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    <p>Here's the solution I finally used. Works only for grayscale images. Its the same algorithm as in the gimp_histogram_get_threshold function in gimphistogram.c </p> <p><a href="http://git.gnome.org/browse/gimp/tree/app/base/gimphistogram.c" rel="nofollow">http://git.gnome.org/browse/gimp/tree/app/base/gimphistogram.c</a></p> <pre><code>(define (auto-threshold imagePath) (let* ( (theImage (car (gimp-file-load RUN-NONINTERACTIVE imagePath imagePath ) ) ) (theDrawable (car (gimp-image-get-active-drawable theImage))) (hist (get-hist theDrawable 0)) ) (get-auto-threshold hist) ) ) ;returns the threshold (define (get-auto-threshold hist) (let* ( (hist_max (vector-ref hist 0)) (chist (make-vector 256)) (cmom (make-vector 256)) (maxval 255) ;end - start (i 1) (tmp ) (chist_max) (cmom_max) (bvar_max 0) (threshold 127) ) (vector-set! chist 0 (vector-ref hist 0)) (vector-set! cmom 0 0) (set! i 1) (while (&lt;= i maxval) (if (&gt; (vector-ref hist i) hist_max) (set! hist_max (vector-ref hist i)) ) (vector-set! chist i (+ (vector-ref chist (- i 1)) (vector-ref hist i)) ) (vector-set! cmom i (+ (vector-ref cmom (- i 1)) (* i (vector-ref hist i))) ) (set! i (+ i 1)) ) (set! chist_max (vector-ref chist maxval)) (set! cmom_max (vector-ref cmom maxval)) (set! i 0) (while (&lt; i maxval) (if (and (&gt; (vector-ref chist i) 0) (&lt; (vector-ref chist i) chist_max) ) (let* ((bvar (/ (vector-ref cmom i) (vector-ref chist i)))) (set! bvar (- bvar (/ (- cmom_max (vector-ref cmom i)) (- chist_max (vector-ref chist i)) ) )) (set! bvar (* bvar bvar)) (set! bvar (* bvar (vector-ref chist i)) ) (set! bvar (* bvar (- chist_max (vector-ref chist i)) )) (if (&gt; bvar bvar_max) (begin (set! threshold i) (set! bvar_max bvar) ) ) ) ) (set! i (+ i 1)) ) threshold ) ) ;returns the raw histogram with values 0-1 as an array (define (get-hist drawable chan) (let* ( (i 0) (hist (make-vector 256)) ) (set! i 0) (while (&lt; i 256) (vector-set! hist i (car (cddddr (gimp-histogram drawable chan i i)))) (set! i (+ i 1)) ) hist ) ) </code></pre>
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    1. COjust what I was looking for, thanks. I had to change the chan param of get-hist to 5 which is GIMP_HISTOGRAM_RGB to get it to match the value as called from the UI.
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      1. USMatthew H.
 

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