[Scummvm-devel] Moving platform specific (down)scalers into the core of ScummVM

[Max Horn]

Hi John,

several backends need downscalers, and would benefit from ARM  
optimized versions of those (and might even benefit from carefully  
optimized C/C++ versions, for that matter). So it  certainly makes  
sense to have some of those in graphics/scalers -- at least as long as  
they are only compiled on ports that want them. (Which is easy enough  
to achieve). In particular, yes, downscalers are useful.

Actually, we listed this a long time ago as one of the desirable  
features for the ominous "small devices meta backend", see <http://wiki.scummvm.org/index.php/Small_Devices_Backend 
 > ;).


The other issue is how to make it as easy as possible for backends to  
use downscalers; i.e. how to interface with them, what API to use. For  
a stand-alone backend, such as the NDS one, this is probably not  
really an issue. Most of your questions seem to be only relevant to  
backends that are based on the SDL backend and wish to reuse as much  
of its code as possible; in particular, those backends ideally would  
like to just tell the SDL backend to use a downscaler and be done with  
it. As you already pointed out, the SDL backend is not well prepared  
for this.

So, we could try to adapt the SDL backend to deal with that. But I  
think that would make an already overly complicated rendering pipeline  
even more complicated... So maybe here's a good opportunity to  
introduce a OSystem_SDL_SmallDevice subclass, which provides a  
graphics pipeline custom tailored towards downscaler mode...  That  
might help to keep the code clean, and would still allow backends  
derived from it to use both up- *and* downscaling (the backend can  
choose whether it invokes the OSystem_SDL or the  
OSystem_SDL_SmallDevice renderer). Alternatively, this functionality  
could be added directly into the SDL backend.

As for the overlay: I think on *any* port, whether it is SDL based or  
not, the overlay should ideally be unscaled, i.e., use the native  
screen resolution. At least in theory -- in reality, we don't define  
themes for resolutions beyond 640x480, and while our GUI is vector  
based, the fact that we don't use vector fonts means we can't scale it  
arbitrarily without custom themes... So, in the real world, if your  
native screen is around 320x200 or 320x240 or 640x480, use that  
natively for the GUI; if it's bigger, then use an overlay size close  
to 640x480.


Bye,
Max