Note wrong lens in 2016-09-25 pi-pan-tilt post; part 2 draft

posts/2016-09-25-pi-pan-tilt-1.md

@@ -55,6 +55,11 @@ run:
 
 [![My shot's not slanted, the ground is](../images/2016-09-25-pi-pan-tilt-1/IMG_20160918_160857.jpg){width=100%}](../images/2016-09-25-pi-pan-tilt-1/IMG_20160918_160857.jpg)
 
+(*Later note*: I didn't actually use the 25mm lens on that shot.  I
+used a 4mm (or something) lens that looks pretty much the same, and
+didn't realize it until later.  It's a wonder that Hugin was able to
+stitch the shots at all.)
+
 The laptop is mainly there so that I can SSH into the Pi to control
 things and to use [RPi-Cam-Web-Interface][] to focus the lens.  The
 red cord is just Cat 6 connecting their NICs together; the Pi is

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

@@ -0,0 +1,48 @@
+---
+title: Raspberry Pi pan-tilt mount for huge images, part 2
+author: Chris Hodapp
+date: October 4, 2016
+tags: photography, electronics, raspberrypi
+---
+
+In my [last post](./2016-09-25-pi-pan-tilt-1.html) I introduced some
+of the project I've been working on.  Those of you who thought a
+little further on this might have seen that I made an apparatus that
+captures a series of images from fairly precise positions, and then
+completely discards that position information, hands the images off to
+[Hugin][] and [PanoTools][], and has them crunch numbers for awhile to
+derive *the very same position information* for each image.
+
+That's a slight oversimplification - they also derive lens parameters,
+they derive other position parameters that I ignore, and the position
+information will deviate because:
+
+- Stepper motors can stall, and these steppers may have some
+  hysteresis in the gears.
+- My pan and tilt axes aren't perfectly perpendicular.
+- The camera might have a slight tilt or roll to it.
+- The camera's [entrance pupil][] may not lie exactly at the center of
+  the two axes, which will cause rotations to also produce shifts in
+  position that they must account for.  (More on this will follow
+  later.  Those shifts in position can also cause parallaxing, which
+  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.
+
+- 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.
+
+[ArduCam]: http://www.arducam.com/camera-modules/raspberrypi-camera/
+[forum-raw-images]: https://www.raspberrypi.org/forums/viewtopic.php?p=357138
+[raspistill]: https://www.raspberrypi.org/documentation/raspbian/applications/camera.md
+[25mm-lens]: https://www.amazon.com/gp/product/B00N3ZPTE6
+[Hugin]: http://wiki.panotools.org/Hugin
+[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