Intermediate Texturing Guide. (Colour, Specular and Normal maps)
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This is a quick a dirty guide about making semi-professional textures that I made for the Open Part Mod.
Not for absolute beginners.
The guide assumes you're familiar with basic 3D and texturing concepts and that you're comfortable with Blender, GIMP and Inkscape or other equivalent programs.
Having said that, I tried to make the guide clear enough so that you can google things if you need to.
Feel free to ask questions, point out errors and share your knowledge to help me improve this guide.
Unwrapping and AO
This shows how I unwrapped the spherical tank. A larger number of sections decreases the amount of texture warping, but increases number of texture seams, which can sometimes be problematic. This is personal preference but I think 8 sections is a good balance. Here I've also kept the middle two face loops completely connected and edited those UVs to be a perfect rectangle, in case I want to easily add a horizontal texture later on, like text or a straight line.
This model is unwrapped without any overlap but you can bake AO on models with overlapping UV. Whether or not it looks good depends on two things:
Whether the overlapping islands are the same and in the same place, which will happen if you duplicate a mesh. This will allow the AO to look the same for all duplicates.
Whether the overlapping islands receive the same ambient light. This means the surrounding geometry must be the same and in the same place with respect to all duplicates. This precludes the AO artefacts which are the result of the program fighting over which light intensity to choose.
Both of these must be true if you want good looking overlapping AO.
Keep in mind that overlapping UVs might (depending your your model) make it more difficult to remove texture seams with texture paint.
When choosing your texture size, which can be done at any point before you baking AO, you should keep in mind pixel density. 128px/m is probably pushing the boundaries of what looks good and it's the minimum you should be aiming for. 256px/m is roughly the pixel density of B9 parts and it's a good balance between memory usage and good looking parts.
The pixel density in the example above is a little under 128px/m. This is partly because of the large amount of empty space between the UV islands. You will have a lot of fun optimising texture space usage especially if you decide to keep rings intact. Blender has tools for automatically equalising pixel density and for optimising texture space usage, but it works best with convex UV islands. It will not place smaller UV islands inside enclosed islands like a ring or even concave islands like something in the shape of a "C".
The example above is only one part, but if you have a project with more than one .mu files or parts you can save memory by filling in the unused texture space with UVs from more than one mesh object, but remember that each mesh object in a .mu file can only have one material applied to it. Sharing a texture between meshes can save memory, but doesn't necessarily save HDD space. In order to keep your textures as small as possible in terms of disk space, make sure that the unused space in a texture is all a single colour. This helps with compression of some file types.
You can also see here the Ambient Occlusion map with the settings I used. The default settings make AO really blurred, so I decreased the Attenuation distance to 20cm. I think that's more realistic that way.
The ring tops and bottoms are continuous, again to keep texture seaming to a minimum. It's not the most efficient but it makes for nicer textures.
It's very difficult to predict the amount of time you'll spend unwrapping a model, since it's directly linked to the complexity of the model.
This shows the way I've unwrapped that bent pipe section. This way isn't always necessary, but keeping all the UVs parallel and perpendicular allows you do easily make textures that look like they're made of interlocking rings (like those shower head bendy hoses for example). This works best when the pipe was made mathematically perfect, with a bezier curve for example.
I downloaded a scratched metal texture from a free website, and decreased the contrast so it's not as noisy. Disregard AO for the time being.
Applying this simple texture results in visible seams. I need to fix this.
Doing this requires Texture Paint (or equivalent tool in whatever app you're using).
You need to save your background texture as a copy with a different name. This will be your stencil.
Make sure your base texture is active in the image view window for the mesh you're using.
I've circled the settings that are important to me, but you can experiment.
The Texture in Texture Paint settings should be your stencil texture and NOT your base/colour texture.
Then just paint on the model on the seams using the stencil, and when you're done save the colour texture in your Image view and import it as a layer in your texture source file.
No more seams!
AO should be a new layer in multiply mode above the base texture. This is the result. A nice foundation.
This took me about 30 minutes, but if you know what you're doing you can do it in 5.
Adding some colour
Firstly I need to know where to apply those colour. Blender allows you to export the UV layout in SVG format, which you'll need to open in Inkscape and merge all the objects into one. This can then be imported into GIMP as paths. If you don't merge all the faces in Inkscape then each triangle will be imported as a separate path. You can of course merge various groups of faces into separate objects in the SVG file, and each merged object will be imported as a separate path.
Images #9, #10, #11, and #12
In my experience (and I have no professional training) there are three main way of applying colour to a texture. They are done using the various Layer Modes.
Multiply (#9). This is to make an existing texture have a different colour while keeping the the noise underneath similar. This is good for making the texture look like it's made of a particular material, like the copper pipe.
Overlay or Hard Light (#10 and #11). This is to make a surface look like it has paint on it. I used this to "paint" the tank grey, and add the stripes.
Normal (#12). This is best for adding something that's meant to be a decal, like the warning labels.
All of these should be between your AO Multiply layer and your background layer. You can obviously experiment with other layer modes for colours, but I think these three are the most important.
I should mention that the less UV overlap you have, the more varied your texture can be. My way of unwrapping allowed me to add two different warning labels and leave all other sides grey.
This is the result. I literally did nothing except what I described here. It's starting to look good, but there are still three main things to do.
This took me a couple of hours but I spent most of that time experimenting with what looks best. If you have everything planned out this can take as little as 15 minutes.
First I need a stencil with which I'll paint a rough mask. I looked for a metal texture with lots of scratches, and edited it to be greyscale, with a white background and prominent scratches.
All you need to do is desaturate, and adjust levels.
Starting with a white background I painted the mask to a new image using Texture Paint as before, very roughly. I fix it later.
In GIMP you need to invert it. As a mask, the black parts determine what's invisible, and white determine what's visible. I edited everything to be more subtle in terms of size, dabbed away a lot with a dirt brush and increased the contrast a bit. The resulting mask is applied to a bright metal texture, which is placed right under the AO layer, because the scratches should affect paint and decals.
I keep a backup of the mask because I'll use it later for the specular mask.
Result. I'm pretty sure I didn't place the damage in all the right places, but this kind of thing is way beyond me. It would take some kind of simulation to determine realistic damage placement.
This can be done in about 30 minutes conservatively.
Specular Map and Normal Map
This is probably the simplest part of this texture so I did them all in one go.
This is the specular map. It's composed of the layers from the colour map. All you need to do is copy all relevant layers, lock alpha of each of the layers and bucket fill each layer with an appropriate grey colour. All colours need to be greyscale. For parts like the copper pipe, I kept the noise, desaturated and adjusted levels. Bright is very reflective, dark is matte.
This is the bump map. It is made in the same way as the specular map, except of course colours have to be edited a bit. Bright is high, dark is low.
This is the normal map. It is generated from the bump map, I made it subtle because scratches aren't very deep and decals aren't very thick.
Image #21, 22, 23
Maps applied to the model. Colour (#21), colour and specular (#22) and all together (#23).
If you have your layers well made, this can take about 30 minutes I guess.
That's all folks!
If you're going to make an actual part you need to apply the specular map (as a mask) to a merged copy of the colour map and export as an image with alpha. I'm not doing that because that's not the focus of this thread.