Pixel Size (Legacy Transformation Widget)
=========================================

.. image:: icons/axes.svg

Converts the detector frame coordinates from pixels to physical units.

Signals
-------

**Inputs**: Dataset

**Outputs**: Dataset

Description
-----------

Provides frame coordinates in micron in other analysis widgets.

To compute pixel size at the sample position, darfix needs the **optical pixel size** parameter.
This is the effective pixel size as seen by the detector optics at the detection plane.

Predefined values based on the current setup at ID03 are defined as **2× magnification** or **10× magnification**.
The user can also select **Manual Magnification** to override this value.
The **optical pixel size** depends on the optical setup of your experiment.

For the aspect ratio, the user can also choose to divide pixel size by :math:`\sin(2\theta)` for the vertical (or horizontal) direction to account for the projection of the beam onto the detector plane.

When input parameters are updated, the widget recomputes automatically the **pixel size at sample position** in horizontal and vertical direction.

This output is then used to compute the scale of axes in other analysis widgets later in the workflow.


.. note::

    Transformation is a prerequisite for :doc:`chi_gradient_correction`,
    which requires the pixel size in microns.

How the pixel size at sample position is computed
-------------------------------------------------

The DFXM setup uses a compound refractive lens (CRL) to image the diffracted beam
onto the detector. The geometry is shown below:

.. image:: img/transformation_geometry.svg
   :width: 480px

The magnification is:

.. math::

    M = \frac{v}{u} = \frac{MAINX}{OBX} - 1

where:

- :math:`OBX` is the sample-to-lens distance
- :math:`MAINX` is the sample-to-detector distance
- :math:`u = OBX / \cos(2\theta)` is the object distance along the beam
- :math:`v = MAINX / \cos(2\theta) - u` is the image distance


The physical pixel size at the sample plane is then:

.. math::

    E_{px} = \frac{d_{px}}{M}

where :math:`d_{px}` is the optical pixel size.