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Once we've got the base diffuse texture unwrapped for an object and we want to start to add on grunge or dirt or light overlays, we need to think carefully about how we unwrap the objects for that extra mapping. Part of this is looking at the size of the element and what kind of texture will be on it and really planning out in that UV space how we'll use it. For this warehouse, I've applied tiling textures to the walls, the sidewall elements, the windows, doors, and doorframes. This is ready for a grunge or dirt overlay.
I'll use an Unwrap UVW modifier using Map Channel 2 and plan out these UVs very carefully. For something like a grunge or dirt overlay, it's going to span the entire size of the wall. For this one, I'll have most of my dirt down here at the bottom where rain and things splashing up are likely to make the wall dirty. Additionally, I need to plan in some wear around the garage door. Because people will be touching it, it'll get stained putting a dirt, and possibly some along the windows, especially on the lower floor where I can really see that texture.
I need dirt on the sill and on the sidewall elements. I'll add an Unwrap UVW Modifier to this object. It shows on my map seams quickly. The first thing I want to do here is change to Map Channel 2 so I don't distort the existing UVs I have. I'll do this in the Unwrap UVW modifier. I'll scroll down to the Channel rollout and change the Map Channel from the default 1 to 2. In this case, Max is going to pop up a warning for us about changing channels.
I'm going to abandon the changes and display the existing UVs. Now when I open up the UV Editor, I'll have my polygons ready for use. I'll open up the UV Editor and I can see my UVs overlaid. This is a little awkward at the moment. Even turning off the checkered background doesn't seem to help. I really can see that they are all overlaid, which is as they should be. That's how I did them originally, with a tiling texture. The first thing then especially in a building is to flatten the mapping out again.
What this involves is right-clicking, choosing Face, and in this case, selecting all pressing Ctrl+A. Under Mapping, I'll choose Flatten Mapping. The default settings will work nicely for this. What this will do is break things apart if they meet at or above 45 degree. When I do this, I get a whole bunch of pieces. What I want to look at in laying out this UV is where do I allocate the most texture size and where can I stack elements even though it is a dirt or overlaid map? In my view, I'm going to overlay the garage doors, as they are all going to share pretty much the same kind of dirt and occlusion.
They all have the same condition. The garage door elements are these slightly taller rectangles. These can all be stacked. To start, I'll grab them all and pull them over onto one door. I can repeat this process with the windows. The windows are these slightly squashed rectangles. All of these can be overlaid, because again, the windows are all the same size and they're all going to share the same condition, basically dirty on the bottom and up around the edges and not so much at the top. Therefore, occlusion will look the same on both as will dirt.
I'll stack these and then show how it looks. What I've done is to stack out the door and window elements. What I can do in mMapping is also slide things out of the way. I really have this whole area around it as my playground for scratch space, we can call it. I'll select everything at the moment and then hold Alt and deselect the major wall elements, these are clearly visible, and we can see which one they are readily. I'll take the other pieces and slide them off to the side. Later after I scale the wall elements out, I'll take these pieces and arrange them by placement on the model, sides with slides, tops with tops, bottoms with bottoms.
So when I do a bake, they all get the right occlusion. Now I'll pay attention to the wall elements. The trick with unwrapping is to make the big pieces as big as possible within this normal or 0 to 1 space. I'll select all four of these and press R for scale. I want to keep my scale uniform. One of the things I do in architecture, especially flat building elements, is to flatten the mapping. This makes sure that the pieces are all the same relative size to each other and that the UVs are as distortion free as possible.
I'll scale this out and press W for move, fitting it into that space as well as I can. In the Arrange Elements rollout, I have some options for packing if I want to try it. Sometimes this works nicely. I'll try out Pack Custom. It's okay, but I'd rather have a little more scale. So it does boil down to a manual process of moving and scaling. I'll scale these out, move them, and fit them just inside that square. The important thing when you are laying out UVs is to get it either inside the square or far outside.
Things on the border is where game engines get finicky sometimes. Now I'll take these elements and switching my Move tool to Vertical, slide them down onto each other, keeping space between, but maximizing that space as much as possible. One of the things I see beginners do and I've seen this quite a lot in teaching is not using the UV space well enough. This is one of the first things that employers look for: how well did you use the texture space. In this case it looks like I need to scale down just a bit, but this maybe tempered somewhat by a judgment call. Which am I most likely to see and where can I get away with a little bit less resolution? I'm going to take the longest wall here and scale it down.
And the judgment call on that is that the shading along the bottom where I would get dirt or something splashing up or just general darkness and grit is constant. That unless I'm going to put something there, I need just basically a line of dirt across. So I can make this element relatively smaller next to everything else. I'll move it down into place. Later I can slide it over. And then I'll fit in more and more complex elements, holding Ctrl to select and pulling them down to fit. That's better.
Now I'll take this wall and make sure it's scaled as large as possible. Don't be afraid to zoom in. Finally as part of my layout, I'll make sure that I'm sliding these elements over. I'll switch over to the Horizontal Move instead and pull this wall just about even with the others. This is ready for the inclusion of the other elements or other use of the texture space here. We might see a roof or other pieces occupying the rest of that.
As a personal preference, I like to flip my elements around. This wall is facing upside down. I'm going to use the Rotate 90 degrees and flip it to orient with the others. That way when I'm painting, I can paint all the bottoms, all the tops, all the windows, and not have to worry about rotating around to catch the opposite side. It's a very big deal to lay out your UVs correctly. How you do it and how you maximize that texture space affects the performance of your game. If we're loading in a square texture, we have to load in all the pixels regardless of they're being used.
So it's better off to use all the space in that texture as thoroughly as possible with as many pieces spread as big as possible to get the most out of that memory.
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