Projects:SlicerFAQ:SamplePoints

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How many sample points should I choose for my registration

• Problem: unsure what the Sample Points setting means or how I could use it to improve my registration.
• Explanation:All registration modules contain a parameter field that controls how much of the image is sampled when performing an automated registration. The unit is often an absolute count, but in some cases also a percentage. Default settings also vary among modules. The number of samples is an important setting that determines both registration speed and quality. If the sample number is too small, registration may fail because it is driven by image content that insufficiently represents the image. If sample number is too large, registration can slow down significantly.
• Fix: If registration speed is not a major issue, better to err on the side of larger samples. Most default settings are chosen to yield relatively fast registrations and for most of today's image represent only a small percentage. The table below relates total sample points and percentages to the most common image sizes. Also consider that sample points are chosen randomly so that some points may fall outside the actual object to be registered. That is not a bad thing per se, some background points are important, but not if they are too far from the edges of the object.

 Defaults Used in Slicer Modules v.3.6.1
 Fast Rigid / Linear: 10,000
 Fast Affine: 10,000
 Fast Rigid: 10,000
 Fast Non-Rigid BSpline: 50,000
 Expert Automated (Rigid): 1%
 Expert Automated (Affine): 2%
 Expert Automated (BSpline): 10%
 BRAINSfit: 100,000
 BRAINSdemon Warp: N/A 
 Robust Multiresolution: N/A
 Surface Registration: 200
Image Size        Total Points	10000	50000	100000	200000	1%	         2%	5%	10%	20%	
128 x 128 x 64     1048576	1.0%	4.8%	9.5%	19.1%	~10000	~20000	~52500	~100000	~200000	 
256 x 256 x 30		1966080	0.5%	2.5%	5.1%	10.2%	~20000	~40000	~97500	~200000	~400000	
256 x 256 x 120		7864320	0.1%	0.6%	1.3%	2.5%	~77500	~150000	~400000	~775000	~1500000	
192 x 192 x 192		7077888	0.1%	0.7%	1.4%	2.8%	~70000	~150000	~350000	~700000	~1500000	
512 x 512 x 48		12582912	0.1%	0.4%	0.8%	1.6%	~125000	~250000	~625000	~1250000	~2500000	
256 x 256 x 256		16777216	0.1%	0.3%	0.6%	1.2%	~175000	~325000	~850000	~1750000	~3250000	
512 x 512 x 160		41943040	0.0%	0.1%	0.2%	0.5%	~425000	~850000	~2000000	~4250000	~8500000