Re: [hatari-devel] Feedback SDL2 support

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Hi,

On torstai 14 toukokuu 2015, Thomas Huth wrote:
> Am Thu, 14 May 2015 14:47:40 +0200
> schrieb Nicolas Pomarède <npomarede@xxxxxxxxxxxx>:
> [...]
> 
> > But maybe we could also remove 16 bit output too and keep only 24 bit
> > full RGB ? I doubt many devices are not capale of 256 colors abd only
> > capable of 65536 colors.
> >
> > And in all cases, shouldn't SDL handle the conversion itself if
> > Hatari outputs a 24 bit RGB image and the display device is only 8 or
> > 16 bit ?

Yes, SDL does conversions automatically (with little help
from application itself).


> > Might be a little less efficient, but that's not even sure if the
> > video card driver has some accelerated function to do this.

Conversion is so simple that properly done (SIMD in SDL), it's memory 
bandwidth bound, not CPU bound, so HW acceleration doesn't help with it.
Only the Hatari part of the conversion (Atari planar -> SDL chunky)
is CPU bound.

Overhead of that extra memory bandwidth usage [1] is *very* visible
with ST emulation on devices that are barely fast enough to emulate ST
(e.g. something like Rasberry PI).

[1] AFAIK 24-bit is typically handled as 32-bit (24-bit can be
    slower than 32-bit due to alignment), so 2x more memory
    bandwidth is needed.


> I've still got an netbook computer (Toshiba AC100) which I use from
> time to time while I'm travelling - and that one can only do 16-bit
> color resolutions. It also has a rather poor CPU, so doing the
> 32-bit to 16-bit convertion additionally would slow down Hatari too much
> on that device. So as long as I've got that netbook, I'm still willing
> to maintain the 16-bit convertion routines.

Btw. While 8-bit palettized resolutions are ugly, Hatari's VDI and
monochrome modes have only 8-bit conversion routines.  Getting rid
8-bit host side code would mean that those would now always go
through second conversion.

How your netbook performance would survive that?


	- Eero



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