TSI: further info

TSI: principle

The TSI consist of a webcam looking downwards to a dome shaped mirror. Images of the camera are analysed for red, green and blue ratios in the built-in linux machine. The resulting numbers, the image and the analysed image mask with detected sky-, thin and opaque cloud areas are stored in the TSI's ring memory.

Data acquisition

Data is received here in cologne every 20sec from the TSI via wget in a bash script and stored in an ascii file. The data of this file comes in the form:


  dd.mm.yyyy hh:mm:ss t_raw_ms      opq thn sun_flag sol_azi sol_ele

With

dd.mm.yyyy hh:mm:ss date and time in UTC
t_raw_ms time in millisecons since 1.1.1970 0:00:00 UTC
opq, thn percentage of opaque and thin clouds
sun_flag flag wether sun is shining or not
sol_azi, sol_elesolar azimuth and elevation as calculated by the instrument

Panorama

The panorama is calculated with the transformation


  pan(x,y) = raw( cx - R*y/w * sin(2π*x/w) , cx + R*y/w * cos(2π*x/w) )

Where:

raw(i,j) is the color of a pixel with coordinates i,j in the raw image
pan(x,y) is the color of a pixel with coordinates x,y in the panorama
cx,cy are the coordinates of zenith in the raw image
R is the radius of the horizon in the raw image
w,h are width and height of the panorama

The transformation is done in a bash script with image magick using the fx command and 'look up tables' (lut). The commands are:


  convert -size $wpan'x'$hpan xc: -channel G -fx "($cx-$rad*j/$hpan*sin(2*pi*i/$wpan))/$wraw" -separate lutX.png
  

  convert -size $wpan'x'$hpan xc: -channel R -fx "($cy+$rad*j/$hpan*cos(2*pi*i/$wpan))/$hraw" -separate lutY.png
  

  ...
  

  convert lutX.png lutY.png raw.jpg   -fx "u[2].p{ u*$wraw , v*$hraw }" pan.jpg
  

  convert raw.jpg pan.jpg -geometry +0+$(($hraw-$hpan)) -composite raw.jpg

The first two convert commands generate the look up tables. They are executed only once a day. The third convert command generates the panorama using the look up table, the forth convert command copies the panorama to the bottom border of the original image. The parameters are:

$wpan and $hpan desired width and height of the panorama.
$cx and $cy position of zenith in the image
$rad radius of the mirror in the image
i,j are by default the coordinates in the source image
v.p is by default the pixel in the second image of the argument list
The second line copies the panorama finally to the lower border of the original image