Beta Test Note: This page has not been updated for series 9 changes.
It started with me wanting to have the "best of the best" mod load. I very quickly hit my esp limit, even with Wrye Bash. I started looking at various stealth mods to see how they worked, and if there was a way to combine them for my own personal use. The more I dug in however, the more I realized that none of them really worked the way I thought they did. A vision came to me of how I wanted sneaking to really work, and shortly thereafter, I found myself knee deep in learning how to mod.
Cracking the Formula
First up on the list was figuring out how the detection formula worked, and then how to hack it. Ultimately, that credit goes to JRoush, who not only deciphered Oblivion's detection formula, but handed me an OBSE .dll plug-in that has allowed me to replace the detection process entirely. To learn more about Oblivion's actual detection formula, you can find it here: Category:Detection
The CAV system
Despite being able to replace the formula (or maybe because of), performance became an issue. The only way to really resolve it was to port as much of the detection formula and supporting calculations into SDR's OBSE .dll plug-in as possible. This required inventing and entirely new way to create, store, modify, and retrieve custom actor values - and thus the CAV system was born and implemented as of SDR's 8 series. This significantly improved performance while managing to allow keeping the vast majority of the new features.
Rewriting the Formula
All in all, although I think the underlying concept for detection had a solid foundation, I felt there was a lot of room for improvement. Either elements of detection weren't dealt with properly, or just not dealt with at all. Following are some of my thoughts on the various bits and pieces that led to the sneaking insanity that is SDR.
In the default Oblivion system, only the lighting on the player factors in. But if you have ever been near a camp-fire and try to stare out in the dark, or be out in the dark staring at the campfire, a more realistic approach is to take into account the lighting on the target as well as the detector. SDR does this.
In the default Oblivion system, you could be standing right in the path of a creature, and it could bump into you and go around you and not even realize you were there. SDR makes that nearly impossible now, and collisions are usually automatic detections.
How Invisibility and Chameleon affected Detection
It bothered me that if you were Invisible, you were 100% undetectable no matter how much noise you made. The original Oblivion documentation implied that they could still hear you, but on the back end that was not the case. SDR fixes that. Invisibility ain't a sure fire method to avoid detection anymore (although it certainly helps!)
Not Just The Boots!
SDR takes into account every piece of armor, clothing and weapon that you have equipped and are actively using when determining sound penalties.
Standing Really Still Is Really Smart
In standard Oblivion, an NPC could be right next to you, and you could turn in place, and they wouldn't hear you. SDR has an option that has you make noise when you turn in place. It's about half of the sound you would make if you were walking. But if you are wearing noisy gear and in line of sight, watch out.
The Algorithm of Sound
In the Oblivion default, light and sound are basically treated the same at the same distance, however in reality, they function very differently. Light continues directly until it is blocked by something, while sound loses energy exponentially as it travels, but can wrap around corners. I did my best to replicate these phenomena.
One of the major advantages that the player had in default Oblivion was the use of the Detect Life spell. Well SDR levels the playing field by incorporating Detect Life into the detection formula and providing methods for other actors to be granted and use Detect Life spells. In addition, I felt that there should be options to customize under which conditions Detect Life works as well as customized Detect Life shaders, because the defaults weren't cutting it for me.
As a natural offshoot to the detection formula changes, I felt there should be some changes in terms of what the player and other actors actually experience.
Torches and Invisibility
It always bothered me that you could be invisible, run around carrying a lit torch, and nobody would notice you. SDR has options that will cancel any normal Invisibility magical effect on you if you are holding a lit torch or are under a Light magical effect. There are also options to have torches and Light spells modify the effectiveness of the Chameleon magical effect. Torches negate, and Light halves it.
New Spells and Audio/Visual Effects
As a natural offshoot of all the detection changes, I felt there needed to be new spells. SDR introduces Blindness, Deafness, and Muffle spells. Blindness and Deafness can be used against your enemies - but they have also been added to the leveled lists, so they can be used against you too. Muffle allows you to quiet your own footsteps.
Changing Other Stuff
I could go on and on, but I'd just be repeating what is covered in the Player's Handbook and elsewhere. So to sum up, there is a LOT going on with SDR that really forces you to rethink how to sneak around.