Mapycus

Overview

mapycusmaximus implements a projection-aware, vector-geometry fisheye based on the Focus-Glue-Context (FGC) model. The transform magnifies a chosen focus (inner radius r_in), transitions smoothly through a glue ring (r_out), and preserves the outer context. It is provided at two levels:

• fisheye_fgc() — numeric coordinates (matrix) with diagnostics
• sf_fisheye() — sf/sfc objects with CRS handling and normalization

This vignette shows how to apply the fisheye to common sf layers, tune parameters, and align multiple layers for a single figure.

Setup

library(sf)
#> Linking to GEOS 3.13.0, GDAL 3.10.1, PROJ 9.5.1; sf_use_s2() is TRUE
library(ggplot2)
library(mapycusmaximus)
theme_set(ggplot2::theme_minimal())

The package ships example data:

• vic: Victoria LGA polygons (projected)
• vic_fish: fisheye-distorted vic (for reference examples)
• conn_fish: sample transfer lines (already warped in data preparation)

Quick start: warp a polygon layer

sf_fisheye() chooses a sensible projected working CRS and normalizes coordinates around a center. With preserve_aspect = TRUE (default), radii are interpreted in unit-like space (approx fraction of the layer’s half-span).

# Focus near a supplied geometry: use the centroid of the combined Melbourne polygon
melbourne <- vic[vic$LGA_NAME == "MELBOURNE", ]

vic_fisheye_demo <- sf_fisheye(
  vic,
  center = melbourne,      # accept sf/sfc; centroid is used in working CRS
  r_in = 0.34, r_out = 0.60,
  zoom_factor = 15,
  squeeze_factor = 0.35,
  method = "expand",
  revolution = 0
)

ggplot() +
  geom_sf(data = vic_fisheye_demo, fill = "grey92", color = "white", linewidth = 0.2) +
  geom_sf(data = melbourne, fill = NA, color = "tomato", linewidth = 0.5) +
  ggtitle("FGC fisheye: Melbourne focus within Victoria")

Choosing a center

You may supply the focus center in several ways:

• As a geometry: center = melbourne (centroid of the combined geometry)
• As lon/lat: center = c(144.9631, -37.8136), center_crs = "EPSG:4326"
• As map units (working CRS): center = c(x, y)
• As normalized coordinates in [-1, 1]: normalized_center = TRUE

# Example: WGS84 center (Melbourne CBD), auto-project to a working CRS
vic_cbd <- sf_fisheye(
  vic,
  center = c(144.9631, -37.8136),
  center_crs = "EPSG:4326",
  r_in = 0.30, r_out = 0.55,
  zoom_factor = 15, squeeze_factor = 0.30
)

ggplot(vic_cbd) +
  geom_sf(fill = "grey92", color = "white", linewidth = 0.2) +
  ggtitle("Center supplied as lon/lat (WGS84)")

Aligning multiple layers

To keep overlays aligned, apply the exact same fisheye parameters to each layer and ensure they share the same working CRS and normalization. A robust pattern is to bind layers together, transform once, then split for plotting; this guarantees a common bounding box for normalization.

centroids <- st_centroid(vic)
#> Warning: st_centroid assumes attributes are constant over geometries
vic$layer <- "polygon"
centroids$layer <- "centroid"
both <- rbind(vic[, c("LGA_NAME", "geometry", "layer")],
              centroids[, c("LGA_NAME", "geometry", "layer")])

both_fish <- sf_fisheye(both, center = melbourne,
                        r_in = 0.34, r_out = 0.60,
                        zoom_factor = 15, squeeze_factor = 0.35)

ggplot() +
  geom_sf(data = both_fish[both_fish$layer == "polygon", ],
          fill = "grey92", color = "white", linewidth = 0.2) +
  geom_sf(data = both_fish[both_fish$layer == "centroid", ],
          color = "#2b6cb0", size = 0.6, alpha = 0.8) +
  ggtitle("Aligned overlays: transform layers together, plot separately")

If layers must be transformed separately, current per-layer normalization can lead to slight radius mismatches. A practical workaround is to compute a scale factor from a reference layer’s bbox and multiply r_in/r_out accordingly, e.g.:

all_points <- sf_fisheye(all_points, center = melbourne,
                         zoom_factor = 1.8, squeeze_factor = 0.30)
scale_radii <- 1 - (((st_bbox(vic)["xmax"] - st_bbox(vic)["xmin"])  -
                     (st_bbox(all_points)["xmax"] - st_bbox(all_points)["xmin"])) /
                    (st_bbox(vic)["xmax"] - st_bbox(vic)["xmin"]))
vic_fisheye <- sf_fisheye(vic, center = melbourne,
                          r_in = 0.35 * scale_radii,
                          r_out = 0.50 * scale_radii,
                          zoom_factor = 1.8, squeeze_factor = 0.30)

Future versions will expose a parameter object or match_to argument so multiple layers can share normalization and radii without manual scaling.

Diagnostics with numeric coordinates

Use create_test_grid() and plot_fisheye_fgc() to understand the mapping in isolation from GIS workflows.

grid <- create_test_grid(range = c(-1, 1), spacing = 0.1)
warp <- fisheye_fgc(grid, r_in = 0.34, r_out = 0.5,
                    zoom_factor = 1.3, squeeze_factor = 0.5)
plot_fisheye_fgc(grid, warp, r_in = 0.34, r_out = 0.5)

CRS and reproducibility tips

• Let sf_fisheye() auto-select a working CRS for lon/lat inputs, or set target_crs explicitly for project standards.
• Prefer center as an sf/sfc geometry or as lon/lat with center_crs for clarity.
• Use normalized_center = TRUE and fixed radii for small multiples across regions.

Caveats

The fisheye intentionally distorts distances and areas within the focus and glue zones. Use it for visual exploration and communication; run quantitative spatial analysis on the original geometry. For publication figures, set revolution = 0 and describe the distortion in the caption.