Horizontal cross-section
========================

.. _fig_horizontal_cross_section:
.. figure:: ./figs/horizontal_cross_section/actorref_hsec.webp
    :figwidth: 100%
    :align: left
    :alt: Horizontal cross-section visualization in Met.3D

    Horizontal cross-section actor with transfer function applied. Base map added beneath.

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

The horizontal cross-section actor renders a 2-D slice of atmospheric data at a
specified pressure level.

* Multiple data fields can be visualized simultaneously.
* Each field can be rendered as line contours, filled contours, pseudo-colour plots,
  or textured stippling/hatching.
* The section can be moved vertically by editing the elevation property or by dragging
  the corner handles, which are visible in actor interaction mode.
* Graticule, coastlines, and country borders are integrated directly into the actor.
* Multiple horizontal sections may be stacked within a single 3-D scene, and their
  elevation can be synchronized with each other.
* Streamlines with attached wind barbs (*streambarbs*) and vector arrows are supported to 
  visualize flow fields.

Elevation
---------

The pressure level of the section is set via **Horizontal section** → **Configuration**
→ **Elevation** (in hPa). The corner handles visible in the scene can also be dragged
vertically.

**Elevation step size** controls how far the section moves per drag increment.

To link the elevation of two horizontal section actors, use **Synchronize elevation with**
and select the other actor. Both sections will then move together.

Rendering
---------

Met.3D supports several rendering modes for each variable: filled contours, line contours,
textured contours (stippling/hatching), and pseudo-colour. These can be combined.

Configure per-variable rendering under **Horizontal section** → **Variables** →
*your variable* → **Rendering**. Options include contour spacing, thresholds,
transfer function assignment, and smoothing.

Graticule, Coastlines, and Country Borders
-------------------------------------------

Graticule lines, coastlines, and country borders are integrated components of the
horizontal section actor, configured under **Horizontal section** → **Configuration**
→ **Graticule**, **Coast lines**, and **Border lines** respectively.

Their behaviour and options (enable/disable, colour, thickness, longitude/latitude
range and spacing, custom shapefiles) follow the same pattern as described for the
:doc:`basemap` actor.

Vector Visualization
--------------------

Met.3D offers two types of vector visualization in horizontal cross-sections:

**Vector glyphs**

* Vectors are displayed as glyphs with adjustable size and scaling.
* Several glyph types are supported, including different arrowhead styles.
* Glyphs can be coloured by a scalar variable or vector magnitude.

**Streambarbs**

Streambarbs extend traditional streamlines by incorporating barb-style wind speed
markers along the path.

* Based on the Jobard and Lefer (1997) algorithm with enhancements, plus alternative
  seeding strategies such as uniform sampling.
* Streambarbs are distributed to achieve even spatial coverage within the domain.
* User-configurable spacing and loop detection options are provided.
* **Domain coverage check grid size**: in domains with disjoint valid regions (e.g.,
  over mountains at low pressure levels), a configurable grid detects unfilled areas
  and reinitializes the seeding algorithm there to ensure complete coverage.
* Like glyphs, streambarbs can be colour-mapped using scalar fields or vector magnitude.


.. _fig_hsec_streambarbs:
.. figure:: ./figs/horizontal_cross_section/hsec_streambarbs.webp
    :figwidth: 100%
    :align: left
    :alt: Horizontal cross-section of streambarbs and wind.

    Horizontal cross-section actor with shaded wind speed and streambarbs.


References
----------

*   Jobard, B., and Lefer, W.: **Creating evenly-spaced streamlines of arbitrary density**,
    `Proceedings of the Eurographics Workshop on Visualization in Scientific Computing, 45–55`, 1997.