Note: if you're new to this, make sure you read the Files and Introduction sections.
Or, just 'Files' might do.
Sonic CD uses a very plain and obvious compression system, which took months to crack only to discover you can 'unzip' these files in later versions of WinZip. Of course, if you don't have WinZip, you will not want to install it just for this guide...that's why I provided a couple of really old DOS programs (if Microsoft decides to sue the SSRG for posting them, they can always be taken them off, after all). And because they're DOS programs, they are short, simple and easy to use, as Cyan H. would put it. They are:- Expand.exe
Uh...these files are zipped, so you might get WinZip after all...but not all versions support the MS compression system, so get Compress and Expand anyway.
Next, you need a tile viewer to see the graphics...there are two of the most popular ones available for download.
Tile Layer Pro is better since you can implent palettes to the graphics directly (you get a clearer picture of what you are doing), but on
the other hand, it runs under Windows 95+, so you can expect lots of slowness, menu navigation and a tough learning curve. I prefer Sprite
View because of it's simpleness, but it's really your choice.
Or maybe not, because all the explanation will be done using that particular program...
Hacking The Graphics
SCD sprite/tile files are, as you probably know now that you've read the file listing on this site, Scmp???.cm_, Tcmp???.cm_ and, in special cases, Tchg???.cm_ for normal levels and sp?cg32.cm_ for special stages. They are compressed, so if you try hacing them while they are in their normal *.cm_ form, you'll end up in a big pile of mess which quite probably won't work. So, use a program called Expand (one more time: it's here) to expand them. (Put them in the same dir with Expand, then do C:\expand scmp???.cm_ scmp???.cmp and yes, that's DOS.) Now open the file in Sprite View (get it above), set the mode to Linear 4 and lines to 16, 32 or 64...depends on how big the object you wanna see is. For info how to do it, consult the powerful and all-knowing Mr F1 Key.
Something like this is what you should see on-screen once you're done with the setup. NOTE: when you first enter the program, you will only see a couple of lines, glitches et cetera...but after you've mastered it, you will be able to get images like this.
Now, to take a deeper look:
Sorry for the poor quality, but what you see is what you ge...wait, I wasn't gonna say this...um...nevermind.
The most important thing here is the number in the bottom-left corner; it is, as the picture suggests, in fact the hexadecimal number of the first line of bytes displayed on top of sprite / tile viewer. That means you can, say, load the file scmp73c.cmp, seek Metal Sonic's sprites, and when the desired sprite is at the very top of the viewer, copy the number on a piece of paper, then seek that address in a hexeditor. Once you have it, copy the bytes from that address to the end of sprite (find the end of it similarly) and paste them over Sonic's original files, replacing them entirely...and that's precisely what I did here. ^_^ Did it work? Well, no, actually, because I corrupted the file by accidently removing one line of bytes (things like this are dangerous to your SCD game, so make sure you have backups ready!). But it did work with my lives icon hack...
Remember: once you have hacked the file, it ain't over yet. You must Compress it now. The concept is similar to Expand, and it's done like this: compress scmp???.cmp scmp???.cm_. Put it back where the original file was and you can now test it! Just enter the stage you just hacked to see the result.
Oh, and here is something to get you started with:
1B18 - 1DA8 : Sonic's standing sprite (?)
51278 - 514E8 : Metal Sonic standing/crouching sprite