nesdev.org

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).

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

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.

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 !!

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

RPM Racing!

It uses mode 5 too.