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Speeding up the search


To ensure the fastest possible search, you should:

  • Choose an appropriate model.

  • Set the search speed to the highest possible setting for your application.

  • Set the search region to the minimum required. Search time is roughly proportional to the region searched, so don't search the whole image if you don't need to.

  • Search the smallest range of angles required.

  • Select the lowest positional accuracy that you need.

  • Set the certainty level to the lowest reasonable value (so that the search can stop as soon as a good match is found).

  • Search for multiple models at the same time, if possible.

If you can't attain the required speed using these techniques, you can use some more advanced techniques discussed in the Pattern matching algorithm (for advanced users) section later in this chapter.

Choose the appropriate model

The size of a model affects the search speed. In general, small models take longer to find than larger ones, although very large models can also be time-consuming. In general, the optimal size is approximately 128x128 pixels if you are searching a large region (for example, most of the image). Small models are found quickly when the search region is not too large.

Adjust the search speed setting

For any model, it is possible to set the speed of the search. Increasing the search speed reduces the search time, however, as you increase the speed, the robustness of the search operation (the likelihood of finding a model) can decrease. When you call MpatPreprocess(), MIL analyzes the pattern in the model, and determines appropriate shortcuts; only shortcuts that are considered safe for a particular model are taken. This also means that higher search speeds might not be any faster for certain models, depending on the particular pattern. Higher search speeds can reduce the positional accuracy very slightly.

You adjust the search speed setting, using MpatControl() with M_SPEED. This function has five settings:

As expected, the M_VERY_HIGH and M_HIGH speed settings allow the search to take all possible shortcuts, performing the search as fast as possible. Higher speed settings are recommended when searching on a good quality image or when using a simple model. Note, the search might have a lower tolerance for rotation when using this setting.

The M_MEDIUM speed setting is the default setting and is recommended for medium quality images or more complex models. A search with this setting is capable of withstanding up to approximately 5° of rotation for typical models.

Use the M_LOW or the M_VERY_LOW speed settings only if the image quality is particularly poor and you have encountered problems at higher speeds. The speed settings are discussed further in the algorithm description at the end of this chapter.

Effectively choose the search region and search angle

You can improve performance by not searching the whole image unnecessarily. Search time is roughly proportional to the region searched; set the search region to the minimum required, using MpatControl() with M_SEARCH_OFFSET... and M_SEARCH_SIZE..., or using a rectangular region of interest (ROI) set using MbufSetRegion(). The latter method is more flexible but has its own limitations, as outlined in the Selecting the search region subsection of the Search constraints section earlier in this chapter.

You can also improve performance by selecting the lowest positional accuracy. In addition, for an angular search, select lowest angular accuracy (MpatControl() with M_SEARCH_ANGLE_ACCURACY) and range required, in combination with the highest tolerance possible.