## Waypoint Projection

Waypoint projection generates a estimate of the distance to the waypoint along
the designated ray and a confidence value. Taken together with the ray, this
forms a polar coordinate of the projected waypoint, which is used by waypoint seeking to control driving.
There are three levels of waypoint projection quality (best to worst):
stereo projection,
flat earth projection,
and line following projection.
In any given cycle, we use the best quality of waypoint projection for which
there is sufficient information. The projection quality of a waypoint can
change from one cycle from the next as range information availability changes.

We return a confidence value [0..1) along with the projected range. This is
used by waypoint seeking to compute a weighted
average of the projected ranges. The interpretation of this value is that a
value with a confidence of 0.5 should be weighted as much as 5 estimates with
confidence 0.1, etc.

### Stereo Projection

If it exists, we use stereo range information. Since in general our line of
sight is no longed aligned with the waypoint ray, the location of the waypoint
in our current stereo field of view becomes ambiguous.
We project the waypoint ray into our current field of view and query the
range all along this line. If we get a range that lies sufficiently close to
the waypoint ray, then we use that intersection point as the projected waypoint
(we return the distance along the original ray of the intersection.) This
projected waypoint may be arbitrarily far from the actual waypoint, see analysis.

The confidence of stereo projection values is generally high, but should take
into consideration the basic stereo range uncertainty, which goes up as the
square of distance. We might also want to take into consideration how close an
intersection we got between the stereo range and the waypoint ray.

### Flat Earth Projection

*flat earth projection* guesses a 3-space location for the waypoint by
intersecting the designated waypoint ray with the current orientation plane of
the robot. The distance along the designated ray to the intersection is
returned as the result of waypoint projection. If there is no intersection
between the waypoint ray and the current plane, then there is no flat earth
projection, and we fall back to line following. We can use the intersection
angle between the designated ray and the current place as a confidence value
(confidence is greatest when the ray is normal to the plane.) The maximum
confidence of flat earth projection is less than that of stereo.

### Line Following Projection

*Line following projection* is is the mode of waypoint projection used
when no range is available. We effectively assume that the waypoint is at
infinity, and drive along the designated ray. This is trivially implemented by
returning an infinite range and a confidence of zero.

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