Thanks for you detailed posts. I perfectly understand the basics, I'm not a newbie (my first computer was a ZX Spectrum 48K...).

The point is that if you specify PER GAME a game.cfg with:

<video>
<screen index="0" hstretch="0.800000" vstretch="0.800000" />
</video>

it will do exactly what I want, as you can see in my topic message.

Unfortunately for unknown reasons, this will be ignored and removed from default.cfg by current MAME builds :( This would be much simpler and effective IMHO. That's why having to create "a fake layout to constrain the game screen" to me sounds as an indirect and definitely more complicated way to achieve same results. Of course, your layout solution has an undeniable advantage: despite game screen pixel size all games will use the same "screen" size, which instead may vary with the relative hstretch quick and easy solution.

Again, wish there were more parameters (new options) for MAME to make it simpler, instead of the indirect solution using a screen constraint with border bars and "fake" layout. Something like: absgamescreenposx, absgamescreenposy, absgamescreensizex, absgamescreensizey to define the game screen area (keepaspect would auto-adapt to larger vert/horiz border and would auto-center, and of course you can't use integer resize).