Navigation mesh points placed above model level

Let’s take a closer look at character in 15_Navigation demo. Initially it stays correctly on the ground:

After clicking and moving it ends up slightly above ground.

In fact, comparing GetPosition() values for these states gives a difference equal to 0.2 on Y asix:

Initial position: -5 0 20
Final position: -4.74867 0.198792 29.8848

In Navigation::CreateScene I’ve add following debug cout:

    // Now build the navigation geometry. This will take some time. Note that the navigation mesh will prefer to use
    // physics geometry from the scene nodes, as it often is simpler, but if it can not find any (like in this example)
    // it will use renderable geometry instead
    navMesh->Build();

    std::cout << "Random points: " << std::endl;
    for(auto i=0; i <20; ++i) {
        auto pt = navMesh->GetRandomPoint();
        std::cout << "\t" << pt.x_ << " " << pt.y_ << " " << pt.z_ << std::endl;
    }

And it looks like all navigation mesh points are placed slightly above the model level:

Random points: 
	-34.8614 0.2 -19.435
	46.8862 0.2 16.3022
	-15.4682 0.34107 -1.71237
	16.1699 0.243568 27.2042
	-14.4599 0.2 17.6979
	-8.4888 0.2 -20.1714
	-9.43277 0.2 -22.4549
	-40.1437 0.2 49.0701
	-21.6182 0.2 45.9047
	-18.2657 0.2 -47.5353
	21.4526 0.2 -18.5636
	32.3839 0.2 -35.4006
	0.882734 0.2 -36.7485
	35.3857 0.2 -44.7061
	36.6319 0.208437 11.8984
	-29.6036 0.2 46.878
	48.0064 0.2 47.2164
	-46.7391 0.2 46.3535
	2.76679 0.379008 14.8697
	-49.0324 0.2 47.8133

What’s the cause and how can I fix this?

The cause is due to the fact that Recast rasterizes navigable geometry into a volume of cells. I’m guessing your cell height is 0.2. When the ground plane is rasterized into the volume it fills the bottom layer of cells, then when it peels off the navmesh its offset by the cell height. Some ways to get around it include using a fine-grained volume resolution to reduce the error, or applying an offset to agent y position. There are geometry hacks that you could attempt by using hidden geometry to pad out the navigable area so that ground planes sit near the tops of volume cells instead of the bottoms. It’s a drawback of the volume raster technique that Recast uses, I’m afraid.