**PURPOSE**- Compute coordinates of center of gravity of an image/volume with respect to the SPIDER image/volume center: (NX/2 + 1, NY/2 + 1, NZ/2 + 1). Also computes radius of gyration for volumes. Example.

**SEE ALSO****CG PH**[Center of Gravity - Phase approximation ||] **CENT PH**[Center image/volume using phase approximation] **SH**[Shift - using bilinear/trilinear interpolation ||]

**USAGE**- .OPERATION: CG [x],[y],[z],[rg]

- [This operation can return up to 4 optional register variables:
Variable Example Receives First [x] Sub-pixel X center of gravity relative to center Second [y] Sub-pixel Y center of gravity relative to center Third (Volumes only) [z] Sub-pixel Z center of gravity relative to center

Fourth (Volumes only) [rg] Radius of gyration

.INPUT FILE: PIC001

[Enter name of image/volume.].THRESHOLD: 0.23

[Only data above this threshold will be used in computation.]

**NOTES**

- The center of gravity determination is very poor with typical
high noise cryo-em particle images without an appropriate threshold.
This threshold is hard to determine apriori. Try 'CG PH' for such images,
but even that alternative central measure may have problems. ]
- For definition of the radius of gyration, see:
*International Tables of Crystallography, vol. III, p327.* - COG Algorithm (1D analog): Sum product of all pixel values by their position,
Then divide by sum of all pixel values.
- USAGE EXAMPLE: To shift an image to its center of gravity
computed for a threshold of T=0.23, one could use the
following operation sequence:

CG [x],[y]

PIC001

.23

SH

PIC001

SHI001

-[x], -[y]

**SUBROUTINES**- CENGR3

**CALLER**- UTIL1

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