Re: [hatari-devel] Pixel aspekt ratio

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Of course there is. NTSC 525 lines, PAL 625 lines, from top to bottom evenly spaced. The reason why Atari decided to "start" the NTSC screen at a specific line, the PAL screen at another line etc to make sure the full usable screen was visible on any screen the user might connect to.

_Some_ monitors included the possibility to "zoom" in on the picture since the decisions were made with TV sets in mind, but that doesn't change what the designed - "correct" - pixel aspect ratio of the platform was.

(0.9/1 apparently)

A discussion of the same on the Amiga (which apparently was very close to 1/1 in PAL):

An emulator doing this for the Atari 800:

And here on how you need to compensate when viewing Atari ST images on a modern screen:

I would very much like to have the option (using SDL2) to scale the vertical resolution by 1.11 in Hatari. I can't have been the only one to have been caught a bit of guard making something on a modern computer which looks good in the emulator but didn't when I ran it on target hardware. As Nicolas points out this will not be pixel perfect so it's not something to enable by default.

(As to what the exact value should be I can't offer more input than the previous quote by an Atari ST pixel artist which seems to have perfectly matched what I saw myself)


On Thu, Nov 12, 2015 at 3:41 PM, Adam Klobukowski <adamklobukowski@xxxxxxxxx> wrote:
I do not know about TVs, but with ST monitors, user could stretch screen width and/or height with a toggle, so there is no way so specify correct aspect ratio, as there is none.

Adam Klobukowski

2015-11-12 12:54 GMT+01:00 Nicolas Pomarède <npomarede@xxxxxxxxxxxx>:
Le 12/11/2015 09:58, Troed Sångberg a écrit :
Recently I realized that graphics I made on my Mac and displayed in
Hatari did not look the same as when I viewed it on my ST. On the ST
everything was stretched out vertically.

I couldn't really understand why, so I started looking for discussions
on Atari ST pixel sizes, and found the following:

"and later on the 16-bit Atari ST (low-resolution 320×200 pixels) each
pixel was roughly .9×1"

Going by the above I stretched the graphics by a factor 1.11
horizontally and all of a sudden a photo taken from the graphics as
displayed on the ST matched perfectly what was displayed in Hatari.

Is this a general thing with modern monitors having square pixels so
that an emulator with 1:1 mapping will always display things "compress"
vertically compared to what was originally designed for target?

If so, should Hatari offer the option to not do 1:1 mapping but stretch
vertically to try to display content as close to what it would look like

Sorry if this is a topic that has been debated to death before, it was
new to me when I stumbled upon it.


that's something I already read about, but I don't think it was debated here.

Problem is : how do you stretch horizontally for example (or vertically in the other direction, that's the same) to not get visible artefacts ?

If we consider a 320x200 ST image should be displayed on a 352x200 area on modern display, this means that at one point 32 pixels will need to be duplicated. Which ones do we chose ? Going from 320 to 352 will sure look ugly.

So, to get proper 1.11 ratio we need a much bigger zoom before, maybe x4 or more. A 320x200 ST image would become a 1420x800 area ; with such an increase you can apply some more effective filters for non integer zooming.

In the end, I think it comes down to another request already made here : being able to zoom the ST image to any non-integer value. Today, we only have x2 zooming, with SDL2 and some additional code, we could have for example x3.3 zooming using HW accelerated opengl and some filters.


Semper Fidelis

Adam Klobukowski

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