Architecture

The plugin has three jobs: register a chart class with OpenCPN, turn ENC cells into tile57's PMTiles so OpenCPN can select them, and draw the chart when OpenCPN asks. This page covers the first two — the GPU draw is in Rendering.

Plugin entry

src/tile57_pi.cpp is the OpenCPN plugin (Tile57Plugin : opencpn_plugin_118, API 1.18). Init() advertises its capabilities to OpenCPN:

INSTALLS_PLUGIN_CHART | INSTALLS_PLUGIN_CHART_GL |
WANTS_MOUSE_EVENTS | WANTS_CURSOR_LATLON | WANTS_PREFERENCES

It reports itself as "tile57 Vector Chart (EXPERIMENTAL)" (v0.1) and warms up tile57's process-global read-only registries on the main thread before the chart-DB scan first creates a chart.

Beyond the chart itself the plugin provides:

• a Build Charts preferences dialog (bulk cell → PMTiles baking);
• an object-query panel on double-click.

Chart classes

The chart lives in src/tile57_chart.{h,cpp}. All the behavior is in the base class ChartTile57; a thin concrete subclass sets the file mask OpenCPN scans for:

Class	File mask	Source
ChartTile57Pmtiles	*.pmtiles	a baked tile57 bundle

OpenCPN's GetFileSearchMask() returns a single wildcard (it does not split on : or ;), so the mask lives on a dedicated dynamic chart class, registered by name through GetDynamicChartClassNameArray. Every chart is a pre-baked bundle; you produce them with the Build Charts dialog (below).

Why PlugInChartBaseExtended

ChartTile57 derives from PlugInChartBaseExtended, not the legacy PlugInChartBaseGL. Extended is what gives quilted vector charts their two-pass GL render:

1. RenderRegionViewOnGLNoText — per-patch geometry, clipped to the patch via a stencil/scissor mask OpenCPN pre-writes;
2. RenderRegionViewOnGLTextOnly — one decluttered text pass across the whole quilt (so labels don't clash at patch seams).

A legacy BaseGL chart is wedged through a per-rect compatibility path that shows seams and doubled text. The plugin maps these two calls onto its renderer's kBase and kText passes (see Rendering).

Scale and coverage

IsReadyToRender() always returns true. A baked bundle opens synchronously in Init() — its metadata and coverage load immediately — so the chart is ready at once. (Reporting "not ready" would make OpenCPN destroy and re-create the chart every frame.)

Coverage comes from the real M_COVR polygons (tile57_chart_coverage) when the bundle embeds them, else the chart's bounding box. Scale selection (GetNormalScaleMin/Max) allows a modest overzoom past native detail and hands off to a coarser cell when zoomed well out.

Baking charts

tile57 renders from PMTiles, so ENC cells are baked to bundles up front — there is no per-view baking. The Build Charts dialog (src/build_charts.cpp) is the pipeline:

• point it at an ENC root; it walks the tree and bakes every .000 cell across a small worker pool, calling tile57's tile57_bake_chart_bytes directly (each bundle embeds the cell's real M_COVR coverage);
• output goes to a fixed cache directory (~/.cache/tile57/charts), written atomically (temp file + rename);
• when the sweep finishes it calls AddChartDirectory + ForceChartDBUpdate — the only place the baker touches the OpenCPN API — so the charts appear without a manual rescan. Its settings persist to a plugin-owned tile57_pi.ini (not OpenCPN's config object, which avoids a macOS dynamic-lookup crash).

Once baked, ChartTile57Pmtiles::Init simply opens the .pmtiles archive directly and the chart renders immediately.

Object query

Double-clicking runs tile57_chart_query against every loaded tile57 chart covering the cursor and renders the returned S-57 attributes as an HTML panel — the plugin's equivalent of OpenCPN's native ENC object query.