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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: filling area by color atack safety
Date: Sun, 17 Mar 2024 13:23:55 +0000
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On 17/03/2024 13:15, Michael S wrote:
> On Sun, 17 Mar 2024 12:54:34 +0000
> bart <bc@freeuk.com> wrote:
> 
>> 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.
>>
> 
> That's what I want.
> Do fir wants something else?
>

His algorithm is the same as that presented in my textbook, where it is 
called FloodFill4.

If I reread the notes I see now the significance of the '4', as it talks 
about 4-connected and 8-connected versions.

Presumably you want the 8-connected version, which will have 4 extra 
calls for the pixels at each corner.