Decals vs 2nd UV Channel?

jordank95

I have a train that Im texturing. Im using a trim sheet that I will probably vertex paint with another dirtier/worn version of the same trim sheet, but for dirt leaks, Im deciding on using decals or adding in a 2nd UV channel. Any advice here on what would be best?

Also, should I make this train in modular pieces, or does it not matter? It won't be used anywhere else. The only reason I can think to make it into modular pieces, is lightmap reasons. 

poopipe

a second uv set is more mesh memory and generally harder to place and manipulate than decals
decals are generally more expensive to render, involve an extra material, can lead to sorting/depth related artefacting and are generally more work to manage. 

modular construction makes sense if you need to repeat a lot of relatively dense geometry and no sense at all if you're looking at simple geometry. 
if we're talking about an FPS viewpoint, building the main structure of each train car as a unique mesh and decorating it with modular parts (eg railings, wheels etc)  is probably a good place to start. 

sacboi

IMHO an insightful wip thread for trims, decals and modular elements, even as a partially switched on modeler I'd learnt a bit so maybe something in there that'll help, as well -  ComEng V-Set Train

gnoop

In my experience you usually need both . second UV  for macro/unique   texture layer  and masks  and    tillable  UVs .  Plus  the decals too.    So they  are complimenting rather than excluding each other.

jordank95

@gnoop thanks. do you have any good examples of this?

gnoop

jordank95   Sorry , to show any screenshots  I have to ask for permissions   but  my guess it's pretty typical   approach  when you do something of macro scale.  Buildings  for example  with unique  uv for  AO and blend masks , while having  repeating base materials .  Splat sharers for terrains  etc.   And decals for wall signatures  , small details , corner  dents etc.     The problem with another unique /macro UVs is  that they need its own tangent base for normal maps so sometimes  people use  same UV as in tillable textures  just scaled up  but  it doesn't work for AO  if your game incapable to do nice looking AO in screen space.     Sometimes you need  unique UV for AO to support  a cheap screen one .  Or for big rocky walls  where even macro tiles would be visible.      Besides decals are usually pretty expensive  and  you need lots of them , really a lot to hide  repeating  tiles  especially for something  having long specifically looking  stripes  like tire tracks  for example  or dried out streams which you cant mess with random  UV shifting in the shader.     

But in the end it's a design decision.   There is no Dogma  or god given rules  regarding that.