Re: filling area by color atack safety

On 17/03/2024 12:46, Michael S wrote:

On Sat, 16 Mar 2024 11:33:20 +0000
Malcolm McLean <malcolm.arthur.mclean@gmail.com> wrote:
 

On 16/03/2024 04:11, fir wrote:

i was writing simple editor (something like paint but more custom
for my eventual needs) for big pixel (low resolution) drawing
>
it showed in a minute i need a click for changing given drawed area
of of one color into another color (becouse if no someone would
need to do it  by hand pixel by pixel and the need to change color
of given element is very common)
>
there is very simple method of doing it - i men i click in given
color pixel then replace it by my color and call the same function
on adjacent 4 pixels (only need check if it is in screen at all and
if the color to change is that initial color
>
int RecolorizePixelAndAdjacentOnes(int x, int y, unsigned
old_color, unsigned new_color)
{
    if(old_color == new_color) return 0;
>
    if(XYIsInScreen( x,  y))
    if(GetPixelUnsafe(x,y)==old_color)
    {
      SetPixelSafe(x,y,new_color);
      RecolorizePixelAndAdjacentOnes(x+1, y,  old_color, new_color);
      RecolorizePixelAndAdjacentOnes(x-1, y,  old_color, new_color);
      RecolorizePixelAndAdjacentOnes(x, y-1,  old_color, new_color);
      RecolorizePixelAndAdjacentOnes(x, y+1,  old_color, new_color);
      return 1;
    }
>
    return 0;
}
>
it work but im not quite sure how to estimate the safety of this  -
incidentally as i said i use this editor to low res graphics  like
200x200 pixels or less, and it is only a toll of private use,
yet i got no time to work on it more than 1-2-3 days i guess but
still
>
is there maybe simple way to improve it?

  >
This is a cheap and cheerful fllod fill. And it's easy to get right
and shouldn't afall over.

 Except I don't understand why it works it all.
Can't fill area have sub-areas that only connected through diagonal?

Suppose you have an image which is a chessboard. You want to fill one of the black squares so that it is red.
If you allow connectivity through the diagonals (so two notionally square pixels that only meet at their corners would be connected), then all the black squares would turn red, not just one.