VDP Resolution

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Okie
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VDP Resolution

Post by Okie » Mon Aug 02, 2021 2:20 am

The normal resolutions are 320x224 ,256 x224 , 320x240 and 256x240.
Did the Sega Interpolate/change this resolution to the one on the CRT screen or did it make a black border around the TV and not change the resolution? How does it handle computer monitors? If the Sega were on CRT analog devices that didn't use pixels does it have an encoder decoder or something? I asked a lot - sorry about that. Thanks.

TmEE co.(TM)
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Re: VDP Resolution

Post by TmEE co.(TM) » Mon Aug 02, 2021 4:57 am

224 vs 240 has 8 pixel borders in top and bottom, but 256 vs 320 is filling the screen horizontally, with non square pixels in 256 (just like Master System and (S)NES stuff. Pixels are longer in the video signal output by VDP compared to shorter ones of 320, there's no really good ways to cleanly approximate it digitally other than using some high enough resoultion such as 1280 where 256 fits in 5 pixels and 320 in 4.
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Okie
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Re: VDP Resolution

Post by Okie » Wed Aug 11, 2021 4:47 pm

Thanks do u mean that 224 Vertical has a 8 pixel border. In my test programs , shown below V28 / 224 has 4 pixels border long on top and 4 pixel border on bottom, not 8 in top and 8 in bottom. Im Using Regen in 320x240 window size. Do you mean that also , 30V / 240 Doesn’t have top bottom border? I’m confused as to the window size vs resolution. How will there be 8 pixels on a tv that is huge. Wouldn’t their be a smaller area of game and a huge border ? Are all the TVs from a certain time period a certain resolutions regardless of tv monitor size? The Sega2f manual says a cell is 8x8 pixel so how can it stretch them in 256? Thanks again! :)
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TmEE co.(TM)
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Re: VDP Resolution

Post by TmEE co.(TM) » Wed Aug 11, 2021 10:26 pm

A TV often does what is referred to as the overscan, which means that only some part of the full image gets shown on the physical screen. There is no direct pixel mapping going on and because of this a game is not supposed to put anything in the edges of the screen since they are simply not guaranteed to be visible on all the TVs. This should be described in the official documentation too.

You're supposed to avoid 16 pixels from each side for text or other things that absolutely must be visible. 8 pixels on every side is usually hidden behind TV's edge but things vary in random amounts, different TVs will show a different amount.

The emulator examples are correct for 320 pixel wide mode assuming a TV that shows all of the visible image and doesn't eat up anything, but the 256 pixel wide ones are wrong, you don't get a narrow pillarboxed image in middle of a screen but something that fills it. The aspect ratio is wrong on the emulator shots you showed as far as 256 pixel resolution goes.

Only reason they are so in that particular emulator (and many others) is because there is no good looking way to stretch 256 pixels to fill the space of 320, not in the low resolutions. I described the least ugly way to show both 320 and 256 pixel modes with roughly correct aspect ratio in my previous post and I'm sure some emulators do such a thing for the best result that reflects how things would look on an actual (CRT) TV.

A CRT TV/monitor is able to show arbitrary resolutions without looking ugly (with some gotchas), but stuff such as LCDs or computer graphics where you have fixed pixels instead of a continuous stream of analog data will have quantisation issues, you just cannot have fractional sized pixels so to speak, think of showing resolution on your computer that is not matching the native resolution of your display, it just looks bad. This is the trouble the emulator had to face.
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