Flashback is supposed to have this shape, that is quare pixels and thus
(256x256) a square resolution. No stretching.

So aspect correction don't do stretching, yes? So my two question remains.

1. What do aspect correction?
and
2. Why only "surface" method at least center this square screen, and other modes is not?

Try a 320x200 game.

wd wrote:

Try a 320x200 game.

I tried already. Many times. 😀 And it seems as "stretching" for me. But I cannot say, that I am good in technical aspects - so I ask that question. Ah, forget it.

Question 2: Do the DOSBox works as intended? If I see "centered" screen in "surface" mode and "left-placed" screen in any other fullscreen mode in Flashback - is it must be so, or something wrong with my system/DOSBox?

its how the other rendering engines work.

Aspect correction in "surface" outputmode is slightly limited in that it can only duplicate lines to at least give a fullscreen appearance for old dos resolution that needs vertical stretching. Should probably use a hardware scaling output if and a fixed resolution for it to look okay.

I had hoped it would have been answered by now, but it is not, so I guess I will have to ask:

What is the purpose of "aspect correction"?

I am not interested in "how", I am interested in "why". What is the visual problem that aspect correction is supposed to fix?

Um someone invented the non-squarepixel 320x200 mode, and if you
render that on a regular mode (windowed say 1024x768) it looks wrong
because there you have square pixels.
Aspect correction stretches the image vertically (if the target output mode
can handle this) so the 320x200 will become 320x240 (nice 4:3 with now-square
pixels) and it looks better depending on the interpolation algorithm.

The "why" is best figured out by drawing a mathematically correct circle
in 320x200, which looks squashed. This is its true look, and when displaying
the same circle in some windowed, non-aspect corrected mode, it's a perfect
circle, which is wrong.
Perfect circles aren't bad, but most games rendered their output so it *looked*
good in 320x200.

Thank you, wd and franpa, now I think, I understand enough from it, and receive answer for my questions. 😀

Added to the Wiki - http://dosbox.sourceforge.net/wiki/index.php? … render%2Faspect

Dunno if that qualifies for the wiki, but Harekiet has to review and update
the whole wiki anyways 😀

We have a wiki?

Bah, don't click that FAQ link on the homepage.

We have a FAQ? amazing... 😀 That dosbox wiki hurts the eys though.

That was before your large wiki-rewrite. Blue-green wiki please.

wd wrote:

and it looks better depending on the interpolation algorithm.

ScummVM has a simultaneously fast and good looking vertical stretching algorithm that may be of some use in surface mode..
http://scummvm.svn.sourceforge.net/viewvc/scu … 024&view=markup

Thanks for the link, it's bilinear interpolation 😉
The easiest thing to get correct aspect ratios is going fullscreen,
this works just fine for the very most output+windib/directx combinations.

1#define	kVeryFastAndUglyAspectMode	0	// No interpolation at all, but super-fast
2#define	kFastAndNiceAspectMode		1	// Quite good quality with good speed
3#define	kSlowAndPerfectAspectMode	2	// Accurate but slow code

I was thinking of their "fast and nice" aspect mode, which is visually close to bilinear filtering, but cheats a bit in order to run faster...