Ok, let's do it quick 'n' dirty!
First of all, I won't go into detail how the external applications work. They're easy to use and you will find lots of documentation, videos, tutorials with a google search. I'll just concentrate on creating a lower level of detail model.
If you don't already have it, you should download
Xnormal and
Instant Meshes. The latter is more optional but if you want a cleaner topology, get that too. Xnormal is a must have and I'll just concentrate on that.
Inside C4D:
Let's assume you have a High-poly mesh.
(pic 1) It should be properly UV mapped, no overlaps, flattened.
Make sure it's in the center at 0,0,0(same with rotation) and export that mesh as an OBJ file. (you can take the original file if it's done properly)
Add a polygon reduction modifier/deformer to your object and set the % amount to how many polygons you want your mesh to be decreased by.
The settings can be mostly set to their defaults. We're creating a lower LOD object and it doesn't have to win a pageant. The slight differences won't be visible in the distance.
When you're happy with the result, convert the current state to a new object.
(pic 2 -80% decreased & untriangulated) Check the UV maps. Sometimes the resulting Map is fine, sometimes you have to create a new one. If you have to create a new one, you don't have to do much, just make sure that the UV map is clean. The island's positions don't matter here. Export that mesh too.
You can add a base map for both objects and compare the result.
(pic 3-high poly mesh)(pic 4 decimated version with same material applied)Start Xnormal:
Load the exported Highpoly object into the high poly tab and do the same with the decimated version in the low poly tab.
If you have a map, such as a base map, then go to the high poly tab, select your object and load in the "base texture". This works for pretty much all kinds of maps. Diffuse, specular, roughness etc.
Just don't add AO, bump maps as they are going to be baked anyway.
Normal maps are the exclusion but for this example, you should only load the diffuse texture.
In the baking settings, set what types of maps you want to render and set the desired resolution, which should be smaller than the original of course.
Typically, The base texture, the normal map and height map, but you can add more types if you need them. You can also set the padding size for the soon to be generated maps, depending on your low poly object's UV layout.
For this example, I've left everything at default values. You can also choose a renderer like one of the GPU renderers which are quicker but don't support all types of maps. The default bucket renderer is fine. The textures won't be large and it won't take more than a few seconds.
Click on the generate button and let the thing do its job.
The maps might look weird, without structure. doesn't matter, it won't be visible in the end.
(pic 5 base map for the low poly object)After the maps have been rendered, go back to C4D and apply the newly created maps to a new material to the low poly object.
(pic 6 - lowpoly with new maps-base/normal, very similar look to the original in pic 3)
Even though we didn't care about proper settings, the result is very close to the original and it won't be noticeable when the new LOD object is farther away from the camera.
If you're doing animation, then you can create a Xpresso script. It just has to toggle the visibility of the high/low poly objects depending on the camera distance to the objects. A simple boole switch setup could do the job.
This is one of many ways to create LOD objects. Doesn't cost extra money for a specific application and the whole thing can be done in 3 minutes.
If the object is more complex, you have to adjust the parameters to get a bit of a cleaner look. But start with the shitty settings first and see if the result's enough for your purpose.
