discard the setting of this bit and the display height will not be changed. When you change core setting related to display, libretro.c will force bitmap.viewport.changed to 11 so bit 1 will be forced to 1 again and the emulator will apply the display height change.

The solution is to replace = 9 in above code by |= 9, in order to preserve any bit set value.

Also, the check for left border change should be a bit less loose as VDP register 0 is not only used to enable/disable left border masking but other things as well (like display height change) so the condition reg[0] != reg0_prev should be replaced by ((reg[0] ^ reg0_prev) & 0x20) to detect only VDP register 0 bit 5 changes.

Note that this bug probably affects all the few other Master System games that use extra height modes, see https://www.smspower.org/Tags/Extra-Height

[KamiLordus]

I checked the other games:
Fantastic Dizzy - seems to work fine (both in sms and gg mode).
Excellent Dizzy Collection - also OK.
Cosmic Spacehead - I noticed problems in the menu. Frame shifting, similar to Micro Machines.

[tuzepoito]

I've tested with Fantastic Dizzy and it does seem to work fine. I notice that out of fullscreen mode, running Fantastic Dizzy changes the window size at the language select screen. However, in Micro Machines, the window size is always the same.