junk (Module within jview)

Description

junk.c is responsible for drawing World Vector Shoreline (WVS) data onto the map display within the jview application. It dynamically selects an appropriate coastline dataset based on the current map’s scale, projects the geographical coordinates to screen coordinates, and then renders the coastline segments. Despite its seemingly arbitrary filename, this module provides essential geographical context for data visualization in jview.

Key Features and Functions

  • draw_wvs(int window): The primary function responsible for rendering WVS data.
    • Pre-condition Check: Verifies the plot_shoreline global flag.
    • Dynamic Dataset Selection: Selects one of six WVS datasets (e.g., wvsfull.dat, wvs250k.dat) based on map_width and map_height (geographical extent), ensuring appropriate detail for the current zoom level.
    • Geographical Query Loop: Iterates through a grid of latitude and longitude coordinates encompassing the current map view.
    • WVS Data Retrieval (wvsrtv): Calls the external function wvsrtv to fetch coastline segments for the specified geographical tile and chosen WVS dataset.
    • Filtering Single-Point Islands: Filters out single-point “islands” which are typically artifacts from data decimation.
    • Projection and Drawing: Transforms latitude/longitude coordinates into map’s display coordinates (segx, segy) using the Project function, and draws these segments as lines using jhc_line.
    • Resource Cleanup: Calls wvsrtv("clean", ...) to free associated resources.

How It Works

The junk module integrates World Vector Shoreline data into the jview application’s map display. When called, it first determines the visible geographic area and dynamically selects a WVS dataset with an appropriate level of detail. It then queries an external WVS retrieval library for coastline segments within this area. These segments are projected onto the jview’s internal coordinate system and rendered as lines on the map. This process provides crucial geographical context, enhancing the interpretability of the displayed data.

Dependencies

  • JV_global.h: Provides access to essential global variables such as plot_shoreline (to control WVS drawing), min_latitude, max_latitude, min_longitude, max_longitude (defining the current map view extent), sub_param, graphic_param (for window dimensions and parameters), and map_header.xstep (for map scaling).
  • wvsrtv: An external function, likely part of a dedicated World Vector Shoreline library, used for retrieving coastline data segments based on geographical coordinates and selected dataset.
  • Project: A crucial function responsible for transforming geographical coordinates (latitude and longitude) into the Cartesian coordinates of the map display.
  • jhc_line: A graphics primitive function from the jview’s drawing library, used to draw lines on an X-windows display.

Notes

Despite its informal filename, junk.c serves a critical function in the jview application by accurately and efficiently visualizing shorelines. Its ability to adapt the level of coastline detail to the current map zoom ensures that geographical context is always present and rendered appropriately, which is vital for users analyzing marine, coastal, or other geographically referenced datasets. The name junk is a misnomer given its essential functionality.