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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: filling area by color atack safety
Date: Mon, 18 Mar 2024 02:30:59 -0700
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Michael S <already5chosen@yahoo.com> writes:

>> 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;
>>> }

[...]

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

It is customary in raster graphics to count pixels as adjacent
only if they share an edge, not if they just share a corner.
Usually that gives better results;  the exceptions tend to need
special handling anyway and not just connecting through
diagonals.