Limit on rms Error for Mapping

This option allows the setting of the maximum allowable rms error for the mapping between the current pixel system and the reference pixel system. The mapping is generated by determining the location of the reference points in the current pixel coordinate system, and then utilising a least squares fit with a chosen function to determine the required transformation. This least squares fit will generally have a non-zero residual sum of squares which may act as an indication of how good the mapping is. A high value of the rms error is an indication of a poor quality image, while a low rms error suggests a good image.

Limit on rms error for reference mapping ?

The absolute value of this entry specifies the upper limit on the allowable rms error for the coordinate system mapping. If the rms error for the mapping is greater than the limit specified by this entry, and this entry is greater than zero, then up to either two or five attempts to recapture the image will be made (the number depending on the response to the next prompt). If the rms error limit is still not satisfied, then the tracking process will continue regardless, though recording this difficulty in the .IND index file. If the rms error is less than twice the limit, then the mapping generated will still be used, otherwise if the rms error exceeds twice the limit, then an identity mapping will be used.

The tracking process assumes the first image captured will be of adequate quality and will initialise the mapping on this image; the rms error will thus always be zero for the first image. Typically the value of the rms error limit should be less than 0.2 (pixels).

If mapping fails, retry a maximum of 1 or 5 times ?

This entry specifies the maximum number of times DigImage will retry acquiring the frame in order to satisfy the reference point rms error limit.

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Stuart Dalziel, last page update: 19 February 1996