Rendering

The renderer draws tile57's draw-ready GPU scene inside OpenCPN's render loop. tile57 portrays a whole view once into vertex, index and quad buffers that are already tessellated, already in S-52 paint order, and already split into ranges that each draw with one pipeline. The plugin uploads those buffers and draws them under a per-frame transform. A pan, zoom or rotation is a uniform change; the scene is rebuilt only when the view leaves its coverage or the mariner settings change.

The code is split by responsibility:

File	Role
src/chart_renderer.*	the host-facing API, the rebuild policy, and the render targets
src/scene_build.*	the CPU build (tile57_chart_gpu_scene) and its worker thread
src/gpu_scene.*	a resident scene: GL buffers, pattern textures, batched draw lists
src/chart_programs.*	the GLSL 1.20 programs and vertex stream bindings
src/chart_atlases.*	the symbol and glyph atlas textures and the halo colors
src/gl_objects.h	move-only owners for GL objects

Passes

OpenCPN calls a quilted vector chart twice per frame, and the renderer maps each call onto a Pass:

• RenderRegionViewOnGLNoText is kBase: everything but text, clipped to the chart's quilt patch.
• RenderRegionViewOnGLTextOnly is kText: text only, drawn above OpenCPN's overlays.

A single unquilted chart uses kAll. One scene serves every pass; the pass selects which ranges are batched.

Scenes and coverage

A scene is portrayed for a view 1.25 times the viewport on each axis, at the view's zoom. It stays valid while the live view keeps the whole viewport inside that box and the zoom stays within 0.3 of the build zoom. Vertices are uploaded relative to the scene's build center so single-precision floats hold their accuracy at harbor zoom.

The first scene for a chart is built on the render thread, because nothing is on screen yet. Every later build runs on a worker thread while the current scene keeps drawing. When a build finishes, the worker asks the canvas for one redraw, and the next render pass adopts the new scene. One request is pending at a time; a newer request replaces one that has not started.

The rebuild policy:

• a settings change or a pan out of the coverage box rebuilds at once;
• a zoom drift past the band waits while a gesture is in progress, up to one full zoom level, then rebuilds when the view settles;
• a zoom-out is built 0.15 levels further out than the view, so its coverage is a superset of every view on the way there.

Draw lists

tile57_gpu_batch turns a scene's ranges into draw calls, merging neighbors that share a pipeline, atlas and pattern. The plugin caches one draw list per combination of pass, text and soundings switches, available atlases, and depth mode.

When a depth buffer is available, opaque fills are drawn first, front to back, with depth writes on, and the depth test drops the fragments later paint would cover. The remaining ranges then draw in paint order with the depth test on and writes off. The supersample target always has a depth buffer; direct rendering uses the host's when it has one.

The vertex model

Every vertex is transformed in the shader as:

screen = R * (aWorld * uScale) + uOrigin + post

• aWorld is the vertex's world position relative to the scene's build center; uScale is framebuffer px per world unit; uOrigin places the center on screen; R is the view rotation.
• post is a screen-space offset in device px: zero for area fills, the half-width for line edges, the corner for a glyph or symbol quad. A map-aligned offset is rotated with the chart; a viewport-aligned one stays upright. A text run flagged to flip turns 180 degrees when its tangent would read into the left half of the screen.
• aScamin and aDispCat gate visibility on the GPU against the live display scale and category switches, so neither forces a rebuild.
• aDepth is tile57's paint-order key.

Colors are per vertex. SDF text carries a halo width per vertex and draws the halo in the scheme's background color.

Atlases

The symbol atlas is baked per pixel ratio and color scheme; the SDF glyph atlases are baked per face (regular, bold, italic). Each is uploaded on first use and shared by every chart in the process.

Antialiasing

When the view is settled, the scene is rendered into a supersampled framebuffer (2x by default) and composited down. During a pan or zoom the renderer draws directly into OpenCPN's buffer, because an offscreen composite tears against OpenCPN's accelerated pan. Supersampling is off on software renderers.

OpenCPN's GL state

OpenCPN mixes fixed-function drawing with the plugin's programs. At the end of every draw the renderer disables its vertex attribute arrays, unbinds buffers and textures, clears the active program, and restores the depth mask.

Environment variables

Variable	Effect
TILE57_SS=<n>	supersample factor (default 2; 1 disables AA)
TILE57_NOSS	draw directly, no offscreen target
TILE57_SIZE / TILE57_CALIB	physical symbol and text size calibration
TILE57_DECLUTTER=<levels>	SCAMIN cull bias
TILE57_DEBUG	log scene builds and the render entry state