@ironic i dont understand ...
no one said all emulators have a 4:3 ratio.
NES is 16:15
SMS is 4:3
GBA is 3:2
GB and GBC are 10:9
And as far as i remember, double dragon is 15:14 (indeed, it's 240x224 -> ratio of 1.07)
How does retroarch stretch then ? Does it stretch to simulate a 4:3 screen, so that, indeed, no pixels are square (but SEGA) ? Or does it simply multiply X and Y byt the same factor, keeping the original ratio, having square pixels, but not simulating an 4:3 output ?
I'm at work, can't experiment for now ..... But i'd say square pixels, so it keeps the original ratio. The double dragon example proves it
NES is 16:15 ratio ?
Never.
NES screen is done to be seen on a 4/3 CRT in full screen, so NES is 4/3 Aspect Ratio.
Wiki :
The aspect ratio of an image describes the proportional relationship between its width and its height
256/240 (or 16/15 ) is not the Aspect Ratio.
This is a NES screen (256x240) X4 so 1024*960
Do you thin aspect ratio is correct ?
This is a true 4/3 aspect ratio of a NES screen.
It is pixel perfect (or integer scale on) so the scanline is perfect.
This is a 4x5 screen (1280x960) but the result is 4/3 screen.
Pixels are not square on TV, NES and others plateformes.
Double Dragon is made to be seen on a arcade monitor, a 4/3 screen in fulsscreen.
So is aspect ratio is 4/3 but not X pixels/Y pixels.
CPS1 games have 384x224 resolution. 384/224 = 1.71 !!
Is done to be seen on a 4/3 screen. 4/3 = 1.33 !!
Why ? Pixels are not square.
Never mind the resolution
A game is done to be seen on a 4/3 screen in full screen ??
Yes !
Ok, this game (256240, 320240, 256192, 384224...never mind....) is 4/3 aspect ratio.
just tried Mario. The core scales the game to 1440x1080 on a 1080p screen. Which means it was upscaled in 4:3. In other words, any console meant to be played would have the same fullscreen viewport ? I tried with SMS, NeoGeo, SNES : always the same viewport size. Which makes things rather easy for non PP display
So the question now is to calculate the screen size when PP is activated
a little reading : http://libretro.com/forums/showthread.php?t=1471&p=15087&viewfull=1#post15087
just tried Mario. The core scales the game to 1440x1080 on a 1080p screen. Which means it was upscaled in 4:3. In other words, any console meant to be played would have the same fullscreen viewport ?
On your link, you can see all consoles use 240 pixels (vertical) but not all viewport is used.
SMS use only 256x192 but the resolution is 256x240.
Your link talks about non-square pixel.
There is a lot resolution and lot of game console, so we must only try to respect 4/3 format on your monitor.
3/4 for Arcade vertical games.
For best image quality, we must use Integer Scale.
(PP or Pixel Pefect can be used only with square pixel).
For all 4/3 games, we must use :
On 1280x1024 monitor => 1280x960 resolution.
On 1280x720 monitor => 960x720 resolution.
On 1920x1080 monitor => 1280x960 resolution.
(LCD pixels aren't exactly square, lets forget that).
But...
Everything I have said is false
Resolution is 240 (vertical) but only 224 pixels are displayed.
So...
The real aspect ratio of a game is 4/3 in fullscreen.
The displayed pixels area is 256x224 ou 320x224 ou 384x224...
On a fullHD (with Perfect Pixels) the screen with Integer Scal must be :
(Vertical) 1080/224 = 4.82 (keep 4).
2244 = 896
896/34 = 1194.6 (keep 1194)
X = 1194 and Y = 896
On my 1280x1024 5/4 monitor.
1024/224 = 4.57
2244 = 896
896/34 = 1194.6 (keep 1194)
X = 1194 and Y = 896
4/3 = 1.333333......
1194/896 = 1.3325....
This is (for me) the "perfect way" to display a NeoGeo (or other 4/3) game on a 5/4 LCD monitor.
For those who say, the honrizontal scale is not an integer scale, i answer :
Remember the PAL/NTSC resolution.
NTSC (525 lines fo 720 × 480) PAL (625 lines for 720 × 576).
On hi resolution, 1280x1024 or 1920x1080, we can use non-integer scale, 99% of people never seen the non-integer pixels.
Without scanline, nothing is visible.
All these "equations" gives a headache ? Sure...
But what we (you) want to do ?
This post is for Overlays but for perfect overlays, a screen ratio must be definied.
@ironic all of your posts give such a headache lol But at least we move forward 
But if i want to sum up what you wrote :
I guess that sets it for consoles ?
