proc analyse img {
     set h [image height $img]
     set w [image width  $img]
     set raw [$img data]
     set clr0 [lindex [lindex $raw 0] 0]
     set clr $clr0
     set str {}
 
     catch {
 	foreach line $raw {
 	    foreach pix $line {
 		if {$pix ne $clr} {
 		    set clr1 $pix
 		    return; #double break
 		}
 	    }
 	}
 	set clr1 $clr0
     }
 
     foreach line $raw {
 	foreach pix $line {
 	    append str [expr {$pix ne $clr}]
 	    set clr $pix
 	}
     }
     set clr0 [expr {[string replace $clr0 0 0 0x]}]
     set clr1 [expr {[string replace $clr1 0 0 0x]}]
     return [binary format ssiib* $w $h $clr0 $clr1 $str]
 }
 
 proc synth {data} {
     binary scan $data ssiib* w h clr0 clr1 str
     set img [image create photo -width $w -height $h]
 
     set clr0 \#[format %06x $clr0]
     set clr1 \#[format %06x $clr1]
 
     set empty [list]
     set data $empty
     set line $empty
 
     set clrs [list $clr0 $clr1]
     set clr $clr0
     set nclr 0
     set i 0
     foreach bit [split $str {}] {
 	if {$bit} {
 	    set nclr [expr {!$nclr}]
 	    set clr [lindex $clrs $nclr]
 	}
 	lappend line $clr
 	if {[incr i] == $w} {
 	    lappend data $line
 	    set line $empty
 	    set i 0
 	}
     }
 
     $img put $data -to 0 0
     return $img
 }