In the LWVNM, we're promoting our
new non-partisan state-wide online Voter Guide,
I got roped into doing the Twitter side of this, using a bunch of
images the communications team got from the national LWV.
The problem is, the images are square, 1500x1500 pixels.
Turns out Twitter won't display square images: according to most
references I found, it crops any image you tweet to 600x335 (16:9).
Read more ...
[ 14:13 Sep 17, 2020
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This is Part IV of a four-part article on ray tracing digital elevation
model (DEM) data.
The goal: render a ray-traced image of mountains from a digital
elevation model (DEM).
Except there are actually several more parts on the way, related to
using GRASS to make viewsheds. So maybe this is actually a five- or
six-parter. We'll see.
The Easy Solution
Skipping to the chase here ... I had a whole long article
written about how to make a sequence of images with povray, each
pointing in a different direction, and then stitch them together
But a few days after I'd gotten it all working, I realized none of it
was needed for this project, because ... ta-DA —
povray accepts this argument inside its camera section:
Duh! That makes it so easy.
You do need to change povray's projection to cylindrical;
the default is "perspective" which warps the images.
If you set your look_at to point due south --
the first and second coordinates are the same as your observer coordinate,
the third being zero so it's looking southward -- then povray will
create a lovely strip starting at 0 degrees bearing (due north), and
with south right in the middle.
The camera section I ended up with was:
location <0.344444, 0.029620, 0.519048>
look_at <0.344444, 0.029620, 0>
with the same light_source
as in Part III.
There are still some more steps I'd like to do.
For instance, fitting names of peaks to that 360-degree pan.
The rest of this article discusses some of the techniques I would
have used, which might be useful in other circumstances.
A Script to Spin the Observer Around
Angles on a globe aren't as easy as just adding 45 degrees to the
bearing angle each time. You need some spherical trigonometry to
make the angles even, and it depends on the observer's coordinates.
Obviously, this wasn't something I wanted to calculate by hand, so
I wrote a script for it:
Run it with the name of a DEM file and the observer's coordinates:
demproj.py demfile.png 35.827 -106.1803
It takes care of calculating the observer's elevation, normalizing
to the image size and all that. It generates eight files, named
Stitching Panoramas with ImageMagick
To stitch those demproj images manually in ImageMagick, this should
work in theory:
convert -size 3600x600 xc:black \
outfile000.png -geometry +0+0 -composite \
outfile045.png -geometry +400+0 -composite \
outfile090.png -geometry +800+0 -composite \
outfile135.png -geometry +1200+0 -composite \
outfile180.png -geometry +1600+0 -composite \
outfile225.png -geometry +2000+0 -composite \
outfile270.png -geometry +2400+0 -composite \
outfile315.png -geometry +2800+0 -composite \
convert outfile*.png +smush -400 out-smush.png
Adjusting Panoramas in GIMP
But in practice, some of the images have a few-pixel offset,
and I never did figure out why; maybe it's a rounding error
in my angle calculations.
I opened the images as layers in GIMP, and used my GIMP script
to lay them out as a panorama. The cylindrical projection should
make the edges match perfectly, so you can turn off the layer masking.
Then use the Move tool to adjust for the slight errors
(tip: when the Move tool is active, the arrow keys will move
the current layer by a single pixel).
If you get the offsets perfect and want to know what they are
so you can use them in ImageMagick or another program, use
This assumes the panorama image is the only one loaded in GIMP,
otherwise you'll have to inspect gimp.image_list() to see where
in the list your image is.
>>> img = gimp.image_list()
>>> for layer in img.layers:
... print layer.name, layer.offsets
[ 15:28 Jul 23, 2019
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