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Any advice on ways to line up UV islands for the different LODs?


Bitsy Buccaneer
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Hi

The workflow I often use is:

1: Create the High LOD mesh

2: UV unwrap and arrange islands.

3: Create my texture for the mesh. (or use a UV test grid )

4: In Blender make a copy of the High LOD mesh and move the copy to a different layer. This copy will be the starting point of   a Lower LOD mesh.

5: In Texture Display mode (so i can see the texture on the copied mesh) I start reducing the vertices of the copied mesh by  selecting an edge loop and then bring up the Delete menu and choose Dissolve Edges. When using this Dissolve tool it usually doesn't effect the UV layout of the mesh. I work in Texture Display mode so that i can see that the Uv layout, the texture on the mesh hasn't been deformed. I repeat for as many of the edge loops as i can, always checking that the texture on the model is still good. If for some reason the texture is deformed then i take a step back with the Ctrl + Z shortcut keys.

When the edge loops are done then i move on to simple edges and individual vertices. Not Deleting but always Dissolving.Its at this stage that the texture is more likely to be deformed. When this happens i go back and select a different vertice; often its a case of picking the verices in the correct order.

Finally when the Lower Lod mesh is good I will triangulate all the faces and check if there are any other vertices that could be removed.

Below is an example of an aereoplane wing. With this mesh I used the Lower LOD mesh for all 3 Lower LOD slots in the mesh Uploader.

The important thing is to be always checking as you are dissolving that the texture is still looking good on the Lower LOD mesh.Lower LOD 1.png

the middle wing is the one just before triangulating.

Lower LOD 2.png

And the result of how the  Lower Lod wing is mapped after all the Dissolving :

Lower LOD 3.png

 

 

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Good workflow from Aquila. It's very easy to forget the UV mapping and texturing before making lower LODs, but it is essential. If you forget, delete the low LODs and go back. That will save you time in the long run.

The trick is to use only UV-map conserving tools in the decimation. Some of these, like Dissolve, always preserve the UV map. For others, like edge slide and vertex merge, it's an option you can check in the panel at the bottom of the toolshelf (left of 3D view). For yet others, such as vertex slide, it's not even an option*. So avoid those unless you want to end up editing the UV map by hand.

The UV map preservation doesn't (usually) work if you do anything with UV seams. In fact, dissolving or edge sliding a seam edge can cause catastrophic destruction of the UV map. So It's worth a little thinking ahead to make sure the seams are on edges that will stay where they are in the LOD meshes. Aquila's example shows an exception to that rule, the wheels, where a whole island gets removed. That's OK because the horrible effects happen when UV faces start connecting between islands.

One other note about the layout - if you are going to use re-usable tileable textures, you need to be careful about the orientation of the islands. For example, pieces of wood have to be oriented so that the woodgrain runs the right way. They also have to be the same scale. This applies whether you are making a second UV map for applying the texture before baking, or whether you are going to use the tiling texture inworld. The latter can save a lot of texture lag when the texture is used on multiple objects.

*ETA: It's been added in 2.74. So vertex slide is ok now,

 

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I started from scratch on the low LOD version. Just seemed more sensible rather than having to learn yet another set of skills and do them correctly umpteen times without messing up. I'm ready to just give up. All I want is a very basic 8 point cylinder of a certain height and diameter to match the nice wee pot. The learning curve on all of this is too much for me.

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Don't give up now !

Starting with a copy of the of the high LOD Wee jar (16 vertices in circumference) and with textured applied in Blender its just a case of selecting every second vertical loop and Dissolving it to get it down to the 8 vertice Lower LOD version.

Then do the Dissolve on as many horizontal loops as possible.

In the gif below I deleted the bottom of the jar and the faces that made up the lip of the lid.

Note: in the gif you don't see the Delete menu popping up all the time because sometimes I used the keyboard shortcut keys for  Repeat Last Action.

Wee jar 1.gif

 

 

 

 

 

When I started with Blender I remember I had a lot of problems understanding how to get a texture made in Gimp to appear on the mesh in the 3D Edit window.

But really its a very simple process. In Edit mode you need to first select the mesh and then select the Texture, and that’s it. The Texture is now assigned to the mesh. You then change the Viewport Shading mode from Solid to Texture to be able to see it on the mesh.

A quick way to select the texture is to simply open the UV/Image Editor and then Open the texture there.

 

 Texture on mesh.gif

 

Once you have this high Lod mesh showing a texture, Tab into Object mode, and make a copy of it, Shft + D. Move the copy to a different layer, keyboard short cut M. The copy is the base for Creating your lower LOD mesh by Dissovling edges.

 

 

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You wrote: 

 

"For yet others, such as vertex slide, it's not even an option. So avoid those unless you want to end up editing the UV map by hand."

I although vertex slide doesn't have that check box to "preserve UVs" like edge slide, I  found  sliding vertices with shortcut  "GG"  doesn't stretch/distort the texture previously assigned to those UVs.  It's as if the option to preserve UVS is built into vertex slide  .option - at least with Blender 2.74.  Please correct me if I'm wrong here.  Maybe my test of this on a subdivided plane was too simplistic.

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Ah. I was using my old 2.71. A "Correct UVs" parameter has been added to Vertex Slide since then. On my 2.74 though, it's off by default so that GG doesn't use it. Once you have checked it with the tool, it then works with GG too. Is there a way of accessing the option via GG? Or maybe there's a setting. Thanks for the correction. I'll note it on the post.

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It's not particularly intuitive.  Press GG and begin to slide the vertex (it will distort the texture)  Immediately left click and press F6 to bring up the properties menu for this vert slide tool.  Check  "Correct UV's"  and go back and position the selected vertex.  This setting will persist until you change it back or start a new session of Blender, however perhaps it can be turned on and  then you can save a session with it turned on along with your start up file.

 

Edited to add:  This contradicts what I wrote previously in that I was under the impression the shortcut key combination "GG" had correct UVs was on by default.  Probably I forgot I used the CTL V command and set this previously when I tested it out.  After your follow-up post I figured out the left clicking bit by trial and error.

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