Sonic 2 input emulation error

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What steps will reproduce the problem?

1. Load "Sonic the Hedgehog 2 (JUE) [!].bin" (1048576 bytes)
2. Play the game until you get to the Aquatic Ruin boss or the Sky Chase zone.
3. Try to do the repeated spindash trick from the top of the Aquatic Ruin boss 
towers to beat him quickly, as one would do on the console, or try to spindash 
from the top of the airplane more than once in a row without walking anywhere 
between spindashes.

What is the expected output? What do you see instead?

I expect to see Sonic repeatedly spindash into the boss, defeating him quickly. 
 Instead, after spindashing into the boss once, trying again results in moving 
nowhere.  Any number of spindash attempts without moving between attempts 
continues to result in Sonic remaining stationary.  Walking a little bit fixes 
it for the next spindash, but the problem is consistently reproducible when 
attempting to spindash into the boss multiple times.

Please provide any additional information below.

This Sonic 2 speed run from the 2012 Speed Demos Archive marathon shows the 
expected behavior on a real Sega Genesis starting at 10m28s into the video: 
http://www.archive.org/download/AwesomeGamesDoneQuick2012/AwesomeGamesDoneQuick2
012_part022_SonictheHedgehog2.mp4

I have personally used this trick on my Sega Genesis many times while playing 
Sonic 2.

Original issue reported on code.google.com by lexlexlex@gmail.com on 9 Mar 2012 at 3:36

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I have no problem doing the repeated spindash (tried against aquatic ruin 
boss), it works as expected as long as you get the timing right (hold down, 
hold C, release C, etc), using either wiimote or genesis controller with 
wiimote adapter, so unless you are speaking about a different thing,  the 
problem is probably with the controller you are using, not the emulator.

Original comment by ekeeke31@gmail.com on 10 Mar 2012 at 2:45

• Removed labels: Priority-Medium, InputEmulation

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This video I just made using Genesis-Next (a port of Genesis Plus GX for 
libsnes) clearly demonstrates the bug.  Notice how I can do the spindash 
whenever I want and I know how to do it.  It's not hard at all.  If I try to do 
it repeatedly without moving, it fails, but in the end, you see me move after 
each spindash and the next spindash works.  This happened exactly how I 
expected it to happen, with knowledge of the bug.  It's not a controller timing 
issue, since it's super easy to do.

You can play the video with MPC-HC and LAV Filters or just VLC 2.0.

http://dl.dropbox.com/u/35695760/sonic2bug-encoded.mkv

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 5:02

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Hmm!  The problem happens in Gens 2.14 also.  Maybe it's the ROM.  :O  You can 
close this issue.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 5:21

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I tried a different ROM and I still see this behavior.  I know it doesn't do 
this on the real system.  Maybe this only happens on certain models of 
Genesis/Mega Drive, but not on the model I have or the model that guy in the 
SDA speed run has.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 5:40

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http://info.sonicretro.org/SCHG_How-to:Fix_screen_boundary_spindash_bug

It seems this is the bug I'm seeing.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 5:49

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I see, it's a bug in the original game indeed.
When i tried the trick, i was on the left pillar but not completely stuck to 
the edge of the screen. As i said earlier, repeated spindash (as with only 
using DOWN and C buttons with good timing) worked perfectly, and exactly as in 
your initial video (Sonic comes back to its initial position after 
spindash,even when boss is moving to the middle of the two pillars...)

Original comment by ekeeke31@gmail.com on 10 Mar 2012 at 10:02

• Changed state: Invalid

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There's no "good timing" necessary.  It simply works the first time or it 
doesn't once you've come back and not moved anywhere since then.  What's done 
in the SDA marathon video can't be done on this ROM with these emulators.  I 
can do it just fine on my Genesis with my real copy of Sonic 2.  It may not be 
an emulation error, but it's definitely not something that you can do here, 
unless you have a different ROM in which the bug is fixed (which is what I 
suspect is the case with the Sonic 2 cart I own).

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 10:09

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If I could dump my cart somehow, I could test this better.  I don't have a ROM 
dumping tool though.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 10:10

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I have tried with both revisions in no-intro romset and what is done in the 
video can be done in both.
I am not sure if the position is *EXACTLY* the same but it works and the boss 
was quickly defeated without moving from my initial position on the pillar.

Original comment by ekeeke31@gmail.com on 10 Mar 2012 at 10:19

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Also, it could be that the speedrun was done with a flashcart and a fixed ROM 
but what is sure is that sonic retro clearly shows it is a bug in the game 
code, not an emulator bug.

Original comment by ekeeke31@gmail.com on 10 Mar 2012 at 10:23

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My cart was bought back when I was a kid, and definitely isn't a flash cart 
with a fixed ROM.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 10:25

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Ohhhh!  It only works from the left!  Here's it working properly:

http://dl.dropbox.com/u/35695760/sonic2bug-part2-encoded.mkv

I must have always naturally done it from the left as a kid, and when I came 
back to playing it as an adult, I naturally tried from the right for some crazy 
psychological reason.  I'm sorry for bothering you with all this!  It's 
definitely my mistake, especially for not noticing he was doing it from the 
left in the SDA marathon video.

I can't apologize enough for wasting your time.  I hope at least you get some 
entertainment out of this.  Lol.

Original comment by lexlexlex@gmail.com on 10 Mar 2012 at 10:52

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No problem. I hate playing sonic 2 but it's always fun to learn new things 
about mega drive games.

Original comment by ekeeke31@gmail.com on 10 Mar 2012 at 10:56