Hough transform: Difference between revisions
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}
$trg put [list $row] -to 0 $rho
}
}
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pack [label .l1.i -image srcImg]
pack [label .l2.i -image targetImg]
# Postpone until after we've drawn ourselves
after idle HoughTransform srcImg targetImg</lang>
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Revision as of 01:13, 22 January 2010
Implement the Hough transform.
The transform maps each point in the target image, , to the average color of the pixels on the corresponding line of the source image (in -space, where the line corresponds to points of the form ). The idea is that where there is a straight line in the original image, it corresponds to a bright (or dark, depending on the color of the background field) spot; by applying a suitable filter to the results of the transform, it is possible to extract the locations of the lines in the original image.
The target space actually uses polar coordinates, but is conventionally plotted on rectangular coordinates for display.
A test image (.png) is located here.
Tcl
If using PNG images,
or
<lang tcl>package require Tk if {[catch {
package require Tk 8.6; # Just for PNG format handler
}] == 1} then {catch {
package require Img
}}
- If neither Tk8.6 nor Img, then only GIF and PPM images can be loaded
set PI 3.1415927 proc HoughTransform {src trg {fieldColor "#000000"}} {
global PI
set w [image width $src] set h [image height $src] set targetH [expr {int(hypot($w, $h)/2)}]
# Configure the target buffer $trg configure -width 360 -height $targetH $trg put $fieldColor -to 0 0 359 [expr {$targetH-1}]
# Iterate over the target's space of pixels. for {set rho 0} {$rho < $targetH} {incr rho} {
set row {} for {set theta 0} {$theta < 720} {incr theta} { set cos [expr {cos($theta/180.0*$PI)}] set sin [expr {sin($theta/180.0*$PI)}] set totalRed 0 set totalGreen 0 set totalBlue 0 set totalPix 0
# Sum the colors of the line with equation x*cos(th)+y*sin(th)=r if {$theta<45 || ($theta>135 && $theta<225) || $theta>315} { # For these half-quadrants, it's better to iterate by 'y' for {set y 0} {$y<$h} {incr y} { set x [expr { $w/2 + ($rho - ($h/2-$y)*$sin)/$cos }] if {$x < 0 || $x >= $w} continue set x [expr {round($x)}] if {$x == $w} continue incr totalPix lassign [$src get $x $y] r g b incr totalRed $r incr totalGreen $g incr totalBlue $b } } else { # For the other half-quadrants, it's better to iterate by 'x' for {set x 0} {$x<$w} {incr x} { set y [expr { $h/2 - ($rho - ($x-$w/2)*$cos)/$sin }] if {$y < 0 || $y >= $h} continue set y [expr {round($y)}] if {$y == $h} continue incr totalPix lassign [$src get $x $y] r g b incr totalRed $r incr totalGreen $g incr totalBlue $b } }
# Convert the summed colors back into a pixel for insertion into # the target buffer. if {$totalPix > 0} { set totalPix [expr {double($totalPix)}] set col [format #%02x%02x%02x \ [expr {round($totalRed/$totalPix)}] \ [expr {round($totalGreen/$totalPix)}] \ [expr {round($totalBlue/$totalPix)}]] } else { set col $fieldColor } lappend row $col } $trg put [list $row] -to 0 $rho
}
}
- Demonstration code
set f [lindex $argv 0] image create photo srcImg -file $f image create photo targetImg pack [labelframe .l1 -text Source] [labelframe .l2 -text Target] pack [label .l1.i -image srcImg] pack [label .l2.i -image targetImg]
- Postpone until after we've drawn ourselves
after idle HoughTransform srcImg targetImg</lang>