If so, here are the margins left for each mode :
1080p has space left everywhere
720p just has 160px left on left as well as on right => we can't let retroarch shrink a 1080p overlay
1280x1024 as 32px left on top as well as on bottom => worthless
What about arcade (forget NeoGeo) ? Looks like those rules fail as we can't set a generic viewport : Mame handles the real resolution of games, and nothing is stretched to 4:3 unless forced or native ? That would mean a per game viewport
One more thing : i thought PP and IntegerScale are the same ... but no ? PP is when we strictly respect the original resolution without stretching it to fit 4:3 ?
I dont agree with this: The integer scale resolution is the same for any console,
@supernature2k indeed you are right ... As it depends on the native console resolution stretched to 4:3
@supernature2k @subs
Here are the screenshots, we were talking about earlier:
System: SNES
Monitor: 19" TFT LCD
Connection: HDMI -> DVI
TV Mode: DMT 35 DVI
Game: F-ZERO
Screenshots: raspi2png
Shader: Scanlines
Integer Scale: OFF
Smooth: ON
Ratio: Auto
Shader: Scanlines
Integer Scale: ON
Smooth: ON
Ratio: Auto
I don't want to bother you... What you are doing seem very complicated to mee ^^
But maybe I can help you.
UXS can now generate Picture with Resolution and Ratio configuration.
Maybe it can scrape a picture dedicated to overlay.
Something basic with wheel or Marquee (maybe 3D or 2D Box) but autogenerated for every game (Console & Arcade)
(Just to let you know this is thing UXS can do in a very near futur : Teaser )
@subs
All you wrote is ok.
If we want to use overlay all around the game screen, it's better to use Interger Scale.
But if we want bigger or thinner overlay, we must disable Interger Scale.
As several consoles haven't square pixels, we must disable Integer scale.
First, we must do a choice. What do we want ?
Is the overlay will be enabled game by game ?
Or console by console ?
I don't know what people wants.
Most people are not interested in all that we say here. They just want nice quality games and fun.
For Arcade games, is more difficult.
There's several resolution but we can keep 4/3 aspect ratio (or 3/4).
Another problem is RetroArch crop the game screen.
Some NeoGeo game use 320x224 and other 304x224 but RetroArch resize the screen.
Some game (like Sega M.S.) have a "no pixel area border" on left.
I think we must define a generic viewport of 4/3 with enough space for overlay.
Pixel Perfect and Integer Scale are 2 differents things (for me).
Integer Scale means that all honrizontal pixels have same size, all vertical pixels have same size,
But honrizontal size and vertical size of a pixel can be different.
On my Bartop, one NES pixel is 5 native pixels (honrizontal) and 4 native pixels (vertical).
Pixel Perfect means all pixels have same size, honrizontal = vertical.
Pixels are square.
Recalbox wiki say Integer Scale = Pixel Perfect.
Why not. It's ok for me. But i prefer use Interger Scale.
Pixel Perfect is for browser or photoshop users...
@screech
Yes, it's very complicated.
Shall we try and compare quality overlays.
I will try your new UXS soon...
This is how I see things according to what I've understood so far :
Of course, this is my personnal opinion. I think we put things plain, time to decide ^^
For consoles, based on the link i posted earlier on the libretro forum, this is what i suggest as resolutions :
299/224 = 1.3348
It's the nearest 1.3333 CRT (4/3) aspect ratio.
If our monitors have a square pixel, everything is good.
If the pixels are not square, we must recalculate the resolution.
On my LCD1280x1024 IBM ThinkVision 17 "5/4:
X = 340mm
Y = 271mm
340/1280 = 0.265
271/1024 = 0.264
A pixel is 0.26mm x 0.26mm
We can say that the pixels are square.
Somebody can calculate pixel size on 16/9 monitor?
Various resolutions will be used:
1280x1024, 1280x720, 1920x1080
The basis for the resolution is 4/3 of 299/224.
With a vertical Integer Scale and the largest image:
1280/1024:
299/224 × 4 = 1196x896
1280 * 720:
299/224 x 3 = 897x672
1920/1080:
299/224 × 4 = 1196x896
With this resolution, image quality will be the best, with and Without scanline shader.
And give area for an overlay.
Arcade games resolution and other:
Arcade Infos Games
For arcade games, overlays are very differents. It's impossible to set a resolution.
Each game must have his resolution.
Very nice job ironic.
Concernning lcd monitors square pixels, let it be a dogma : they are square, can't bother for every recalboxer ...
