Step 3: Tune the Lightmap
Once you have picked a resolution, the next step is to tune how the lightmap is distributed across the model to make sure the parts of the model that need the most density have it. In general, you will want the most density in the trunk, low branches, and roots. This is the area most often seen by the player and the most prone to obvious artifacts if the density is too low. Leaves often have to have comparitively low density to fit the model in a reasonably small lightmap.
Use a combination of the following tools to tune the lightmap density distribution:
Reset - Select Tools -> Reset lightmap to set all lightmap scalars to 1.0 except the leaves. They are set to 0.0 so that each leaf gets exactly one texel and no more to start with (we will up that later).
Material Scales - Each material has a property called Unwrap scale. Increase or decrease this value to change the relative size of each piece of geometry that uses this material (very useful for caps).
Generator/Node Scales - Each generator/node combination that can house geometry has a Lightmap:Scale property. Change this value to change the size of the selected object's space in the lightmap. The distribution curve on branches can be used to shift the distribution of lightmap space on the object itself (without changing the actual lightmap area used). This curve is used to counteract the fact that branch lightmap density naturally increases near the tip as the geometry narrows but uses the same texture space.
Lightmap Resolution - If you just cannot make it work well in the resolution you have selected, you may need to bump it up by selecting a higher resolution like you did in Step 2. The layout will drastically change so you may need to do a reset and start over if it comes to this.
You can go into node editing mode and scale up individual branches, fronds, and leaves to tackle particularly troublesome areas.
Best Practice: Keep the leaves at zero until the branches are good. Then scale up the leaves to fill in all of the empty space.
Our algorithm aggressively tries to fit everything in the resolution you have selected. If you scale one component up so large that not everything will fit, the algorithm will adjust and change sizes to make it fit. If this happens, you may suddenly lose density where it was fine before. This effect can be counteracted by lowering the scale you just edited to get it back in range.