What no Lugi?

Because he'd have to hack all that functionality in. It's not a simple matter of copying over some magic code to make Luigi with all the multitude of moves and powerups from SMW playable alongside Mario. Just hacking in a 2nd player character running and jumping around with proper collision detection is probably hard enough.

@ Trevor
Notice how in this stage, there were already Rex enemies around? My guess is that he was able to have a playable Rex here because Rex enemies were already loaded into the memory.

Of course, I guess he could have just had two Marios, with one of the palette swapped, but I dunno. Like Jordan said, I'm sure the choice was made because hacking ROMs ain't easy - I'd imagine hacking a single-player game into having Co-op is especially taxing.

Well, I didn't mean to make it sound like it was easy to hack a game, but really it doesn't make much sense in this case to not have Luigi.

Let's go with the scenario you put forth the7k. You theorized that using the Rex sprite was most logical because there were already Rex sprites in the level and thus already loaded. However, notice that the playable Rex is recolored and every non-playable Rex is the same color as one another and non are colored the same as playable Rex. Despite simple being a palette swap, the playable Rex counts as a separate sprite.

But, lets say it still counts as the same sprite for argument sake, why couldn't he have simply used Luigi? After all Luigi is simply a recolored Mario sprite and Mario as with the Rex (how do you pluralize "Rex"?) in the level are already loaded into memory.

As for your theory TheStripe, well, I can't say much about it since I can't say for sure if the camera is set to follow the Mario sprite specifically. But I would assume it just naturally follows whomever is the main player. Since it is naturally meant to be played single player, with two players simply taking turns, player one as Mario and player two as Luigi, I assume it just follows whomever's turn it is. So if player one was meant to be the one playing the camera would follow them, and vice versa.

@Trevor: All of mario's animation may get swapped out at times, and therefore not be completely available. I don't know the SNES as well as the NES, but with the NES, you had a finite number of sprites (8x8 pixels) that could be in memory at a time. It was a fairly small ammount. Some games (like megaman) would have run out of space with the main character's animations if all moves were in memory all the time. So megaman's walking animation sprites get swapped into the sprite sheet when he starts moving, and the standing sprites get swapped out. At least, this is something I noticed when running that rom in nesticle back in the day (that emulator would let you peer into a lot of the nes' graphical memory.

So it's very possible that two mario characters could exist, but if one was moving, the other would look like he's walking in place, because the walking art exists where the standing art used to.

I'm not sure if that even made sense to me. eh?

Wow. That's awesome.

Pretty neat. Still looks about as ridiculous as NSMBW does.