Aspect ratio - why circle is not circle?

[rotacak]

I like to ask a question about the aspect ratio. I have a 16:9 monitor and when I will set in a winuae in a filters 16:9 monitor (that is my monitor) then it will give me the best result.

But still when I will measure the circle sizes (from dpaint) there are small differences. The Amiga picture is little wider. See the picture - screenshot from a dpaint and that dotted circle is from the photoshop.

Why it is deformed? It was like that even on a real Amiga?

[Dunny]

Your scaling is off. Pay attention to some of the horizontal pixel rows in your image - some are taller (fatter) than others, but all the "fat" ones are on the same rows. So it's not the Amiga - WinUAE is at fault here, but knowing Toni it's your usage of the settings.

[Toni Wilen]

First I'd like to know if it is "correct" even in real world. It is not good idea to assume anything from era of old CRT TVs or if there is some PAL/NTSC adjustment side-effects and so on.

[Dan]

I would explain this so:

Dpaint on amiga has another algorithm to draw Circles than Photoshop (which is on the PC) has.

For example, DPaint has to ensure that the circle is really a full circle, without breaks in the circle.
If you have taken a screenshot from winuae, then this screenshot is probably scaled in a way.
So if you draw a Circle, from a Photoshop, over a scaled screenshot, it wont give you the same results.

Rotacak, i would ask you to save the picture with the circle, in dpaint, as iff file, and then to load that file into Photoshop.

If photoshop can not load iff files, then convert the iff to BMP, without changing the resolution. Then make another circle comparison.
(if you want to do this, then make sure that the circle is NOT dotted, but has to be continuous)

(sidenote, the term Subpixel just came as a thought in combination with Photoshop )

[dodke]

DPaint might not take into account the pixel aspect ratios of PAL or NTSC and uses simply 1:1 which is 'good enough' for computer graphics and probably looks better in low resolution anyway. It's not for print.
(As you might know PAL 4:3 image on Amiga low res is generally 320x256, NTSC is 320x200 and 1:1 pixel aspect would be 320x240)

[Mclane]

Also take in to account that peoples screen are rarely ever set up properly on a crt...

[Foebane]

Originally Posted by Mclane

Also take in to account that peoples screen are rarely ever set up properly on a crt...

Exactly, CRT raster displays can move and stretch and squash around the CRT screen very easily. I'm sure that most of them (especially Amiga monitors) had controls to move the X & Y and likewise "size" controls for both axes.

[Retro-Nerd]

This is how it looks on a real Amiga (640x256), Sony Trinitron 4:3 TV.

[Rotareneg]

The original "official" aspect ratio for Amiga NTSC pixels was 10:11, later revised to 11/13. I think PAL was always assumed to be 1:1 although 320x256 doesn't exactly match the 4:3 display either, but I guess they figured it was close enough to not matter.

Under the Prefs menu in DeluxePaint there is a "Be Square" option that compensates for Amiga NTSC pixels not being quite square.

[Retro-Nerd]

This "Be Square" option is set as a default though. Just checked it.

[Rotareneg]

In DP IV and up, but in DP III and older it wasn't.