This question is kind of complicated to formulate, but i´ll give my best.
In my app 6 textures form one texture-object.
Height (A) (for parallax-mapping)
So, i COULD combine them like this:
Diffuse / Alpha
Normal / Gloss
Emissive / Height
Or, since Diffuse and Height certainly have the same resolution, but Emissive and Alpha maybe have lower resolutions:
Diffuse / Height
Normal / Gloss
Emissive / Alpha
This way, i can reduce the number of texture-accesses. HOWEVER i will do some different passes, for example a z-only pass, and in such a pass i only need Alpha. Now, since i combined Emissive and Alpha, the card now needs to load and access 4 times the memory, it would need to access, if i had only Alpha in the texture.
Also, when doing Parallex-Mapping, i would need to access Diffuse/Height, instead of only Height, which is certainly slower.
What i mean is, that i DO reduce the number of necessary texture-access, but some accesses are certainly a lot slower because of unneeded data.
Since i do have enough texture-units, i could simply not combine any textures (except for Normal/Gloss which really makes sense). But then i would need more texture-accesses.
Therefore, i´d like to ask, if anyone knows, if this is better or maybe worse. At the moment i cannot test, which is faster, since my app is still at a very early stage, but i´d like to at least have a clue right now.
I hope, that most gfx-cards act quite equal in this case, but if it matters, i am working on a Radeon 9600XT.
One additional idea: If i seperate all textures, i could also save on filtering-computations, because i can use aniso-filtering on my Diffuse-Texture and bilinear on my Alpha-Texture, etc.
Thanks in advance.