Is it possible to change the base of a shape key?

[Healisoulus]

(Warning: screenshots with mild 3D nudity https://imgur.com/a/UqJNu4a ) I am "fixing" a model for Source FilmMaker, and I'm in a bit of a situation where I want some of the model's shape keys to use a different base, yet their "destination" to remain the same. I'll try to explain it further: the model is technically an augmented version of a very similar model (aka "template") - both were built on the same base; my model has larger breasts by default, yet it also has a shape key called Breast - Default, which adjusts the breasts so that they look like on the template. Now, alongside that there's also the same shape keys that the template had: Breast - Implants, Breast - Natural, etc. Breast shape key "family".

The issue is that when I use any of the flexes from Breast family - other than Breast - Default, the results look as if Breast - Default was already engaged when it's actually not, and when it IS engaged, the results look just wrong.

It is important to note that if I set Relative to of all non-Breast - Default shape keys from the family to Breast - Default instead of base, in Blender everything looks just as I want it to look - no issue, everything's fine. However, I assume it's only a cosmetic fix, since after exporting and compiling the model with Blender Source Tools (it's an .mdl afterall for Source FilmMaker), the issue persists in SFM.

My guess is that I should use Blend from shape here, but I can't figure out how, and I also may be very wrong. Maybe also Add Rest Position... ah, I simply don't know. How should I fix this, please?

[ZergSquad]

Bruh its a blender forum, not SFM.

but u lucky a bit
u fail because sfm model no such option as "relative" and all flexes being applied based on first flex 0.
So for exporting u need to have IN BLENDER first flex relative to nothing and other relative to first flex.
By achieve this (at least as i remember i did with my blender negative lvl skills) just ctrl+D mesh, choose the flex u want to be based. Or make "shape key from mix" if u have few ones on 1.000 as base.
Then delete all other shape keys leaving only this "base" one, delete this shape key LAST ONE and add new flex again. And then just transfer other shape keys from other mesh.
should be fine

PS also for SFM its better to make your "BASE" shape breast as larger as u can. You would need every time to pull flexes to make it smaller in sfm, but in general you would get BETTER shading in SFM because its just about how SOURCE ENGINE shading work

[Healisoulus]

Yeah, I wasn't sure whether I should post this in Blender or SFM hub, eventually decided to go with the former. In any case, thank you very much, I will fix this soon :D

[Healisoulus]

Initially I tried following ZergSquad's advice, but didn't reach my goal. Then I tried to fiddle with correctional flexes/shape keys, eventually that worked but not in the way I wanted. So here is what finally gave me the results I needed:

1. set a non-Breast - Default shape key relative to Breast - Default
   
2. drag it to 1.000 with everything else being at 0.000
   
3. click New Shape From Mix
   
4. set the old shape key relative to base again
   
5. rename the old shape key to something different
   
6. name the new shape key to the original name of the old one.
   
7. set all shape keys back to 0.000
   
8. (optional) repeat from step 1 for more shapekeys, if needed

Problem solved :)