:Author: Marco Cammarata :Date: 22/11/2022 :Keywords: description of conventions used by pyFAI Conventions =========== This page describes the conventions used by pyFAI. Some useful information (orientation, position of observer, etc.) can be found in :ref:`geometry` Mask ---- PyFAI considers masks with values equal to zero 0 as valid pixels (mnemonic: non zero pixels are *masked out*). This is a different convention with respect to other programs used at ESRF like Lima that used no zero pixels as valid. Pixel Coordinates ----------------- The origin is set at the corner of a pixel as shown in the figure below for a simple 3×4 pixel matrix. .. figure:: img/pixel_coordinates.svg :align: center :alt: Position of the origin with respect to the pixel matrix Note some specificities: * Each pixel *n* starts at the coordinated *n* (included) and goes to the coordinate *n+1* (excluded). The center of any pixel is at half integer pixel coordinate. This convention differs by half a pixel from the one used in matplotlib where pixels range from *n-½* to *n-½*. Care must be taken when displaying images with matplotlib (when assessing beam-center for example): there is ½ pixel offset. * The origin is at the bottom and differs from the *camera* convention where the origin is at the top. As a consequence, the sign of the ᵪ-angle gets inverted. * The detector is seen from the sample and differs from the *camera* convention where the observer is behind the camera. As a consequence, the sign of the ᵪ-angle gets inverted. * The former 2 inversions cancel each other.