Boundary Fill Algorithm

1. Introduction

Boundary fill is used when the boundary of a region is clearly defined with a specific color. The algorithm fills pixels until it reaches this boundary.

• Uses boundary color as stopping condition
• Common in drawing applications

2. Basic Idea

Starting from a seed point inside the region, the algorithm fills neighboring pixels until it encounters the boundary color.

• Seed-based filling
• Stops at boundary color

3. Working Principle

If the current pixel is neither the boundary color nor the fill color, it is filled and its neighbors are processed recursively.

• Checks boundary color
• Uses recursion or stack

4. Types of Boundary Fill

• 4-connected Boundary Fill
• 8-connected Boundary Fill

5. Steps of Boundary Fill Algorithm

• Select a seed point inside the region
• Check pixel color
• Fill pixel if not boundary or fill color
• Recursively process neighbors

6. Boundary Fill Pseudocode

BoundaryFill(x, y, boundaryColor, fillColor):
    if color(x, y) == boundaryColor or color(x, y) == fillColor
        return
    set color(x, y) = fillColor
    BoundaryFill(x+1, y)
    BoundaryFill(x-1, y)
    BoundaryFill(x, y+1)
    BoundaryFill(x, y-1)

7. Advantages

• Easy to implement
• Works well for closed boundaries
• Boundary color based control

8. Disadvantages

• Fails if boundary has gaps
• Stack overflow for large regions
• Boundary color must be unique

9. Boundary Fill vs Flood Fill

Boundary Fill               Flood Fill
-------------------------   ----------------------------
Stops at boundary color     Replaces target color
Requires boundary defined   No boundary required
Sensitive to boundary gaps  Works on color regions

10. Applications

• Paint programs
• Polygon filling
• Region coloring in graphics

Practice Questions

1. What is boundary fill algorithm?
2. Explain 4-connected boundary fill.
3. Differentiate boundary fill and flood fill.
4. Write boundary fill pseudocode.
5. List advantages and disadvantages.

Practice Task