# How to understand feature "Shift" in module "Volume Rendering"?

**URL:** <https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623>\
**Category:** Development\
**Created:** [March 10, 2020, 8:05pm UTC](https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623 "2020-03-10T20:05:00Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![Illusion](https://avatars.discourse-cdn.com/v4/letter/i/e19b73/32.png) [@Illusion](https://discourse.slicer.org/u/Illusion)\
**Post date:** [March 10, 2020, 8:05pm UTC](https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623/1 "2020-03-10T20:05:00Z")

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Hello,

There are two questions that I was not able to figure out. Please help me. 😉

1. I was trying to get the 3D coords of the skin-surface of the back surface from a vertebral MR, to get the posterior-anterior axis given the inferior-superior axis and left-right axis.

Is there a module to help do that?

1. When showing the volume rendering, the feature “Shift” (as shown in the screenshot below) changed the valid volume. How should I understand the disappeared volume when Shift is enlarged? And how the extra pure-black volume was added when Shift is reduced?  
 ![image](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/e/f/ef89238a0acce95f5b184b615da408a696031f97.jpeg)

 ![image](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/2/0/20cf1b168de42c6d737937482c4ad4eea7777264.jpeg)

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [March 11, 2020, 12:42pm UTC](https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623/2 "2020-03-11T12:42:08Z")

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It is equivalent with adding/subtracting a constant to voxel values.

What it actually does it shifts volume rendering transfer functions, which you can find in the “Advanced” section.

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<div class="post-metadata">

**Author:** ![Illusion](https://avatars.discourse-cdn.com/v4/letter/i/e19b73/32.png) [@Illusion](https://discourse.slicer.org/u/Illusion)\
**Post date:** [March 11, 2020, 7:52pm UTC](https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623/3 "2020-03-11T19:52:15Z")

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Hi Iassoan,

Thank you for your reply!  
It seems that the “Advanced” section is kind of advanced for me. 😅

Could you please post some link or tutorial, or even source code about how the feature “Shift” is applied on voxel processing mathematically?

Thank you in advance!

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**Author:** ![lassoan](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/lassoan/32/13_2.png) [@lassoan](https://discourse.slicer.org/u/lassoan)\
**Post date:** [March 12, 2020, 10:24pm UTC](https://discourse.slicer.org/t/how-to-understand-feature-shift-in-module-volume-rendering/10623/4 "2020-03-12T22:24:01Z")

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Shift is equivalent to adding/subtracting a constant value from the voxel values. You can read more about volume rendering in the [VTK Textbook](https://vtk.org/vtk-textbook/).