• Isotropic: The speckle pattern should not exhibit a bias in any particular orientation.
• Random: It is actually hard to achieve a pattern regular enough to cause false matching, but if you are to print
repeating patterns it can occur. Even using templates/stencils with repeating dots is typically irregular enough due to
the paint seeping through the stencil being irregular.
Below are examples of patterns that lack these characteristics to some degree.
Repetitive Low contrast Anisotropic
SPECKLE SIZE
Speckles should neither be too small nor too large. In practice, there is a wide range of how large a speckle pattern may be,
and still achieve excellent results. However, having an optimal pattern will give the best flexibility.
If the pattern has speckles that are too large, or if it is too sparse, we may find that certain subsets may be entirely on a
region of black or region of white. This prevents good correlation, because everywhere in that region is an exact match. We
can compensate for this by increasing the subset size, but this is done at the cost of spatial resolution.
Subset too small for pattern – errors may result in
black areas
Larger subset will work, but reduces ability to
measure fine gradients
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