How to create a "decal map" for UVs to add decals to a car

I’d like to use UVs to add a bunch of decals to an F1 race car model. While it’s fairly simple to add one or two things to simpler objects, it gets tricky (at least for me) when you want to add a bunch of things. I’ve searched and searched for good tutorials on this, but what I mostly find are ways to use UVs to make textures, but almost nothing to add multiple decals.

I think the proper way to do it would be to make a “decal sheet” - one big PNG. Unwrap my object and then place the UV islands over each decal, placing the rest over the base color so it takes the color of the material. I believe this is a common technique in gaming, and there are some old CSS/HTML tricks that use to use a similar concept.

The real problem I am finding is how to size things. I did a proof of concept, and while it generally works, the issue is that some of the islands are wonky in size, they are out of aspect, upside down, etc.

I have used seams to basically circle the area for each decal, which sort of helps. I know you can use UV display options to show islands that are messed up, but not sure how to fix them. Here’s a couple images of my test. In general I got things to be where they need to be, but there are some funky things happening - especially with the 27 where there are two areas next to each other (I didn’t put a decal in the area below it yet).

If anyone knows of some good tutorials using this technique, or can advise some tips that would be great.

If you want them on a sheet and want to get the sizes right, the way I would tackle it is as follows.
Create a blank image as your map, assign it as a texture, go into texture paint, block out the rough size and shape of the decal, you don’t need to be tidy or precise, just give yourself an indicator of placements and save the result. In your image editing software, use this as a background layer and place the decals appropriately, hide the background layer and save. You should have exactly what you need with the sizes and locations.

Alternatively or additionally, decals can be placed/scaled/rotated with object coordinates, using a helper object like an empty, and then bake. Face snapping with align rotation can help placing the empty.

That sounds interesting. Do you know of any tutorials on this technique? I assume you don’t mean placing an image plane with subD/Shrinkwrap like some of the add ons do.

That sounds interesting. Do you know of any tutorials on this technique? I assume you don’t mean placing an image plane with subD/Shrinkwrap like some of the add ons do.

Nope, one I remember was for Blender internal render. It’s very similar with Cycles, replacing the default UV mapping for an image node with texture coordinate node object output, and telling which object in the object field.

The image colors can be mixed, with the (decal) image alpha output as the mix factor. Obviously, the images you use will need alpha transparency for that to work.

Cycles viewport is bad in previewing this, need to use the rendered view to know the actual result, and this is in 2.79b.