Explanation of 2nd part of yoshi clip (continued)

[From mraglar (23-Jan-2013)]

Ok, there were more factors that determine whether the trick will work or not. I have only tracked down one of them but at least I've found the actual chance that it will work.

It turned out that the success rate is also determined by the vertical subpixels of Yoshi. In this game, there's 16 subpixels per pixel and are stored as an unsigned byte meaning that they only come in multiples of 16 and can thereby are expressed by the numbers 0, 16, 32, ...,240. Here is a list of the chances to succed for every subpixel that Yoshi can have, after executing the first part.

0-16: 0%
32-64: 25%
80-96: 50%
112-240: 75%

This explains why I first thought the chance was 75%. Since in all my tests, before I looked up the subpixels, I seemly always got values between 112 and 244 and thereby thought they didn't matter, oh well. If my math isn't completely out of order the chance of succeding is:

(2/16)*0.25 + (3/16)*0.50 + (9/16)*0.75 = 53.125%

But that's only correct if all subpixels have an equal chance of occurring, which might be the case if you miss it once and then have to try again. However as for the first attempt, granted that you succeed with part 1, the chance is always 75%. At least for FoI2.

Why? Well Yoshi's subpixels are a bit special, they only change value when Mario isn't on Yoshi. At the start of the level Yoshi vertical subpixels are always 0 and will thus stay that until you try the glitch (granted that you don't unmount him before that). And all my tests show that from the time that Yoshi is unmounted to the time when he is mounted inside the wall, his subpixels always change by:

(current_subpixels + 128) modulo 256

Meaning that the first time you you perform it you will land inside the wall with a vertical subpixel value of 128 which is in the range for the 75% chance.

It doesn't end here though, because if the water physics and Yoshi's subpixels were that only things that determined the outcome then, for all subpixelvalues from 128 to 244, the chance of succeding would be 100%, this isn't the case though and I honestly don't know why. Could maybe be that Mario is at a too high position when Yoshi is mounted but if this where the case then I can't see why it always works every 3 out of 4 frames since Mario's vertical subpixels changes value every 4th frame (according to the water physics) and thereby I would expect some inconsistency in the success rate.


[previous message: http://pastebin.com/JgugCHFw]