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The head origin is defined as the midpoint between the LFHD and RFHD markers (also denoted 'Front'). Note that after the head XYZ axes are defined and the static head offset angle is applied, a (-0.84, 0, -0.3) mesh origin offset multiplied by a headscale value (2 * distance between temples) is applied.
The midpoint between the LBHD and RBHD markers ('Back') is also calculated, along with the 'Left' and 'Right' sides of the head from the LFHD and LBHD midpoint, and the RFHD and RBHD midpoint respectively.
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