Fix bounding volume for tiny ground primitives

[hpinkos]

Fixes #4326

The bounding box minimum wasn't quite low enough for really really tiny ground primitives which resulted in z-fighting

var viewer = new Cesium.Viewer('cesiumContainer');
var r1 = new Cesium.Rectangle(-0.2865044365070024, 0.49644278153576343, -0.28647774383347757, 0.496452001539263);
viewer.scene.primitives.add(new Cesium.GroundPrimitive({
    geometryInstances : [new Cesium.GeometryInstance({
        geometry : new Cesium.RectangleGeometry({
            rectangle : r1,
            vertexFormat : Cesium.PerInstanceColorAppearance.VERTEX_FORMAT
        }),
        attributes: {
            color: Cesium.ColorGeometryInstanceAttribute.fromColor(new Cesium.Color(1.0, 0.0, 0.0, 0.5))
        }
    })],
    appearance : new Cesium.PerInstanceColorAppearance({
        closed : true
    })
}));
viewer.scene.camera.setView({
    destination: r1
});

[mramato]

Unit test?

[hpinkos]

ready

[pjcozzi]

15.0 and please add a comment here.

This could also have a significant performance impact for horizon views. How did you come up with 15? I would much rather this depend on the view parameters since 15 meters, for example, could be millions of pixels when zoomed in close.

[hpinkos]

What view parameters? This function takes a granularity and an ellipsoid.

There's no specific reason I chose 15. For the code example I pasted above even subtracting 1.0 fixed the problem. From my understanding problem happens when the shadow volume ends exactly at the height the terrain does, so maybe 1.0 would work for all cases? I honestly don't know. @bagnell @tfili what do you think?

[pjcozzi]

View parameter - where is the camera and what is the FOV. This extrusion should happen in the vertex shader on a frame by frame basis so we don't create huge volumes when zoomed that create a high fillrate load.

[hpinkos]

Ah okay. Well that kind of goes beyond my realm of knowledge, so someone else will have to take over here.

[pjcozzi]

Also, update CHANGES.md.

[mramato]

@tfili can you review this, or should it be @bagnell?

[pjcozzi]

@mramato as discussed above, this needs a new approach, not a review. @bagnell should be able to fix this when available.

[mramato]

Ah, okay sorry for the noise; I missed that comment about the vertex shader.

[hpinkos]

Closing this. I added a comment to #4326 pointing here for more information