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Delphi - floodfill without canvas


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Sometimes a region with a certain property has to be 
filled. Eg the brightness is above a threshold. The image
is assumed a field where the pixels are :

 MyField=array[0..width-1,0..height-1]of byte, integer;

The result is assumed also a field as above, the result 
pixel is set to 1 when it belongs to the area.

The property test is external and returns true when the
property meets the criteria :

 function test(x,y):boolean;

 eg : function test(x,y):boolean;
       result:=(Image.canvas.pixels[x,y] > threshold);

an additional function inside(x,y) has to be provided,
which checks a new pixel being inside the image.

procedure FloodFill(var image,FResult:MyField; startx,starty:integer);

procedure ff(a,b:integer);
 if (FResult[a,b]=0)and(test(image[a,b])) then begin   // valid pixel
   FResult[a,b]:=1;	  // set the pixel as used
   if inside(a-1,b-1) then ff(a-1,b-1);
   if inside(a-1,b)   then ff(a-1,b);
   if inside(a-1,b+1) then ff(a-1,b+1);
   if inside(a,b-1)   then ff(a,b-1);
   if inside(a,b+1)   then ff(a,b+1);
   if inside(a+1,b-1) then ff(a+1,b-1);
   if inside(a+1,b)   then ff(a+1,b);
   if inside(a+1,b+1) then ff(a+1,b+1);
 else FResult[a,b]:=-1;   // invalidate pixel
end;  // local proc ff

 Fresult.clear;        // set the result field to zero


The algorithm runs into a corner and fills from there. Should the boundary be just one pixel wide, the algorithm may jump across it diagonally. This is a core structure and may be extended to :

Feedback is welcome

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last updated: 23.june.99

Copyright (99,2000) Ing.Büro R.Tschaggelar