More updates to 2016-10-04 draft

posts/2016-10-04-pi-pan-tilt-2.md

@@ -28,15 +28,50 @@ information will deviate because:
   is much more annoying to account for.
   [No, it's not the nodal point. No, it's not the principal point.][npp])
 
-That is, the position information we have is at best a guess; it's not
-sufficient on its own.  However, these tools still do a big numerical
-optimization, and a starting position that is "close" can help them
-along, so we may as well use the information.
+That is, the position information we have is subject to inaccuracies,
+and is not sufficient on its own.  However, these tools still do a big
+numerical optimization, and a starting position that is "close" can
+help them along, so we may as well use the information.
+
+Also, these optimizations depend on having enough good data to average
+out to a good answer.  Said data comes from matches between features
+in overlapping images (say, using something like [SIFT][] and
+[RANSAC][]).  Even if we've left plenty of overlap in the images we've
+shot, some parts of scenes can simply lack features (like corners)
+that work well for this.  We may end up with images for which
+optimization can't really improve the estimated position, and here a
+guess based on where we think the stepper motors were is much better
+than nothing.
+
+(TODO: Stick a photo here to explain features?  Link to my CV text?)
+
+If we look at the [PTO file format][pto] (which Hugin & PanoTools
+use), it has pitch, yaw, and roll for each image.  Pitch and yaw are
+precisely the axes in which the steppers move the camera (recall the
+pictures of the rig from the last post); the roll axis is how the
+camera has been rotated.  We need to know the lens's angle of view
+too, but as with other parameters it's okay to just guess and let the
+optimization fine-tune it.  The nominal focal length probably won't be
+exact anyhow.
+
+Helpfully, PanoTools provides tools like `pto_gen` and `pto_var`, and
+I use these in my script to generate a basic .pto file from the 2D
+grid in which I shot images.  The only real conversion needed is to
+convert steps to degrees, which for these steppers means using 360 /
+64 / 63.63895 = about 0.0884, according to [this][steps].
+
+With no refining, tweaking, or optimization, here is how this looks in
+Hugin:
+
+(supply screenshot here)
+
+TODO:
 
 - We're using Panotools and our apparatus together; they can
   cross-check each other.
-- Our position info also turns readily into a .pto file which Hugin
-  can visualize.
+- Conversion for steppers; axes that Hugin/Panotools use and what I
+  use
+- dcraw conversion?
 
 [ArduCam]: http://www.arducam.com/camera-modules/raspberrypi-camera/
 [forum-raw-images]: https://www.raspberrypi.org/forums/viewtopic.php?p=357138
@@ -46,3 +81,7 @@ along, so we may as well use the information.
 [PanoTools]: http://wiki.panotools.org/Main_Page
 [entrance pupil]: https://en.wikipedia.org/wiki/Entrance_pupil
 [npp]: http://www.janrik.net/PanoPostings/NoParallaxPoint/TheoryOfTheNoParallaxPoint.pdf
+[steps]: https://arduino-info.wikispaces.com/SmallSteppers?responseToken=04cbc07820c67b78b09c414cd09efa23f
+[SIFT]: https://en.wikipedia.org/wiki/Scale-invariant_feature_transform
+[RANSAC]: https://en.wikipedia.org/wiki/RANSAC
+[pto]: ???