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Last updated on Oct-18-2019 Download

Software interlacing on the NES ?

Software interlacing on the NES ?
by on (#19147)
Is it possible to do software interlacing with the NES to increase its actual resolution from 256x240 to 256x480 ? I mean, since TVs exept the video signal to be interlaced at 30FPS NTSC (or 25FPS PAL) and that the NES does not interlacing and have lower resolution but higher framerate, is it possible to trick the TV to make it thing there is a interlaced signal, and typically rendering from one nametable the odd frames and the second nametable the even frame ? Is this making sense ?

by on (#19149)
The NES already tricks the TV into displaying a non-interlaced (progressive) image. I think you'd have to trick the NES into generating 263 scanlines (NTSC) every other frame if you wanted to get an interlaced image. I don't think it's possible. Even when displaying the NES on a recent TV which can't be tricked and always uses interlace, you wouldn't know which fields were which.

by on (#19151)
Quote:
Even when displaying the NES on a recent TV which can't be tricked and always uses interlace, you wouldn't know which fields were which.

I was under the understanding that modern TVs (I mean digital TVs) doesn't interlace any longer, but display both frames at once to look better. Effectivly you wouldn't know wich field is wich, but you could display a test image on reset and having something displaying "If this image doesn't proprely show up, reset your console until it does then press start". That way, the user would in average have to reset one time because there is two field. And that would be a little price to pay to gain better graphics.

However, since I don't have any modern TV and that the NES has been designed to run on old TVs and that emulators doesn't support interlace, I don't think it's possible.

by on (#19155)
I can assure you that newer TVs do interlace video when they are told to - in HDTV "1080i", the "i" stands for "interlaced". Also, I have personally connected my NES to an HDTV and was able to verify that it did not try to merge multiple frames together - when Mario got hit and shrunk, he flickered like he was supposed to.

The only sets that seem to have trouble are LCD and Plasma TVs - CRTs can interlace just fine.

by on (#19161)
The Super NES and Sega Genesis were capable of interlacing, but few games used interlaced modes because they would need double the CHR memory to get an equivalent picture.

by on (#19171)
tepples wrote:
The Super NES and Sega Genesis were capable of interlacing, but few games used interlaced modes because they would need double the CHR memory to get an equivalent picture.

Not exactly. Look at SFC games such as "Maka Maka" and the first Ranma 1/2 game. They interlace the picture (for god knows what reason) but have the same graphics in each field.

Games such as RPM Racing do interlace the image for a higher resolution and use up more of the SNES' background map. (Not necessarily CHR memory)

by on (#19174)
Quote:
They interlace the picture (for god knows what reason) but have the same graphics in each field.

Does the emulators support that ? Wikipedia states that Axelay interlace in Level 4 and I notice nothing special, same goes with Ranma 1/2.

Seiken Densetsu 2 and 3 does use mode 5 in their main menu, wich has 512 pixels horizontally instead of 256. But the horizontal resolution doesn't change, so I don't think there is any interlacing here.

by on (#19189)
I remember one of the Yuu Yuu Hakusho games displaying the Maker's (was it Namco?) logo in interlaced mode...

Is it possible that we don't notice anything different on emulators because of the way they handle interlacing (maybe they just mix the fields or even ignore one of them)?

by on (#19198)
My guess is that an SNES emulator would always generate 448 lines of output, updating all lines at 60 Hz in non-interlace mode and updating every other line in interlace mode. This is just a guess though - maybe someone who worked on/studied SNES emulator source code can offer some insight here.

by on (#19209)
Bregalad wrote:
Quote:
They interlace the picture (for god knows what reason) but have the same graphics in each field.

Does the emulators support that ? Wikipedia states that Axelay interlace in Level 4 and I notice nothing special, same goes with Ranma 1/2.

Try out Syvalion and see if the introductory text is shown in highres-interlaced in your emu. If it is, try out either Maka Maka or Ranma 1/2. You'll notice that with fast-moving graphics, the images begin to deinterlace.

by on (#19275)
What about just bankswitching the CHR rapidly to get an interlaced effect?

by on (#19276)
Dwedit wrote:
What about just bankswitching the CHR rapidly to get an interlaced effect?

This is one of the ways interlacing could be done, on the software side, if interlaced output was supported on hardware.

