namespace eval ::histo {
    namespace export histo

    package require Tk

    proc histo {image {cmd -display}} {
        # get options
        switch -glob -- $cmd {
            -dis*   { set cmd -display }
            -exp*   { set cmd -export }
            default {
                error "use\n\t[histo image ?-display|-export?"
            }
        }
        # get the image size
        set width [image width $image]
        set height [image height $image]
        if {$width * $height == 0} {
            error "bad image"
        }
        # count colors components
        for {set i 0} {$i < 256} {incr i} {
            foreach c {r g b} {
                set counts($c:$i) 0
            }
        }
        set max 0
        for {set y 0} {$y < $height} {incr y} {
            for {set x 0} {$x < $width} {incr x} {
                foreach {r g b} [$image get $x $y] break
                foreach c {r g b} {
                    set n [incr counts($c:[set $c])]
                    if {$max < $n} {
                        set max $n
                    }
                }
            }
        }
        if {$cmd == "-display"} {
            # display
            # compute the coef
            set coef [expr {256.0 / $max}]
            # create toplevel
            set t _$image
            toplevel .$t
            wm title .$t $image
            # draw the histogram
            set c .$t.c
            canvas $c -width [expr {256 * 3 + 40}] -height [expr {256 + 40}]
            set x0 9
            set y0 267
            $c create rectangle $x0 9 [incr x0 258] $y0 \
                    -outline black -fill white
            incr x0 8
            $c create rectangle $x0 9 [incr x0 258] $y0 \
                    -outline black -fill white
            incr x0 8
            $c create rectangle $x0 9 [incr x0 258] $y0 \
                    -outline black -fill white
            set y1 272
            set x0 10
            for {set i 0} {$i < 17} {incr i} {
                $c create line $x0 $y0 $x0 $y1
                incr x0 16
            }
            set x0 276
            for {set i 0} {$i < 17} {incr i} {
                $c create line $x0 $y0 $x0 $y1
                incr x0 16
            }
            set x0 542
            for {set i 0} {$i < 17} {incr i} {
                $c create line $x0 $y0 $x0 $y1
                incr x0 16
            }
            incr y0 -1
            set xr 10
            set xg 276
            set xb 542
            for {set i 0} {$i < 256} {incr i} {
                    set yr [expr {10 + 256 - round($counts(r:$i) * $coef)}]
                set yg [expr {10 + 256 - round($counts(g:$i) * $coef)}]
                set yb [expr {10 + 256 - round($counts(b:$i) * $coef)}]
                $c create line $xr $y0 $xr $yr -fill red
                $c create line $xg $y0 $xg $yg -fill green
                $c create line $xb $y0 $xb $yb -fill blue
                incr xr; incr xg; incr xb
            }
            $c create text 10 276 -anchor nw \
                    -text "image: $image, max count: $max"
            pack $c
        } else {
            # export
            for {set i 0} {$i < 256} {incr i} {
                foreach c {r g b} {
                    lappend list$c $counts($c:$i)
                }
            }
            return [list $listr $listg $listb]
        }
    }
}

# example
# -----------
# to download the image:
# http://perso.wanadoo.fr/maurice.ulis/tcl/image1.png

package require Tk
package require Img
wm withdraw .
namespace import ::histo::histo
histo [image create photo -file image1.png]