Recently I've been playing with the MD's shadow and highlight mode, so to show what I've learned, I made a little hack! Sonic the Ghosthog! This little hack makes Sonic a ghost with new abilities. Phase through enemies when walking at slow speeds or standing still And when you phase through Caterkillers, you scare them to pieces! Ghost Sonic also gets a double jump! (Since flight would be too overpowered) And as a ghost, Sonic logically doesn't need to breathe underwater. And because of the way S/H works, indoor areas like MZ and SBZ are thematically enhanced, as they cast objects in darkness. This may also prove as a new challenge as you could lose Sonic in the darkness. Which is especially the case in the Special Stages since I could not make any of the plane A tiles high priority. I also made some cute art for this hack! Now if you have any questions or confusion about something in the hack, 90% of the time the answer will be due to the limitations and functions of S/H. For example, Green Hill's dirt is mostly darkened because low priority tiles are shadowed, if I had made it high priority to make it render normally, all the objects would go behind it, making it near impossible to play. Included is the source code, and in the zip there is also an HTML with a beginner's guide to using S/H. Hopefully that will help some of you learn S/H for yourselves. There's also an Easter egg in the release ROM you won't find in the source code~
The transparent effect on Sonic is pretty neat, and I really appreciate giving the tools to encourage people using the S/H effect, which has some neat possibilities if you plan around the limitations. The HTML is laid out really well. One thing to note is that the sprites section of your guide doesn't seem to make a distinction between using S/H (transparent sprites) and being affected by shadowed planes. It should be clear that priority only matters in the latter case. You can make a high priority sprite transparent. You could also mention how highlights on a low priority sprite on a shadowed plane will be darkened back to normal. You can easily fix this by making every sprite high priority. The limitation with this is that the only thing able to cover sprites is sprites with higher sprite priority ($18(a0) in Sonic 1). The concept of making Sonic a ghost has a lot of gameplay opportunities. I think the idea of Sonic only being able to interact with objects indirectly could result in some interesting scenarios. For example, Tails could be controlled by the CPU while the player as Sonic would have to scare away or possess enemies or keep Tails on the fastest path in order to stop Tails from dying or allow Tails to reach the end more quickly.
Thanks for the pointers and suggestions. There are a few things I'd like to fix up with this project (Such as various things involving priority, and a typo or two in the HTML), so I'll be sure to keep this in mind. I do believe I've conveyed the important parts of S/H at least, so hopefully at this state it's still a valid resource.
I finished playing this hack and it was fun to play. This hack can offer a lot of new ideas (as ProjectFM said) and about to these I have come up with a new skill that you could add. After collecting all the chaos emeralds, by pressing a button and collecting a certain number of rings (example 15), sonic can become untouchable without walking at slow speeds or standing stillby by enemies and go through some objects (e.g. the columns of Marble Zone) for a few seconds.
The ideas for new abilities is nice, but I only added the ones I did to further sell the hack itself, rather than leaving it as an awkward S/H demo that just makes Sonic translucent, and not much else. The source is there so if you'd like to add new moves yourself, you're more than welcome to.
Practice makes perfect! If you set your mind to it, you can become familiar with Sonic 1's engine and add new moves to the game. As for the hack itself it's a very interesting concept, one that I'm shocked no one has thought of doing before!