Image Borders

Some of the oneIPL image processing functions operate on user-defined border types. It means that the values of border pixels must be assigned in accordance with the border_type (or border) and border_val (optional) parameters, specified in the algorithmic spec.

oneIPL supports the following border types:

• Replicated border

• Wrapped border

• Mirrored border

• Mirrored border with replication

• Default border

• Constant border

• Transparent border

• Border in memory

The borders must be controlled via enum.

enum class oneapi::ipl::border_types : int

Type of border. Enum values are consistent with IppiBorderType.

Values:

enumerator repl

Replicated border type.

enumerator wrap

Wrapped border type.

enumerator mirror

Mirrored border type, left border: 012… -> 21012…

enumerator mirror_repl

Replicated mirrored border type, left border: 012… -> 210012…

enumerator default_val

Constant value border type, border value is max value of corresponding type.

enumerator const_val

Constant value border type.

enumerator transp

Outer pixels are not processed.

Replicated Border

This type of border shall correspond to the repl value in the border_types enumerator. When using a replicated border, values for border pixels shall be obtained from the source image boundary pixels, as shown in the figure below. Squares marked in red shall correspond to pixels copied from the source image ROI. Squares with green values shall correspond to border pixels, which must be replicated from the boundary pixels of the source image.

Wrapped Border

This type of border shall correspond to the wrap value in the border_types enumerator. When using a wrapped border, values for border pixels shall be obtained from the source image boundary pixels, as shown in the figure below. Squares marked in red shall correspond to pixels copied from the source image ROI. Squares with green values shall correspond to border pixels that must be taken from the source image.

Mirrored Border

This type of border shall correspond to the mirror value in the border_types enumerator. When using a mirrored border, values for border pixels shall be obtained from the source image boundary pixels, as shown in the figure below. Squares marked in red shall correspond to pixels copied from the source image ROI. Squares with green values shall correspond to border pixels, which must be mirrored from the source image pixels.

Mirrored Border with Replication

This type of border shall correspond to the mirror_repl value in the border_types enumerator. When using a mirrored border with replication, values for border pixels shall be obtained from the source image boundary pixels, as shown in the figure below. Squares marked in red shall correspond to pixels copied from the source image ROI. Squares with green values shall correspond to border pixels, which must be mirrored from the source image pixels. The difference of this border type from the mirrored border is that the anchor cell value shall be replicated to the border pixels.

Default Border

This type of border shall correspond to the default_val value in the border_types enumerator. When using a default border, values for all border pixels shall be set to the constant value equal to corresponding data type default value. In the figure below, this default value is marked as V. Squares marked in red correspond to pixels copied from the source image ROI.

Constant Border

This type of border shall correspond to the const_val value in the border_types enumerator. When using a constant border, values for all border pixels shall be set to the constant value specified in the spec parameter of the corresponding function. In the figure below, this constant value is marked as V. Squares marked in red correspond to pixels copied from the source image ROI.

Transparent Border

This type of border shall correspond to the transp value in the border_types enumerator. When using a transparent border, border pixels are not processed.

Border in Memory

If ROI is inside image across any boundary and external pixels from source image are available, the values shall be taken from external pixels if crop flag is crop == crop_types::off. If crop == crop_types::on, the border in memory is not used and pixels must be generated according to the border_types enumerator value. In the figure below, squares marked in red correspond to pixels copied from the source image ROI. Squares with black values correspond to source image pixels in memory.

Borders in Neighborhood Operations

Filtering functions described in this section shall perform neighborhood operations. They shall operate on the assumption that for each pixel to be processed, all neighborhood pixels required for the operation are also available.

The neighborhood for each given pixel must be defined by the filter kernel (or mask) size and anchor cell position. For more information about anchors and how to define the anchor cell position refer to Neighborhood Operations.

As the following figure illustrates, if the input pixel is near the horizontal or vertical edge of the image, the overlaid kernel may refer to neighborhood pixels that do not exist within the source image and are located outside the image area.

The set of all boundary source image pixels that require such non-existent pixels to complete the neighborhood operation for the given kernel and anchor is shaded yellow, while the collection of all scanned external pixels (called border pixels) is shaded gray.

If the neighborhood operation is needed to be applied to the part of the source image, or ROI, then the necessity of extending the image area with border pixels shall depend on the ROI size and position within the image. There are two options to handle borders in case of ROI, which can be switched by crop_types enumeration in algorithm specification:

enum class oneapi::ipl::crop_types : int

Crop types.

Values:

enumerator on

Crop is applied.

enumerator off

Crop is not applied.

If boundary pixels are part of ROI, some area of the source image must be extended.

Crop type	Description
on	All ROI borders must be treated according to the specified border type
off	Pixels outside ROI must be used for calculations when available. Otherwise specified border type is applied

The figure below illustrates behavior when crop == crop_types::off. If ROI does not cover yellow (internal boundary) pixels, then internal image pixels outside ROI shall be used.