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Print Page - Modifying orbiting VFX
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Library of the Ancients => Final Fantasy IV Research & Development => Topic started by: Pinkpuff on June 14, 2015, 07:26:02 AM
Title: Modifying orbiting VFX
Post by: Pinkpuff on June 14, 2015, 07:26:02 AM
Ok I know I'm asking for a doozy this time... but is there any way to delete one of Earth's moons from the "Mysidian Legend IN SPACE" visual effect?
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 14, 2015, 08:13:28 AM
Probably... just the IN SPAAACE effect? That is, you don't need it removed from any of the other space visual effects, right? I recently looked into trying to fix the earth graphics in the "moon flies away" VFX in the opening of TfW. No luck there, but in messing around with it I was able to change the graphics of just about every other object in the effect, the experience of which may have been enough to help in this situation (assuming the planetary graphics are encoded similarly). I'm off today and could use a break from the toil I've gotten myself into lately. No promises, but I'll look into it.
Title: Re: Modifying orbiting VFX
Post by: Pinkpuff on June 14, 2015, 11:48:16 AM
Yes, just that one effect.
And thank you so, so much for looking into it. I wrote out the other moon out of the story without realizing what I was getting myself into until it was too late...
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 14, 2015, 05:56:12 PM
UPDATE This should be a snap, actually. I've already zeroed in on all of the data and assembly involved in drawing the planets. It will require a custom routine, because the same assembly and some of the same data are used to draw all of the objects in the effect, but no problem. Shouldn't take too long, but it will take a little trial and error to get it just right.
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 14, 2015, 10:58:28 PM
UPDATE 2 Okay, so "a snap" was a bit of an exaggeration of the simplicity of this project. I mean, I'll get it, but it's going to take a smidge longer than I originally thought. As of now, the second moon is gone right up to the point when the earth finishes its first pass in front of the sun, but then it reappears for parts of the pass behind the sun. I think I know why this is happening, and fixing it won't be impossible, but it's going to take a bunch of custom routines (because the moon sprites that I'm blanking out are in different places in RAM depending on where the earth is, I think because the larger moon images require more tiles). Anyway, gonna take a few days, but I'll get there...
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 16, 2015, 04:11:48 PM
UPDATE 3 Getting there. I didn't get to work on it yesterday at all, but I was able to chart some numbers today before work. I now have everything I need to make this work, it's just a matter of writing the routines. I'll post them all when the patch is up and running (as well as the patch). It's all pretty straightforward and observable, though the custom routines will certainly not be an exercise in efficiency. It all boils down to sprites being copied from the 300 section of RAM to VRAM, and the data at 300 are written depending on variables (believe it or not) at 7E:2000.
Like I was just saying, helping other ROM hackers out often leads to learning stuff that will help me out, and this is no exception. I'm sure that by studying this stuff, I'll finally be able to figure out how I screwed up the earth graphics in my own hack. :childish:
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 17, 2015, 11:55:18 AM
SUCCESS!
So, OK, if you want, you can skip past all the BS that follows and just download the patch linked herein (apply to an un-headered 1.1 US ROM), but for those interested in the process involved in creating the patch, here goes...
So, as previously mentioned, the sprite data are loaded into VRAM from RAM beginning at 00:0300. The data at 300 are loaded from a couple of different banks in the 12- and 130000s (LoROM... exact location I did not record, and is kinda irrelevant here). The values here are constantly refreshed to control the constantly moving planets, moons and sun. In order to decide which sprite data to use, the game references constantly descending two-byte values beginning at 7E:2000 (I guess they figured, no battle is going on at the time, so why not use this RAM?). A separate routine controls the descent of these values, but I didn' use it at all, so again, kinda irrelevant. Each sprite ends up having four values in the RAM 300 bank. I'm pretty sure they represent x-pos, y-pos, then the standard "vhopppcc cccccccc" sprite data. Since I'm not a computer and cannot easily interpret the x,y values with any accuracy, the trick to cracking all of this was looking at the vhopppcc cccccccc bytes in conjunction with the tile spread in Tile Layer (which technically begins at ACE00, though the first two lines of tiles look like gibberish...). So, freeze frame, compare tile values with the tile map... from there, it's just a couple of tries before you find the tiles that represent the moon you want to delete. When you want "no sprite," the game uses F0F0F0F0. I don't know why not FFFFFFFF, but eh, whatever.
First problem that arose: the "revlolving moons" don't actually make a full revolution. The sprites at any given (moon) location control half of a full revolution, then skip back to the opposite side for a second pass. Most likely, this was an easier thing to program, because one portion of RAM could be devoted to the smaller sprites (that pass behind the earth) and the other to the larger sprites (that pass in front). So, by observing the constantly descending two-byte values at 2000, I noticed that a (complete) half-revolution always begins when the (one-byte) value at 2000 equals FF. So, pick a previously unused byte (in this case, I chose 7E:2400), and make it toggle between 00 and 01 every time 2000 hits FF. Then, if 2400 = 00, blank out one moon; if 2400 = 01, blank out the other moon. Voila! full revolution!
Second problem that arose: one of the two moons (the behind moon, I think) always uses the data at 300-30F, but the other moon uses different data depending on whether the Earth is in front of or behind the Sun. I originally thought this was because of the moons, but now I think it's actually because of the size of the earth. Anyway, doesn't matter really. So, I had to figure out 1) exactly where in the 300 bank the changing moon's sprites were located in both situations, and 2) what other predictable values in RAM (in the constantly-rotating 2000 area) I could use to tell the game which of those locations to blank out. Over the course of the full visual effect, the moons make eleven full revolutions (or, more appropriately, 22 half-revolutions) around the earth. So, I had already made my life easier by setting up that alternating byte at 2400. I set a breakpoint to 2400 and recorded the values at 2000 for each (only the first 12 bytes do anything at all, so that's what's shown here):
TWENTY-SECOND CHANGE: FF EB FF EA 5F FD B3 FD 53 FF 80 01 After analyzing these data, what I noticed was, whenever the earth and moons are behind the sun (or at least, out of view), the lowest bit at 2009 is 1, and whenever the earth and moons are visible in front of the sun, the lowest bit at 2009 is 0. So then, the path was clear: load 2009; AND 01; if A = 00, do one thing, if A = 01, do the other.
So there you have it - One Moon in the Mysidian Legend IN SPACE visual effect!
Title: Re: Modifying orbiting VFX
Post by: Grimoire LD on June 17, 2015, 04:18:10 PM
Good heavens! That all looked tremendously complicated. Fantastic work though Chillyfeez!
Title: Re: Modifying orbiting VFX
Post by: chillyfeez on June 17, 2015, 05:11:18 PM
Complicated... Yes. I admit I didn't expect it would take all that when I set out, but all-in-all it was a rewarding experience
Title: Re: Modifying orbiting VFX
Post by: Pinkpuff on June 18, 2015, 03:00:32 PM
OMG Chillyfeez you are my hero once more! Thank you so much! This looks and behaves perfectly. The game will be ready for general release very shortly. Only a few minor fixes and corrections to make.