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Thread: When did they fix the column scroll bug?

  1. #1
    Master of Shinobi
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    Default When did they fix the column scroll bug?

    The VDP in the Megadrive can do a column scroll mode, one where it scrolls a 16 (or was it 32) pixel long "column" up or down. Some games used it to scroll the screen with a vertical parallax effect, while some others used it in combination of row scroll (change the scroll value at every scanline), in order to achieve rotation effects. The Pirate Ship in Streets of Rage 2 is an example where they use both scrolls, to rotate the background.

    Column scroll however had a bug in early hardware. The leftmost column did not scroll properly. I'm not sure if it was the leftmost as in the first "column" worth of pixels starting from the active display, or if it is the 0th "column" of pixels, ie. the one immediately behind the first column worth of active pixels. But the point is that the effect was buggy. Load up that level of SOR2 on a model 1 Megadrive to see it. Kawasaki Superbike also did background rotation like that, you can see the bug there as well, for example on this video:
    https://www.youtube.com/watch?v=5Gji1xHgB4w

    Emulators have the bug "fixed", except maybe Exodus (I did not check).

    I was testing a VA4 Model 2 Genesis today, and used Streets of Rage 2, as it is the Megadrive game I'm most familiar with. I noticed that on the pirate ship level, the background was NOT bugged like that. The background rotated just as it does on emulators. So at some point, during the model 1 and model 2 transition, they fixed the issue.

    Does anyone knows exactly which revisions of the VDP are affected? The MD1 ones from VA0 to VA6 all are. MD2 VA4 uses the 315-5940 combined ASIC (68k+z80+vdp+ym2612), and that one has it fixed. What about the 315-5487 (model 2 VA0, early VA1), 315-5660 (model 2 VA1, VA3), 315-5700 (model 2 VA3), 315-5786 (model 2 VA2)?

  2. #2
    Comrade as in friend. Master of Shinobi ComradeOj's Avatar
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    Interesting! I was never aware of a bug. I have a bunch of different revisions. I'll do some testing and report back.

    Also, it is 16px. Not 32.
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  3. #3
    Comrade as in friend. Master of Shinobi ComradeOj's Avatar
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    Quick follow up:

    I tried a homebrew test program that uses the vertical column scrolling mode. I ran it on my VA6 and VA6.5 model 1s, nomad, and VA2.3 model 2. All USA models. In all cases, the effect looked the same as an emulator. I also tried the kawasaki superbike game. It looked completely normal, not like in the video.
    Modded consoles:
    Master System (v7040) with s-video & direct AV out
    Model 1 with 10mhz overclock & halt switches
    Model 1 with 10mhz 68010
    Model 2 VA2.3 with unfiltered Mega Amp, & s-video
    Model 3 VA1 with compatibility fixes & s-video
    32X with s-video
    Visit my web site at www.mode5.net
    Or my collection of homebrew Genesis games, programs, and music on SEGA-16!

  4. #4
    Hero of Algol
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    https://forums.sonicretro.org/index.php?showtopic=26788

    Also:
    Quote Originally Posted by Stef View Post
    Gynoug does display garbage on first row in first level during earthquake... I observed that later model 2 hardware fixed the problem where it was present on jap MD1.
    You can see the bug in this video :

    http://www.youtube.com/watch?v=8pNGK1lDdIk#t=85s
    Quote Originally Posted by kool kitty89 View Post
    Huh, interesting. I wonder if it's all model 2s (and the VA7 model 1) that fix that, or more specific later models. (certainly might have been fixed in the new CMOS VDP ASIC introduced in the VA7)

    Also, sheath's recording of your Star Fox demo shows it even more obvious here:
    https://www.youtube.com/watch?v=PNGWykZ0ju0 (looks more dramatic than in Gynoug though)
    Quote Originally Posted by Stef View Post
    Indeed its very visible in my starfox demo, i can use sprite to hide the column but that is not my first preoccupation
    Quote Originally Posted by TmEE View Post
    MD2 VA4 has the bug fixed, and thus Genny3 too.

  5. #5
    Hero of Algol
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    But that bug can be hidden by software if you "mask" that first column.

    One game that nobody ever mentioned in such discussions but has it covered is Pitfall: The Mayan Adventure.
    This is one of the very few games which implemented waterfalls using the column scroll hardware feature (Jurassic Park: Rampage Edition, Vectorman and Vectorman 2 - all three developed by BlueSky Software - also did that) instead of simple color cycling.

    The Pitfall levels with waterfalls (Xibalba Falls and Yaxchilan Lagoon) have a slightly diminished horizontal view area (compared to the other levels) in order to hide that bug (all levels run using the H40 mode) (pictures compare the first level to the two waterfall ones):






    Notice how the waterfall levels' view area is moved to the right. Do you know how many pixels has that horizontal offset? *Exactly* 16 pixels.
    Yeah, George Allan knew his shit.
    Last edited by Barone; 05-14-2016 at 12:02 AM.

  6. #6
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    One of the levels in Castlevania also hid the effect like that. The tower was tilting left and right as I recall, but they made it so the tower is not the entire width of the screen, just the middle. I always thought it was there to hide this.

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