ROI (3D)

• Center X
• Center Y
• Use 2D Coordinates -
  • X and Y can be specified using 2D coordinates.
  • The Z coordinate will necessarily still need to be a 3D coordinate
• Center Z - defines the 3D center point in 3D coordinates
• Search Z - autodetect the Z value by scanning different values:
  • Nominal - middle Z scan value
  • Range - spread around the nominal value
  • Step - increment
• Template Height, Width - defines the size of the used template in 3D coordinates
• Search area Height, Width - defines the size of the correlation region used to match the template with the other images in 3D coordinates. Must be larger than Template Height, Width.

Include in on-screen-description - requires description to be active under Visualisation

• Match - (for Correlation search only - see Advanced) - match between images
• 3D Coordinates - the position found
• Deviation - deviation found (see Advanced)

Advanced

Match settings

• Pixel size in resampled images - the approximate ratio of original to resampled pixels used in template match. Always used in the final search, but can be overridden for initial loops (this to reduce processing time). Override is given in the next option
• Set template size in Z scan to (pixels) - use this size for the template in initial scan to estimate the Z value
• Stretch image contrast - optimise image contrast. This is most effective for blob search, and has less effect for correlation search
• Percent level - contrast stretch setting
• Min distance -  contrast stretch setting
• Stretch defaults - set Percent level and Min distance to default values

Correlation search

Resampled images are correlated, and maximum correlation factor determines the match position.

• Min score for match (0-255) - cross correlation match threshold
• Ignore images with lower Std ratio than - ignore featureless images
• Run first search with template size (pixels) - run two loops to locate points, first a coarse search for approximate location, then a refined search for accurate placement. This can give a significate speed improvement.
• Template selection
  • Auto - use best image as template in cross-correlation algorithm
  • Camera 1-4 - use fixed image as template

Blob search

Blobs are searched within the resampled images, and the center of gravity of a blob is used to determine the match position.

• Use thresholds - manually set thresholds
• Auto (dark) - see description below
• Auto (bright) - see description below
• Minimum threshold - for manually set thresholds
• Maximum threshold (inclusive) - for manually set thresholds
• Position factor - see description below
• Bimodal - see description below
• Morphology (e=erode, d=dilate) - erode or dilate blobs
• Blob smooth factor - blob smoothing
• Largest/Darkest/Brightest/Single blob only - only a single blob will be used for positioning; you can here select which. If you choose Single blob only, the search will fail if more blobs are within the search area.

Auto thresholding

A threshold is set based on the resampled image's histogram.

• If Bimodal is checked, the histogram should have two distinct tops (i.e., two major gray levels). A threshold is calculated assuring maximum separation.
• If Bimodal is not checked, the histogram is assumed not bimodal. A threshold is still found to separate the image, but since tops in the histogram are not assumed, the centre of gravity of the histogram on each side of the threshold is used as a "top" instead.
• The Position factor determines the reported threshold. 0 means the optimum threshold, -1 means the darker top, +1 means the brighter top. Anything in between leads to linear interpolation between a top and the threshold.

Factor for calculating deviation

There are two deviation measures calculated for each point:

• The first (aka. Deviation or 3D Deviation) measures how well the observed points coincide in 3D space. A 3D point is found as the intersection of two or more 3D lines, one line from each camera. The lines are the "inverse projections" of the 2D points into space. When two lines do not meet exactly in space, the deviation is half of the shortest distance between them. i.e., by how much the point result deviates from the lines. When more that two cameras are used, the deviations are averaged. Deviation is given in physical coordinates and reflects 3D camera calibration
• The second (2D deviation) is calculated by mapping the found 3D point back to the camera image, and measuring the distance from this point to the incoming 2D point. A large error here could mean that the tools producing the 2D points have missed.

The weight percent lets you use either deviation measure or both:

• Weight percent deviation vs. 2D deviation -  100% means use 3D deviation only; 0% means 2D deviation only. Or you can select a combination. Generally this is sound, since the 2D and 3D measurements are in the same dimension (usually mm).

Constraints

• Max deviation for accepted point - acceptance criterion for each point (see Deviation above)
• Ignore camera points with deviation ratio over [dB] - ignore cameras where 3D deviation ratio to the best camera is over the threshold - measured in dB
• Ignore camera points with deviation over - used to remove "bad" points before estimating position in space
• Minimum number of active cameras - only succeed if at least this number of cameras is accepted. This is especially useful in combination with the Template selection (above)

Resampling

The best match in the Z scan is displayed to the left.

For Correlation search, the used template and the search areas from the other cameras are displayed to the right. The template will be the smaller image.

For Blob search, the same size is used for all resampled images. All blobs found in the images are drawn in red, and if only one blob is found, or if you select the largest/darkest/brightest as the result, it will be drawn in green. The center of gravity for this blob is the position used to locate a 3D point.

You will also see the effect ov Contrast stretching in these images. Below the left image is original, the right image has been contrast stretched.

Original: Contrast stretched:

Curves

A set of curves is supplied for z scan analysis. For each step in the Z scan loop, the image correlation factors are averaged and plotted (Match). Another plot (Differences) show how the correlation factors for the different comparisons differ.

Visualization

Results