Bregalad was asking if the NES could output such a signal, and apparently it can't, meaning that no software technique could be used to accomplish that.

I'd like to try one of these methods anyway (CHR swapping, alternating name tables, etc), just to see how it looks on hardware. Maybe, just maybe, even flickering it might look OK. Although I guess it could hurt the eyes of the viewers if you did the effect for too long.

by on (#19277)
I once came across a NES program with Memblers' name on it, some kind of NSF player with a graphical display, that did "interlacing" that would have worked if the NES outputted an interlaced signal.

by on (#19282)
I've seen other demos that claimed to use interlacing effects too (by Chris Covell, I think), but I have no idea how those look on a NES connected to a TV.

Also, I recently played a game that was trying to display a picture behind some (scrolling, perhaps) text. The text was in one of the name tables and the pictures on the other. Each frame would display one of the name tables. I know it didn't look to good in the emulator I tried, and trying to read that flickering text was quite annoying. If it looks similar on a TV, I don't know why someone would make a game with that effect.

by on (#19295)
tokumaru wrote:
I've seen other demos that claimed to use interlacing effects too (by Chris Covell, I think), but I have no idea how those look on a NES connected to a TV.

Several of my demos do "interlacing" the CHR-swapping way. On a TV, it looks okay; on a high-persistence monitor, it looks great. But it still is a bit flickery.
tokumaru wrote:
Also, I recently played a game that was trying to display a picture behind some (scrolling, perhaps) text. The text was in one of the name tables and the pictures on the other. Each frame would display one of the name tables. I know it didn't look to good in the emulator I tried, and trying to read that flickering text was quite annoying. If it looks similar on a TV, I don't know why someone would make a game with that effect.

Batman.
Who knows why?

by on (#19296)
ccovell wrote:
Several of my demos do "interlacing" the CHR-swapping way. On a TV, it looks okay; on a high-persistence monitor, it looks great. But it still is a bit flickery.
tokumaru wrote:
Also, I recently played a game that was trying to display a picture behind some (scrolling, perhaps) text. [...] I don't know why someone would make a game with that effect.

Batman.
Who knows why?

Batman for Game Boy had a similar effect to blend the word "BATMAN" and the bat-symbol. They got away with it because the original Game Boy's LCD, like most LCDs of the time, was high persistence. The credits of Link's Awakening use the same effect.

The effect could be improved by switching between images per scanline, displaying odd lines from image A and even lines from image B in one frame and vice versa in the other. This would kill most of the flicker if images A and B have little correlation (as in the Batman case); it won't help if flicker blending is used as a method of temporal dithering (as in Chris Covell's NES demos and in PocketNES).

by on (#19303)
Do the dot crawl differences caused by turning on the screen late have anything at all to do with interlacing?

by on (#19305)
Not on an NTSC NES. The presence (480i) or absence (240p) of interlacing depends on what happens at the start of vblank (sync signal delayed by 1/2 scanline or not), not what happens at the end of vblank.

by on (#19309)
Syvalion does definitely interlace, with part of it using mode 5, because SNES9x does output high resolution both horizontally and vertically (unlike Seiken Densetsu 2/3 wich only increase the resolution horizontally).
However, I don't see any SNES game interlacing using just normal mode 1 increasing the resolution only vertically.
And is it possible to trick the ouput signal to be interlaced on the NES ? I saw Cris Covell's demoes, but they did seems to interlace, but since no emulators actually interlace both frames, and most probably the real NES won't do it either, this isn't really interlacing.
Flickering does definitely look a lot better on a real TV than what it does under most emulators, but still, it doesn't look smooth at all, at least not like interlacing does/is supposed to do.

EDIT : If the SNES has a flag that allow the SNES to output the exact same signal, but interlaced, this means that the fix images won't budge, but that the animated images may flicker a weird way. Is that what Axelay/Level 4 and Ranma 1/2 does ?
This could be great for games using mode 7 for 3D effects, because with the exact same maps and all, the game could change the matrix coeficiant by a fine value each scanline, so that the rendered image would have more precision vertically, and look a lot better, without wasting any tiles nor map space !!

by on (#19325)
Bregalad wrote:
However, I don't see any SNES game interlacing using just normal mode 1 increasing the resolution only vertically.

RPM Racing!

by on (#19335)
It uses mode 5 too.