Runahead causes collision issues with some Game Gear games

[msheehan79]

Enabling Runahead on some Game Gear games causes collision issues, for example on Fantasy Zone shots go through enemies and you cannot be killed.

This was tested with RetroArch 1.2.1 nightly build on Windows x64, using the latest Genesis-Plus GX build f64e0a1 - all default settings for RA and the core (apart from enabling Runahead)

2 games that are easy to reproduce the issue would be Fantasy Zone and TaleSpin.

[ekeeke]

See ekeeke#596

Any game that relies on VDP sprites collision flag will have issue when rendering is skipped (which runahead features afaik does for a few frames) as this flag is set during line rendering (when sprites are rendered more exactly).

[msheehan79]

@ekeeke that makes sense, thanks for that issue link. Is there any sort of resource or list of known games that use this flag? It might be helpful to document here for future reference if so.

Also not sure if relevant or not, but the SMS Plus GX core does not exhibit this issue, so that is an alternative for those who want to use runahead features with these affected games.

[ekeeke]

Is there any sort of resource or list of known games that use this flag? It might be helpful to document here for future reference if so.

I am not aware of such list.

Also not sure if relevant or not, but the SMS Plus GX core does not exhibit this issue, so that is an alternative for those who want to use runahead features with these affected games.

Looking at libretro SMS Plus port, it does not support frameskipping so i am not sure runahead feature properly works with it (no support for RETRO_ENVIRONMENT_GET_AUDIO_VIDEO_ENABLE callback).

[msheehan79]

Looking at libretro SMS Plus port, it does not support frameskipping so i am not sure runahead feature properly works with it (no support for RETRO_ENVIRONMENT_GET_AUDIO_VIDEO_ENABLE callback).

It does seem to work on that core based on my testing using the pause + frame advance keys - I do see a drop in response frames based on the runahead configuration. Not sure if if the missing callback has some other impacts though.

In any event, thanks for the info on this. While I expect this likely won't be fixed based on how runahead works I'm hesitant to close this as it may be useful to keep open for awareness at the very least.