Re: filling area by color atack safety

On 17/03/2024 14:37, Michael S wrote:

On Sun, 17 Mar 2024 13:23:55 +0000
bart <bc@freeuk.com> wrote:
 

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.
>
>

 '4' variant does not appear useful for changing colors of drawn shapes,
like lines or circles. Nor would it work for changing color of text
except when font is unusually bold.
 

An 8-connected flood fill is typically too much, and a 4-connected flood fill is often too little.  Neither is perfect for all cases, but I think the 4-connected version is the most commonly used.
And as they stand, both are useless for images that are come from realistic pictures (as distinct from drawings), since real colours change gradually.
That's why graphics programs have feathered selection and masking, fuzzy fills, and so on.