[magick-users] Autolevel Correction

[Wolfgang Hugemann]

Hello,

I transferred Fred Weinhaus' autolevel script to Windows (i.e. VBS), 
because I often have to lighten bunches of under-exposed nighttime 
police photographs, see:

http://www.unfallrekonstruktion.de/ImageMagick/original.jpg
http://www.unfallrekonstruktion.de/ImageMagick/corrected.jpg

The VBS code, when translated to a DOS batch file, basically reads as 
follows:

convert original.jpg -channel R -separate r.gif
convert original.jpg -channel G -separate g.gif
convert original.jpg -channel B -separate b.gif

convert r.gif -contrast-stretch 0%,1% rc.gif
convert g.gif -contrast-stretch 0%,1% gc.gif
convert b.gif -contrast-stretch 0%,1% bc.gif

convert rc.gif -gamma 3.56 rcg.gif
convert gc.gif -gamma 4.23 gcg.gif
convert bc.gif -gamma 2.84 bcg.gif

convert rcg.gif gcg.gif bcg.gif -channel RGB -combine corrected.jpg
........

Fred's basic idea is to split the three channels R,G,B, adjust each of 
them separately and combine them again to a color photograph. As the 
above example shows, this works just fine. (Anthony: you may use the 
above photographs as an example, as the licence plates cannot be seen.)

The gamm-values mentioned above are just abitrary examples. In the 
skript, I calculate them according to Fred's formula:

gamma = ln(mean_value)/ln(0.5)

This garantuees that in the corrected picture, the mean value is about 
half of the maximum pixel value (i.e. quantum range).

A few remarks however:

1) It is not really clear to me what the difference between 
"-contrast-stretch" and "-linear-stretch" exactly is.

2) @ Fred: The mean value which is needed for the gamma formula can be 
computed without using the "quantumrange" value, just by
identify photo.jpg -format "%[fx:mean]"

3) @ Fred: I think that "-contrast-stretch" does a better job on this as 
your computing of the "-level" values in your script. As far as I 
understood, "-contrast-stretch" actually lets you specify what 
percentage of the pixels will turn to pure black and pure white. (Correct ?)

4) The gamma adjustment according to the above formula could be easily 
implemented in IM itself, for instance if the user supplies a gamma 
value of zero, or by some "auto-correct" or "middle" option. Or is there 
any chance that I can get the result of the "identify" statement into 
just one "convert" command, so that I could do the above transformation 
in a simple batch file, instead of having to deal with a tedious VBS?

Greetings from Münster
Wolfgang Hugemann

P.S.: I'll come back on that lens distortion issue in short term...