40 lines
2.0 KiB
Markdown
40 lines
2.0 KiB
Markdown
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---
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title: Boundary Fill
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---
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## Boundary Fill
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Boundary fill is the algorithm used frequently in computer graphics to fill a desired color inside a closed polygon having the same boundary
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color for all of its sides.
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The most approached implementation of the algorithm is a stack-based recursive function.
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### Working:
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The problem is pretty simple and usually follows these steps:
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1. Take the position of the starting point and the boundary color.
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2. Decide wether you want to go in 4 directions (N, S, W, E) or 8 directions (N, S, W, E, NW, NE, SW, SE).
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3. Choose a fill color.
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4. Travel in those directions.
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5. If the pixel you land on is not the fill color or the boundary color , replace it with the fill color.
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6. Repeat 4 and 5 until you've been everywhere within the boundaries.
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### Certain Restrictions:
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- The boundary color should be the same for all the edges of the polygon.
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- The starting point should be within the polygon.
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### Code Snippet:
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```
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void boundary_fill(int pos_x, int pos_y, int boundary_color, int fill_color)
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{
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current_color= getpixel(pos_x,pos_y); //get the color of the current pixel position
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if( current_color!= boundary_color || currrent_color != fill_color) // if pixel not already filled or part of the boundary then
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{
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putpixel(pos_x,pos_y,fill_color); //change the color for this pixel to the desired fill_color
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boundary_fill(pos_x + 1, pos_y,boundary_color,fill_color); // perform same function for the east pixel
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boundary_fill(pos_x - 1, pos_y,boundary_color,fill_color); // perform same function for the west pixel
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boundary_fill(pos_x, pos_y + 1,boundary_color,fill_color); // perform same function for the north pixel
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boundary_fill(pos_x, pos_y - 1,boundary_color,fill_color); // perform same function for the south pixel
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}
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}
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```
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From the given code you can see that for any pixel that you land on, you first check whether it can be changed to the fill_color and then you do so
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for its neighbours till all the pixels within the boundary have been checked.
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