I've been wanting to do a write up on this subject but I found this killer example:http://www.polycount.com/forum/showthread.php?t=89682
At the bottom of page 2 there is a post with the texture sheet. Basically he's making all of those space station assets from one 512x256 diffuse and normal map. And he's hiding the decal map in a seperate channel just cause he's hyper efficient I guess
Anyway, do this, save draw calls. It's not necessary to be this extreme but it's the same mindset we used making the clobberdome map in SMC which is 95% on 3 sheets (structures, padded surfaces, floor).
Grabbed this for your convenience^^^^
Also I want to talk about it a little. So basically he's milking this for ALL ITS WORTH and its pretty genius. You could really use all three of these seperate and maybe add 350-750kb of vram depending on the compression you use. Let me just outline what each channel is doing so everybody gets it. Remember each channel could be a seperate map and you'd still be really efficient, this guy is also a shader wizard and that helps
RGB+A Map: This is the diffuse (color) map on everything. It looks screwy combined because he's really just using the map to store multiple maps nested in the different channels, lets explore...
-Red Channel: Specular map, this adds shiny-ness, its probably adding to the green channel in the shader.
-Green Channel: The actual seam pattern where he's getting all the greebles and details from. Most of the clever UVing is done on this channel.
-Blue Channel: Decal Map, color is likely being handled in the shader.
-Alpha: The computer screens, likely strictly additive in the shader so the black doesn't show up. Color again in the shader afaik.
RGB: This is the normal map, but again he's doing more.
-R+G: The normals. UV's here will match UV's in the Green Chanel above to add bumpiness.
-B: This is a material mask so the shader can manipulate the white space seperately from the black space. Meaning he can tweak the specularity or tint on one section independent from the other. That's how he's making some walls orange, but the vents and seams stay grey.