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Convert a registry symbol size to a rendering target's units

Source: R/gq_symbol.R
gq_symbol_size.Rd

The registry's radius field is a diameter, in millimetres. The name is a misnomer inherited at extraction: gq_qgs_extract() reads the QGIS SimpleMarker option literally named size — the marker's overall extent — and stores it under radius. Every point QML in the corpus confirms it by carrying scale_method = "diameter" alongside.

Usage

gq_symbol_size(
  radius,
  target = c("tmap", "mapgl"),
  shape = "circle",
  scale = 1
)

Arguments

radius

Registry size in millimetres. May be a named vector (a classified layer's per-class sizes), NA, or NULL for a layer that defines no mark at all. Names are preserved, because tm_scale_categorical() matches values by name.

target

One of "tmap" or "mapgl".

shape

Registry shape, for target = "tmap". Defaults to "circle", which is what tmap itself draws when no shape is given.

scale

Uniform multiplier applied after conversion. The knob for a dense map that needs every symbol smaller — one number applied once, rather than a hand-tuned value per layer, which is what this function exists to replace.

Value

A numeric vector in the target's units, or NULL if radius is NULL.

Details

Each target measures symbols differently:

"tmap"

Depends on shape. tmap sizes symbols in grid "lines" (5.08 mm), but R's graphics engine then applies a per-pch factor, and base R normalises those by area where QGIS normalises by extent. A circle draws 3.81 mm of ink per size unit, a square 3.38, a triangle 5.13. Passing the wrong shape leaves the symbol 11–35% out.

"mapgl"

MapLibre's circle-radius is a true radius, in CSS pixels — so the value is halved and converted at 96 px per inch. shape does not apply: a MapLibre circle layer has only circles.

A constant circle-radius is deliberate for mapgl, not a simplification: it holds a fixed screen size at every zoom, which is exactly QGIS marker semantics. An interpolated expression would be a departure from the registry, not a correction to it.

See also

gq_symbol_shape()

Examples

# A 3 mm circular marker, drawn as 3 mm of ink
gq_symbol_size(3, "tmap")
#> [1] 0.7874016

# The same millimetres as a square need a different size, because base R
# normalises pch by area and QGIS by extent
gq_symbol_size(3, "tmap", shape = "square")
#> [1] 0.8885814

gq_symbol_size(3, "mapgl")           # circle-radius in CSS pixels
#> [1] 5.669291
gq_symbol_size(3, "tmap", scale = 0.5)
#> [1] 0.3937008

# Per-class sizes keep their names
gq_symbol_size(c(BARRIER = 3, PASSABLE = 2), "tmap")
#>   BARRIER  PASSABLE 
#> 0.7874016 0.5249344