It seems like Batman must move each tile by two pixels then? The effect on a real NES appears to operate at half the refresh rate (looks like 30 fps while the game scrolls at 60).
Probably. Well, it's not really moving the tiles. As was said earlier, the NES background is like a flat sheet of paper, there's no foreground and background. Basically wherever you see those clouds, the game is playing an animated GIF in those tiles. I'd guess that they're moving so quickly so that you don't notice the slight weirdness that they move along at the same speed as everything else when the background scrolls.
Battletoads does a similar thing. This is what its full set of background graphics for
level 2 looks like in action:
In the example of Sword Master, you can see that the far background area that scrolls is actually quite small, only 32x32 pixels, but nonetheless it's a lot of space when it comes to NES tiles.
If I remember correctly, Sword Master does its mountain animation by swapping out a whole bank of tiles, which is faster and easier than writing to graphics RAM but also wasteful, since they're using whole banks on that one scroll animation.
Additionally, notice that the places where the mountains are, there are no knobby bits that stick outward that would overlap, as there are above and below the mountains:
If they wanted to overlap the parallax effect with anything, they would need to use sprites synced up with the background, which I believe a couple of games do.
In any case I'm a little skeptical that this effect could actually be done on a NES:
You wouldn't do it like tiles of an animated gif. There is simply too much background there to animate within the NES's limited graphic memory.
If you wanted to do that kind of full screen parallax on a real NES, you would do it like you see in Shatterhand:
This isn't accomplished via the "animated gif" method mentioned above. This is done by timing the rendering of the image very carefully, waiting until you've drawn a certain chunk of the screen from the top down, and then suddenly adjusting the scroll of the image. Imagine you draw a scene like that on paper, and then you cut the paper into three strips, and move the three strips independently of each other so that the bottom part moves the fastest etc.
And the way the NES works, when tiles scroll off one edge of the screen, they immediately scroll back in on the other edge, unless you replace them with new ones. In fact you can see this happening in the Shatterhand gif, that big building scrolls off the left side and comes back in on the right. Meanwhile, with all the interactive stuff at the bottom of the screen, the engine is very quickly replacing stuff that scrolls off on the left with new stuff to come in on the right.
If you wanted to transition to a new parallax background, you could do so, just like with those "foreground" bits. You change them to something else when they scroll just off screen. But the transition would move at the same speed that this chunk of the background is scrolling! It would be really slow, not quick like the end of the tunnel in Steel Assault is. And with the other parallax at the very top, assuming they'd use the same strip method, the transition would follow the speed of those strips as well and be really uneven and weird.
A lot of these NES games with parallax don't really try to transition to anything else. Shatterhand for example just fades out and fades back in on an interior scene.
One way I can think of to try an effect like that on the NES would be to have a full screen vertical stripe of sprites representing the end of the tunnel, and move that along the screen, covering up the fact that you are quickly changing the background tiles behind it as it moves. But that's a lot of sprites, and a lot of tile replacement going on every frame.
Some NES games actually do use a "vertical stripe" method like that to cover up tile replacement artifacts, but they're always there and not used as transitions.
I believe Alfred Chicken is one. Notice how in the transition from title screen to gameplay, there is suddenly a black bar on the right side, except for a very tiny strip visible at the top of the screen. But that is super wasteful of a limited sprite budget, since you only have 64 available, and can only display 8 horizontally before you need to start flickering them.
While we're at it, another problematic part of this mythical NES Steel Assault would be this bit:
Like I said, you'd be doing parallax by scrolling each strip of the screen independently, but at this point there are actually TWO strips - the fast moving train and the slow moving background, and the train has transparency over the top of the background. Basically the only way you could do that on the NES is if the whole train was a massive sprite, which is not possible.
However,
after leaving the tunnel, it's totally doable, because there is just solid black behind the train part. The black part would be moving at the same speed as the train but you have no way of telling that, because it's just black.
Finally, the HUD at the top wouldn't be able to exist as it does, floating over the background. It would all have to be sprites, and it would just use up too many of them. You can see in all of the other examples, Shatterhand, Little Sampson, and Batman, that the floating HUD is extremely minimal. Same with the Mega Man games. All sprite HUDs, and they count against your drawing limit so you want to use them sparingly. Games like Sword Master, Mario 3, and Castlevania 1 and 3 have that solid black HUD at the top or bottom.
Actually the solid HUD is
itself a parallax effect! It's scrolling at a different speed than the rest of the screen. That is, a speed of 0.
Anyway this is not a criticism of Steel Assault at all, they've been very clear about how they're not following the NES specifications exactly, but I just wanted to try to clarify on some stuff.