For arcade, as I said earlier, the aim of the topic is to go step by step, and find some rules and scales for overlays. I'd love to offer some masks (png / psd) for people (like @screech) who want to generate overlays
so far we haven't spoken yet of handhelds, but they all have a lcd screen, just need to find their resolutions, and their integer scale values for 1080p and 5:4
@ironic the SMS is the only one with a resolution different from 299x224 : 256/192, which is 4/3. So this perticular overlay should be slightly different than others. We also means quite some consoles, namely vectrex as you've worked quite much on its overlays
As, quite often, most consoles have several resolutions, should we force a 4/3 custom ratio in retroarch or can wa consider afterall that those consoles would anyway render to 299x224 ?
For the pixel aspect ratio of any screen : you can do it much faster. The panel size ratio MUST equal the resolution aspect ratio.
In yur case, you just pray that 340/1280 = 271/1024 => 1280/1024 = 340/271. 5/4 = 1.25, 340/271 = 1.262
Anyway, no need to check monitors, that's a user problem that we can't solve through recalbox as it depends on parameters (the real lcd panel size) that we can't programmatically get.
Arcade Overlays :
Let's try to scale an overlay :
Dragon Ninja (256x240)
This is a pixel perfect 256x240
To see how it really must be, scale it to 4/3 (320x240)
The Bezel is square, how can we scale it to fit 4/3, 5/4 or 16/9 screen ?
I don't know.
Here, the game screen is very very small...
We can find some 16/9 truncated Bezels for 16/9 as :
(This one is 1.32 Ratio)
The viewport of the bezel is too small. Such a game 320x240 once stretched to 4:3 will be "integer-scaled".
For 1280x1024 : factor 4 => 1280x960
For 1920x1080 : factor 4 => 1280x960, but with bigger borders
This is how I wee things. This example prooves once again that 1280x1024 is not worth for overlays unless you want the viewport to look like a stamp
https://github.com/libretro/mame2003-libretro/pull/53
There has been some corrections about aspect ratio in mame2003. This last pr reverts some stuff. I suppose it is relevant so I post it here.
Oh la la, très très jolie Overlay/Bezel, bravo 
our friends over at HFS made a nice wiki and if you go to http://wiki.hfsplay.fr/doku.php?id=tutos:install_hfsbox15k#l_installation_des_drivers_runtimes_directxand scroll a little down, you have all resolutions for CRT consoles. The rest of the topic is very interesting to read too, despite being in French
The complete list of MAME 0.78 resolutions / La liste complete des resolutions MAME à mettre en exergue avec https://forum.recalbox.com/topic/3475/recalbox-sur-tv-crt-en-rgb/308
1020x1020
1024x768
1024x772
1024x780
1024x800
1024x808
1024x840
1024x848
1024x912
1040x880
1050x900
1056x864
160x200
208x232
224x224
224x232
224x240
232x224
240x176
240x192
240x216
240x224
240x232
240x240
240x248
240x256
248x208
248x224
248x240
248x256
250x280
254x232
256x184
256x192
256x200
256x208
256x216
256x222
256x224
256x230
256x232
256x234
256x240
256x248
256x256
256x288
256x448
256x480
260x240
264x224
264x232
272x216
272x224
272x232
272x236
272x256
276x240
280x216
280x224
280x240
284x240
288x208
288x216
288x224
288x256
292x230
292x240
296x224
296x240
300x260
304x224
304x232
304x240
304x248
304x256
318x222
320x152
320x200
320x204
320x224
320x232
320x240
320x256
320x416
336x238
336x239
336x240
338x236
338x240
352x240
352x256
360x224
360x240
360x244
368x224
368x232
368x240
368x448
376x224
376x240
376x248
380x224
384x224
384x240
384x254
384x256
384x280
394x240
396x256
400x248
400x254
400x256
404x254
410x256
416x224
420x400
432x224
448x224
448x240
464x224
480x440
480x480
488x384
496x232
496x384
496x480
500x440
512x192
512x224
512x236
512x238
512x240
512x256
512x288
512x352
512x384
512x400
512x448
512x480
520x400
540x400
580x400
580x500
580x570
638x240
640x232
640x240
640x480
672x240
720x768
768x240
832x448
864x224
900x600
@supernature2k @subs
Here are the screenshots, we were talking about earlier:
System: SNES
Monitor: 19" TFT LCD
Connection: HDMI -> DVI
TV Mode: DMT 35 DVI
Game: F-ZERO
Screenshots: raspi2png
Shader: Scanlines
Integer Scale: OFF
Smooth: ON
Ratio: Auto
Shader: Scanlines
Integer Scale: ON
Smooth: ON
Ratio: Auto
