Flood fill algorithm using C graphics

The flood fill algorithm is a technique used to fill a connected region with a specific color. It starts from a seed pixel and recursively fills all adjacent pixels that have the same color as the original pixel.

Syntax

void floodFill(int x, int y, int newColor, int oldColor);

Note: This example requires Turbo C/C++ or a compatible graphics library with graphics.h support. Modern compilers may need additional setup for graphics functions.

Algorithm Steps

The flood fill algorithm follows these steps −

• Check if the current pixel coordinates are within screen boundaries
• If the current pixel color matches the old color, replace it with new color
• Recursively call flood fill for all four adjacent pixels (up, down, left, right)
• Continue until no more pixels match the old color

Example: Rectangle Flood Fill

Here's a complete implementation that draws a rectangle and fills it using flood fill algorithm −

#include <graphics.h>
#include <stdio.h>
#include <conio.h>

void floodFill(int x, int y, int newColor, int oldColor) {
    // Check if current pixel matches the old color
    if (getpixel(x, y) == oldColor) {
        // Set new color at current pixel
        putpixel(x, y, newColor);
        
        // Recursive calls for adjacent pixels
        floodFill(x + 1, y, newColor, oldColor);  // Right
        floodFill(x - 1, y, newColor, oldColor);  // Left
        floodFill(x, y + 1, newColor, oldColor);  // Down
        floodFill(x, y - 1, newColor, oldColor);  // Up
    }
}

int main() {
    int gdriver = DETECT, gmode;
    
    // Initialize graphics mode
    initgraph(&gdriver, &gmode, "");
    
    // Define rectangle coordinates
    int left = 50, top = 50, right = 300, bottom = 300;
    
    // Draw rectangle outline
    rectangle(left, top, right, bottom);
    
    // Set fill parameters
    int seedX = 55, seedY = 55;
    int newColor = 12;  // Light red
    int oldColor = 0;   // Black (background)
    
    // Apply flood fill
    floodFill(seedX, seedY, newColor, oldColor);
    
    getch();
    closegraph();
    return 0;
}

How It Works

Key Points

• The algorithm uses recursion to fill connected pixels of the same color
• getpixel(x, y) retrieves the color of pixel at coordinates (x, y)
• putpixel(x, y, color) sets the color of pixel at coordinates (x, y)
• The seed point must be inside the area to be filled
• Stack overflow can occur with very large regions due to deep recursion

Conclusion

The flood fill algorithm is essential for area filling in computer graphics. It efficiently fills connected regions by recursively processing adjacent pixels, making it useful for paint bucket tools and region coloring applications.