# Flood Fill Algorithm vs. Boundary Fill Algorithm: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on February 10, 2024
Flood fill algorithm fills an area with a chosen color by spreading from a seed point, while boundary fill algorithm fills until it reaches a specified boundary color.

## Key Differences

The flood fill algorithm starts from a seed point and floods an area with a specific color, propagating in all directions. The boundary fill algorithm also begins at a point but fills the area until it encounters a boundary of a different color.
Flood fill checks for a target color to replace. In contrast, boundary fill looks for a boundary color to stop the fill process, not filling those pixels.
Flood fill is used in scenarios like filling a continuous area in a paint program. Boundary fill is suitable for filling bounded shapes where the boundary color is known and consistent.
Variants of flood fill include the 4-way and 8-way algorithms, affecting how it spreads. Boundary fill also has variants, such as 4-connected and 8-connected, influencing how it detects boundaries.
Flood fill can be less efficient in complex images with many different colors, while boundary fill might struggle with irregular boundaries or varying boundary colors.

## Comparison Chart

### Starting Condition

Begins at a seed point and spreads
Starts at a point and fills until a boundary

### Color Testing

Replaces a target color
Stops at a specified boundary color

### Typical Usage

Filling continuous areas
Filling bounded shapes

### Algorithm Variants

4-connected or 8-connected boundary detection

### Efficiency and Complexity

Less efficient in multi-colored areas
Challenges with irregular or varying borders

## Flood Fill Algorithm and Boundary Fill Algorithm Definitions

#### Flood Fill Algorithm

Flood fill operates until it reaches different colors.
The flood fill stopped spreading when it hit a border of a different color.

#### Boundary Fill Algorithm

Boundary fill is used in computer graphics.
Boundary fill was perfect for filling in the complex pattern without overflow.

#### Flood Fill Algorithm

Flood fill replaces a target color.
The flood fill algorithm replaced all the red pixels with blue.

#### Boundary Fill Algorithm

Boundary fill uses a boundary color to stop filling.
Boundary fill stopped when it encountered the black outline of the shape.

#### Flood Fill Algorithm

Flood fill is used in graphic applications.
I applied the flood fill algorithm to fill colors in a digital drawing.

#### Boundary Fill Algorithm

Boundary fill is suitable for bounded shapes.
We used boundary fill to color a circle with a defined edge.

#### Flood Fill Algorithm

Flood fill algorithm fills an area from a seed point.
We used the flood fill algorithm to change the background color in the image.

#### Boundary Fill Algorithm

Boundary fill algorithm fills an area up to boundaries.
The boundary fill algorithm precisely filled the shape without crossing its borders.

#### Flood Fill Algorithm

Flood fill spreads out in all directions.
The flood fill algorithm smoothly filled the entire selected area.

#### Boundary Fill Algorithm

Boundary fill can have 4-connected or 8-connected variants.
The 4-connected boundary fill algorithm effectively filled the square shape.

## FAQs

#### What's the purpose of the boundary fill algorithm?

To fill an area bounded by a specific color.

#### How does flood fill determine where to fill?

It fills areas with a target color, spreading out from a seed point.

#### What does the flood fill algorithm do?

It fills a connected area starting from a seed point.

#### What stops the boundary fill algorithm from filling?

Encountering a specified boundary color.

#### Can flood fill handle non-continuous areas?

No, it works best with continuous areas of the same color.

#### Does flood fill work diagonally?

It depends on the variant: 8-way flood fill can fill diagonally.

#### Is boundary fill suitable for shapes with uneven borders?

It might struggle with irregular or unclear boundaries.

#### Where is boundary fill commonly used?

In graphic applications for filling well-defined shapes.

#### What's a key consideration when using boundary fill?

The boundary color should be consistent and clear.

#### What's a common use of flood fill in software?

In paint programs for filling large, uniform areas.

#### What happens if the seed point in flood fill is mischosen?

It may fill an unintended area or not fill anything at all.

#### What's a limitation of the flood fill algorithm?

It may not work well with areas having multiple different colors.

#### How does color selection affect flood fill?

The algorithm replaces all pixels of the selected target color.

#### Can boundary fill handle multiple boundary colors?

Generally, it's designed for a single, consistent boundary color.

#### Is flood fill algorithm resource-intensive?

It can be, especially in large areas with complex color patterns.

#### How does the boundary fill algorithm react to gaps in the boundary?

It might fill outside the intended area if there are gaps.

#### What challenges does boundary fill face?

Difficulty in filling areas with irregular or varied boundary colors.

#### Can flood fill be customized?

Yes, through different spreading mechanisms like 4-way or 8-way.

#### How does the choice of seed point affect boundary fill?

It determines where the fill starts and can affect the fill area.

#### Are flood fill and boundary fill algorithms used in 3D graphics?

They're primarily used in 2D graphics, but concepts can be extended to 3D.