The block outputs either a matrix of match metric values or the zero-based location of the best template match. The block outputs the matrix to the Metric port or the location to the Loc port. Optionally, the block can output an NxN matrix of neighborhood match metric values to the NMetric port.

Since the Template Matching block does not pad the input data, it can only compute values for the match metrics between the input image and the template for where the template is positioned such that it entirely falls on the input image. A set of all such positions of the template is termed as the valid region of the input image. The size of the valid region is the difference between the sizes of the input and template images plus one.

The output at the Metric port for the Match metric mode is of the valid image size. The output at the Loc port for the Best match index mode is a two-element vector of indices relative to the top-left corner of the input image.

The neighborhood metric output at the NMetric port is of the size NxN where N must be an odd number defined in the block mask.

To perform template matching on a subregion of the input image, select the Enable ROI processing check box. This check box adds an input port ROI to the Template Matching block. The ROI processing option is only available for the Best match index mode.

The ROI port requires a four-element vector that defines a rectangular area. The first two elements represent the zero-based row and column coordinates for the upper-left corner. The second two elements represent the height and width of the ROI. The block outputs best match location index relative to the top left corner of the input image.

The block computes the match metric at each step of the iteration. Choose the match metric which best suits your application. The block evaluates the best metric value based on the metric choice. The best metric is the global optimum over the support of the "valid" subregion of the input image intersected by the ROI, if provided.

The matrix of the match metric values always implements single-step exhaustive window iteration. Therefore, the block computes the metric values at every pixel.

When in the ROI processing mode, the block treats the image around the ROI as an extension of the ROI subregion. Therefore, it computes the best match locations true to the actual boundaries of the ROI. The best match location indices returned are relative to the top-left corner of the input image.

If you choose to return the matrix of metric values in a neighborhood around the best match, an exhaustive loop computes all the metric values for the NxN neighborhood. This output is particularly useful for performing template matching with subpixel accuracy.

When you select Best match location as the output option, you can choose to use either Exhaustive or Three-step search methods.

The Exhaustive search method is computation intensive because it searches at every pixel location of the image.

The Three-step search method is a fast search method employing the following steps:

1. The search starts with a step size equal to or slightly greater than half of the maximum search range.

2. The block compares nine search points in each step. They comprise the central point of the search square, and eight search points located on the search area boundaries.

3. The block decrements the step size by one, after each step, ending the search with a step size of 1 pixel.

4. At each new step, the block moves the search center to the best matching point resulting from the previous step.

Three-Step Search

The ROIValid and NValid ports represent boolean flags, which track the valid Region of Interest (ROI) and neighborhood. You can use these flags to communicate with the downstream blocks and operations.

Valid Region of Interest

If you select the Output flag indicating if ROI is valid check box, the block adds the ROIValid port. If the ROI lies partially outside the valid image, the block only processes the intersection of the ROI and the valid image. The block sets the ROI flag output to this port as follows:

• True, set to 1 indicating the ROI lies completely inside the input image.

• False, set to 0 indicating the ROI lies completely or partially outside of the input image.

Valid Neighborhood

The neighborhood matrix of metric values is valid inside of the Region of Interest (ROI). You can use the Boolean flag at the NValid port to track the valid neighborhood region. The block sets the neighborhood NValid boolean flag output as follows:

• True, set to 1 indicating that the neighborhood containing the best match is completely inside the region of interest.

• False, set to 0 indicating that the neighborhood containing the best match is completely or partially outside of the region of interest.