LODs are a bit of a mess

[Trass3r]

The documentation is not clear about what LOD 0 is, sometimes it's even contradictory:

jmonkeyengine/jme3-core/src/main/java/com/jme3/scene/Mesh.java

Lines 524 to 544 in 8ba6574

	* @return The number of LOD levels set on this mesh, including the main
	* index buffer, returns zero if there are no lod levels.
	*/
	public int getNumLodLevels() {
	return lodLevels != null ? lodLevels.length : 0;
	}
	/**
	* Returns the lod level at the given index.
	*
	* @param lod The lod level index, this does not include
	* the main index buffer.
	* @return The LOD index buffer at the index
	*
	* @throws IndexOutOfBoundsException If the index is outside of the
	* range [0, {@link #getNumLodLevels()}].
	*
	* @see #setLodLevels(com.jme3.scene.VertexBuffer[])
	*/
	public VertexBuffer getLodLevel(int lod) {
	return lodLevels[lod];

One has to dig deep into the code to find out that it just ignores the main index buffer when LODs are set:

jmonkeyengine/jme3-core/src/main/java/com/jme3/renderer/opengl/GLRenderer.java

Lines 3416 to 3419 in 8ba6574

	if (mesh.getNumLodLevels() > 0) {
	indices = mesh.getLodLevel(lod);
	} else {
	indices = mesh.getBuffer(Type.Index);

Also setting an LOD level to null seems to be valid:

jmonkeyengine/jme3-core/src/main/java/com/jme3/renderer/opengl/GLRenderer.java

Lines 3446 to 3449 in 8ba6574

	if (indices != null) {
	drawTriangleList(indices, mesh, count);
	} else {
	drawTriangleArray(mesh.getMode(), count, mesh.getVertexCount());

But one runs into crashes since it's not consistently handled like that:

jmonkeyengine/jme3-core/src/main/java/com/jme3/scene/Mesh.java

Line 864 in 8ba6574

return computeNumElements(lodLevels[lod].getData().limit());