# Computation time does not scale with SetMetricSamplingPercentage setting in ITK V4

**URL:** <https://discourse.itk.org/t/computation-time-does-not-scale-with-setmetricsamplingpercentage-setting-in-itk-v4/1798>\
**Category:** Algorithms\
**Tags:** registration\
**Created:** [April 17, 2019, 8:34pm UTC](https://discourse.itk.org/t/computation-time-does-not-scale-with-setmetricsamplingpercentage-setting-in-itk-v4/1798 "2019-04-17T20:34:25Z")\
**Posts on this page:** 1\
**Showing post:** 9

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**Author:** ![zivy](https://discourse.itk.org/user_avatar/discourse.itk.org/zivy/32/1726_2.png) [@zivy](https://discourse.itk.org/u/zivy)\
**Post date:** [May 2, 2019, 7:04pm UTC](https://discourse.itk.org/t/computation-time-does-not-scale-with-setmetricsamplingpercentage-setting-in-itk-v4/1798/9 "2019-05-02T19:04:21Z")

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Hello @eewujian,

ITK uses a rejection sampling approach which implicitly assumes that you are interested in a significant portion of the image.

For additional details see [this related discussion](https://discourse.itk.org/t/fixed-mask-not-working-as-expected/1238/6).

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_[View the full topic](https://discourse.itk.org/t/computation-time-does-not-scale-with-setmetricsamplingpercentage-setting-in-itk-v4/1798)._