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How do Sonic 2 multiplayer mode work? DPS processor? Enligthen me.

#1 User is offline ashthedragon 

Posted 07 March 2017 - 09:41 AM

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Hello there.

I've been discussing with a guy on youtube who claims that Sonic 2 is much more technically advanced to Sonic 3 ( and he claims this game made the franchise go to the drain, until Sonic & Knuckles got released and saved the day) because it used an extra co-processor similarly as how the Super FX chip makes the Snes able to run games like Starfox. He claims that the DPS Processor that Sonic 2 cart has ( and no, it's not he SNES audio DPS but a " Digital Signal processor", he claims) makes the Sonic 2 multiplayer work, making the two multiplayers screens are in the same resolution as the 1p game. That is what makes the image look crushed. He claims Sonic 3 is much worse and a major downgrade because the multiplayer mode stages are shorter and have half the resolution, compared to the 1p mode and Sonic 2's.

I know many of this is bullshit as the only game with extra co processors in Mega Drive/Genesis is Virtua Racing, but it has startled my curiosity. How do Sonic 2 multiplayer mode work? Why is it different in Sonic 3? IMHO Sonic 3's look much better, but is it really half the resolution while Sonic 2 is not? I vaguely remember something about Sonic Team working hard to make multiplayer work in Sonic 2, but they were not happy with the results , as it was like that because some bug ( Don't take my word as granted, it's all from memory and I don't remember it well).

#2 User is offline MainMemory 

Posted 07 March 2017 - 10:44 AM

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Sonic 2's 2 player mode uses the Megadrive's Interlace Mode 2, which allows the system to output a 320x448 image by alternating between even and odd lines every frame. This allows the game to display two full screens' worth of graphics, at the cost of effectively halving the frame rate to 30. This effect is especially noticeable if you get a shield, or hold yourself on a plunger in CNZ, you'll see that each line is only updated every other frame, leading to a striped appearance.

Sonic 3's competition mode on the other hand uses the normal 320x224 progressive scan mode, using graphics specifically designed for the lower resolution.

Sonic 2's 2 player mode tries to essentially run two copies of the main game at once, cutting down on objects so it just barely works, while Sonic 3's competition mode is specifically designed to work with two players running at the same time.

#3 User is offline ashthedragon 

Posted 07 March 2017 - 11:06 AM

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Oh, so that's it! Thanks for the info MainMemory!

#4 User is offline GerbilSoft 

Posted 07 March 2017 - 11:56 AM

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For what it's worth: The only commercial Mega Drive title to use a coprocessor was Virtua Racing, which contained the "Sega Virtua Processor" on the cartridge.

Also, the SVP's address mapping conflicts with 32X (0xA150xx), so you can't use it if a 32X is connected.

EDIT: Apparently you already mentioned this. :v: Either way, what MainMemory said is correct here - it's using interlaced mode, which allows for higher resolution but at a cost of flickering, squished vertical aspect (if not accounted for; S2 doesn't account for it), and it's somewhat more difficult to manage.
This post has been edited by GerbilSoft: 07 March 2017 - 11:58 AM

#5 User is offline Felik 

Posted 07 March 2017 - 12:29 PM

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But if Sonic 2 uses this method then why any zone other than EH, CN or MC has broken graphics when forced to run in 2p mode?
Wouldn't all zones look good if no additional work is needed to be applied to level graphics when run in 2p mode?

#6 User is offline MainMemory 

Posted 07 March 2017 - 12:44 PM

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Interlace mode 2 has a different method of displaying tiles than the other modes, all tiles are treated as being 8x16, instead of 8x8. Because of that, the even numbered rows of sprites and backgrounds must use the next 8x8 tile from the odd numbered row just above it, as well as the same palette and x flipping. Most of Sonic 2's levels were not designed with that limitation in mind, and in fact would be impossible to make compatible without losing some of the detail.

#7 User is offline Felik 

Posted 07 March 2017 - 01:41 PM

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Would that mean that these three specific zones (and Hill Top) were designed with this in mind?
If so how can this be spotted with these four zones? They look pretty normal (without any limitations in presentation) to me.

#8 User is offline MainMemory 

Posted 07 March 2017 - 01:43 PM

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The most obvious clue is in the tile list. All three levels start with two empty tiles, rather than one.

#9 User is offline Clownacy 

Posted 08 March 2017 - 08:50 AM

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Don't those zones also have a lot of duplicate tiles, because the 8x16 format needs both tiles next to each other?

#10 User is offline Flygon 

Posted 08 March 2017 - 09:00 AM

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Wasn't that mapping-wise, rather than the tiles literally being stored adjacent inside the VRAM? My memory on this is really fuzzy.

#11 User is offline MainMemory 

Posted 08 March 2017 - 09:13 AM

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Yes, the tiles have to be literally adjacent in the VRAM. If the first row uses tiles 0 and 8, the second row must use tiles 1 and 9.

#12 User is offline Neo 

Posted 08 March 2017 - 11:55 AM

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Don't think of it as "both tiles need to be next to each other", think of it as "tiles are twice as tall in 2P mode", because that's what the abstraction is. So blocks are just two tiles side by side, and sprite mappings that are 1 or 3 cells tall are illegal.1 Then, the fact that tiles on the bottom row of a block need2 to be equal to the top row + 1 is just a necessary constraint to have the stage look correct in 1P mode.

1. This is why Sonic and Tails' art has so many empty tiles, and why Super Sonic appears glitched in 2P mode -- he was never meant to show up there, so he can use odd-height mappings to save a bit of ROM space.

2. The obvious exception being when the tile is flipped vertically in the block definition -- the entire 8x16 tile is flipped, so it's the top row that's equal to the bottom row + 1.

#13 User is offline Sodaholic 

Posted 08 March 2017 - 12:08 PM

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View Postashthedragon, on 07 March 2017 - 09:41 AM, said:

I've been discussing with a guy on youtube
There's your problem right there. It's frustrating how ill informed many people are, and many of them are vocal in the comments section of Youtube.

I remember one time I was debating a guy about framerate interpolation for games with capped framerates. There are already ports of games that increase the framerate this way, by interpolating object/camera positions and rendering new 3D frames. For some reason he couldn't understand that's what I was talking about, and kept insisting that I was talking about blending existing frames despite my insistence that was not the case.

#14 User is offline Rlan 

Posted 08 March 2017 - 05:22 PM

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What other games used the Megadrive's " Interlace Mode 2" mode?

#15 User is offline GerbilSoft 

Posted 08 March 2017 - 06:19 PM

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View PostRlan, on 08 March 2017 - 05:22 PM, said:

What other games used the Megadrive's " Interlace Mode 2" mode?

Combat Cars is the only other licensed MD title I know of that uses IM2.

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