Is it possible to generate 2nd UV for lightmaps on multiple SM

I am trying to import from 3dsMax a FBX with several objects and i want them separately so i can put materials individually.
Should I generate second channel in 3dsMax ? can anyone tell me how to do that without overlapping?
I tried unfold and it always gets some overlapping.
I also tried with packed UV’s but i got some distorted shadows after baking

I forgot to mention that i used SM editor to generate another channel, i set the lightning coordinate properly and after backing i still got distorted shadows

You should generate the second UV channel in your 3d editor for better results. Overlapping UV may be due UE combining all your objects in a single one at the import (there is a checkbox for this)
For a very interesting overview of all the lightmapping process check this:

(it is based on UDK, but almost everything is the same in UE4)

Hey VladV -

Yes, you should generate a second channel in 3DS Max for the best Lighting results. Unfortunately the automatic routes that can speed up unwrapping do not work the best for preserving the integrity of your models’ UV islands and therefore placing the shadows where the shadows should go.

Here is how I would handle the unwrap:

  1. Decide what my ultimate lightmap resolution will be in the engine, the default is 32, but I tend to like to push that a little up to 64 or even 128. If your using mobile 32 is probably fine.
  2. Assuming you are combining your meshes into one in the engine, you will need to select all the meshes and apply an UnwrapUVW. If you are not combining them as one mesh in the editor, you will repeat the following steps for each mesh item.
  3. In the UnwrapUVW modifer, Look for Map Channels section. It should by default be set to Channel 1 which is where you do all your texture unwraps for the materials. Click the up arrow next to the Channel 1 to advance to Channel 2.
  4. A Window will pop up and ask you whether you want to Abandon or Move the Unwrap. Assuming that you like the islands you created in your initial unwrap and that the follow some since of shape to the mesh, you will want to MOVE the islands.
  5. Once the Map Channel now says 2, Open your UV Editor.
  6. In the UV Editor, under preferences, turn on your grid.
  7. Still under preferences, Grid Size should be set to (1/[Resolution Size of your Lightmaps]) So for 64 it would be 0.015625 and 128 would be 0.0078125. Max will round up to its lowest significant digit in the window, but it will hold the original number entered.
  8. Click OK to accept and close the window.
  9. You should now see a very fine grid laid out over your 0,1 space. Each square in that new grid represents a pixel on the lightmap when it is rendered. Since a pixel is the smallest renderable unit, each square will either get a light component or a shadow component (think black or white).
  10. Keeping Step 9’s knowledge at hand, layout your UV islands across you 0,1 space minding the general rules of leaving at least 2-3 grid boxes between each island and remember the edges of the map wrap so the top and bottom edge are connected as are the left and right.

Size of each island will probably not be nearly as big as on your texture unwrap and that’s ok as long as they are consistently not as big. Do not worry to much about organic lines and trying to lay them out perfectly according to the grid lines as long as you respect the 2-3 box space. Organic Shapes tend to already have more shading along them that you would not see on hard edge surfaces.

This is a time consuming process, but if you do it your shadows will look so much better and you will have far less problems with light builds.

Let me know if this helps

Eric Ketchum

Thank you for this answer. Very well explained.
You also answered the next question I posted with images with my shadows and my uv’s.

I am looking for a fast and efficient way to get my SM in unreal in a decent form because I’m working with large and complex architecture buildings.

Modularity will be your friend in this case, and if you do decide to work modular then lightmapping the second UV channel will be even more important to remove the seams in between the pieces. Also remember you can save out UV unwraps from the UnwrapUVW modifier so you can unwrap something then load that unwrap on something else with very similar geometry - This works really well if you have a piece that has been copied. Here is a great article written on the subject of modularity from the UE3 days, but the workflow still holds true.