Chi Gradient Correction
========================

Apply chi COM optical correction to remove the geometrical gradient
introduced by the DFXM imaging optics.

The correction subtracts a position-dependent angular shift along the
horizontal direction:

.. math::

    \chi_\text{corr}(y) = \chi_\text{meas}(y) - \frac{\xi \cdot y_s}{2 \sin(\theta)}

where:

- :math:`y_s` — horizontal position relative to image center (meters)
- :math:`\theta` — half Bragg angle :math:`= \theta_\text{obpitch} / 2`
- :math:`\xi = 1 / d_1`

with :math:`d_1 = \frac{obx}{\cos(\theta_\text{obpitch})}`

 (algorithm by Marcelo Demetrio De Magalhaes).

Signals
-------

**Inputs**: GrainProperties

**Outputs**: GrainProperties

.. note::

    This widget has **no visualisation**. Connect a second :doc:`grainplot` to its output
    to display the corrected maps.

Prerequisites
-------------

:doc:`transformation` must be placed upstream to provide the pixel size in microns,
which the correction algorithm requires.