[Arjen Markus] (13 september 2004) A recent discussion on the comp.lang.tcl newsgroup which arose after a question of mine :), raised the issue of vector versus raster operations in geographical information systems. While I have not found the time yet to explore this in any detail, I have found out that Tk's [canvas] can serve you well if you have a need to do this kind of computations. Let me explain the idea: * Quite often an analysis of geographic data involves finding out the (weighted) area of overlap between two polygons. Just an example: you have a projected highway which produces a lot of noise - how large is the region of houses that suffer too much noise? * Doing these computations on ''rasterised'' polygons (that is: polygons that are replaced by rectangles or squares) is much easier than using the vector representation (that is, the set of coordinates of all the vertices of the polygons). * Tk's canvas is excellent in displaying complicated polygons. It also provides query functions like 'find overlapping". So, the script below tries to estimate the overlap of two rectangles. Special notes: * The canvas is not visible * The rectangles fall partly outside the canvas's area * The rectangles are filled, because otherwise the "find overlapping" function considers them lines, rather than filled areas ... * A more elaborate scheme would go into pixels once a square was found that holds both rectangles ---- # det_area.tcl -- # Determine the area of overlap # package require Tk catch { console show } # # Create a canvas without showing it # Create two overlapping items # canvas .c -width 100 -height 100 -bg white # # It matters whether the items are filled or not # for the find operation # .c create rectangle -100 -100 20 20 -fill green .c create rectangle -10 -10 40 40 -fill red proc calcArea {} { set area 0 for { set j -10 } { $j < 4 } { incr j } { for { set i -10 } { $i < 4 } { incr i } { set x1 [expr {$i*10}] set x2 [expr {$x1+9}] set y1 [expr {$j*10}] set y2 [expr {$y1+9}] set items [.c find overlapping $x1 $y1 $x2 $y2] # if { [llength $items] != 0 } { # puts "Rectangle: $x1 $y1 $x2 $y2 - $items" # } if { [llength $items] == 2 } { puts "Rectangle: $x1 $y1 $x2 $y2" incr area 1 } } } return $area } puts [calcArea] ---- ---- ***Screenshots Section*** [http://farm5.static.flickr.com/4053/4714495015_85f4f034f0.jpg] [gold] added pix ---- [[ [Category Graphics] | [Category Geography] ]]