The scene graph and the paint model • vellum

This article covers the two pieces you spend most of your time with in vellum: the scene graph (units, viewports, and the tree they form) and the paint model (gradients, patterns, and masks) shared across every backend.

The scene graph

A vellum scene is a tree. The root is the page created by vl_scene(); every push() adds a vl_viewport() child and descends into it; every draw() appends a grob at the current level; pop() climbs back up. The tree is retained rather than drawn-and-forgotten, which is what enables the queries in vignette("retained-mode").

Because it is a tree, viewports nest, and a child’s geometry is expressed relative to its parent. That is the whole mechanism behind panels, insets, and faceting: push a viewport for a sub-region, draw inside it in local coordinates, then pop.

vl_scene(6, 2.4, bg = "white") |>
  # a full-width band
  draw(rect_grob(height = 0.6, gp = vl_gpar(fill = "#eef2f6", col = NA))) |>
  # an inset viewport occupying the middle third
  push(vl_viewport(x = 0.5, width = 1 / 3, height = 0.8)) |>
  draw(rect_grob(gp = vl_gpar(fill = "#3a7bd5", col = NA))) |>
  draw(text_grob("inset", gp = vl_gpar(col = "white", fontface = "bold"))) |>
  pop()

Units

Coordinates and sizes are vl_unit() vectors: a value paired with a unit name. Each element carries its own unit, so one vector can mix coordinate systems, and a grob can even use different units on its x and y axes.

The units you reach for most:

"npc" (the default): normalised parent coordinates, 0 at bottom/left and 1 at top/right of the current viewport.
"native": the enclosing viewport’s xscale / yscale, so data values map directly. This is what you use for plotted data.
"mm", "cm", "in", "pt": absolute physical lengths that keep their size regardless of the viewport.

A bare number is interpreted in the grob’s default units (usually "npc"), so x = 0.5 and x = vl_unit(0.5, "npc") are the same thing.

vl_unit(1:3, "native")
#> <vellum_unit[3]>
#> [1] 1native 2native 3native
vl_unit(c(0.5, 1), c("npc", "in"))
#> <vellum_unit[2]>
#> [1] 0.5npc 1.0in

"native" units need a viewport with scales to resolve against. Set xscale and yscale when you push:

vl_scene(5, 3, bg = "white") |>
  push(vl_viewport(
    width = 0.86, height = 0.82,
    xscale = c(0, 10), yscale = c(-5, 25)
  )) |>
  draw(rect_grob(gp = vl_gpar(fill = "grey97", col = "grey70"))) |>
  draw(lines_grob(
    x = vl_unit(0:10, "native"),
    y = vl_unit((0:10) * 2, "native"),
    gp = vl_gpar(col = "steelblue", lwd = 2)
  )) |>
  pop()

Absolute and relative units compose within a viewport, and font- or string-relative units ("char", "line", "strwidth") resolve to millimetres at construction. Mixing a relative and an absolute unit in a single arithmetic expression (say vl_unit(1, "npc") - vl_unit(2, "mm")) is deferred and reported rather than silently guessed, so an ambiguous offset fails loudly instead of drawing in the wrong place.

The paint model

Any fill in vl_gpar() can be more than a flat colour. The same three paint types work identically on the raster, SVG, and PDF backends (with the documented exception that the PDF backend does not yet rasterise patterns).

Gradients

linear_gradient() and radial_gradient() interpolate between colour stops. Their geometry is given in a coordinate system ("npc" by default) and is resolved against the viewport at draw time, so a gradient transforms with its grob just like the outline does.

vl_scene(6, 2.2, bg = "white") |>
  push(vl_viewport(x = 0.28, width = 0.44)) |>
  draw(rect_grob(
    width = 0.8, height = 0.7,
    gp = vl_gpar(fill = linear_gradient(c("#1b2a4a", "#3a7bd5")), col = NA)
  )) |>
  pop() |>
  push(vl_viewport(x = 0.72, width = 0.44)) |>
  draw(circle_grob(
    r = 0.34,
    gp = vl_gpar(fill = radial_gradient(c("#f6d365", "#fda085")), col = NA)
  )) |>
  pop()

Patterns

vl_pattern() fills a shape by tiling a grob (or a list of grobs). The tile is authored in the unit square and repeated across a cell whose size you choose.

tile <- list(
  rect_grob(gp = vl_gpar(fill = "#ecf0f1", col = NA)),
  circle_grob(r = 0.32, gp = vl_gpar(fill = "#e74c3c", col = NA))
)

vl_scene(4, 2.4, bg = "white") |>
  draw(rect_grob(
    width = 0.84, height = 0.84,
    gp = vl_gpar(fill = vl_pattern(tile, width = 0.18, height = 0.3), col = NA)
  ))

Masks and group opacity

A mask is a grob whose coverage modulates the visibility of a viewport’s contents. Wrap it with as_mask() and pass it to vl_viewport(mask = ...). Here a linear gradient is clipped to a circular alpha mask.

vl_scene(4, 2.4, bg = "white") |>
  push(vl_viewport(
    mask = as_mask(circle_grob(r = 0.42, gp = vl_gpar(fill = "white", col = NA)))
  )) |>
  draw(rect_grob(gp = vl_gpar(fill = linear_gradient(c("#7f53ac", "#647dee")), col = NA))) |>
  pop()

Related to masks is group opacity. Setting vl_viewport(alpha = ...) composites the viewport’s contents as a single isolated layer at that opacity, so overlapping elements do not accumulate the way per-element vl_gpar(alpha = ) would. That distinction (compositing a group versus fading each mark) is exactly the kind of control a grammar layer needs from its backend.

Recap

A scene is a retained tree of nested viewports and grobs, built with push() / draw() / pop().
vl_unit() vectors express geometry; "npc" is relative to the viewport, "native" follows the data scales, and "mm" and friends are absolute.
vl_gpar(fill = ) accepts gradients and patterns, and vl_viewport() accepts masks, group opacity, and blend modes, all consistent across backends.

Next, see vignette("retained-mode") for what the retained tree lets you do after it is built. ```