---
title: "The ggseg_atlas object"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{The ggseg_atlas object}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r}
#| label: setup-opts
#| include: false
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)
```

```{r}
#| label: setup
library(ggseg.formats)
```

Everything in the ggseg ecosystem starts from a single object: the `ggseg_atlas`.
Whether you are making a 2D cortical flatmap with ggseg or spinning a 3D mesh in
ggseg3d, the atlas is the container that holds the geometry, the region metadata,
and the colour palette together. This vignette walks through the structure so you
know exactly what you are working with.

## What is a ggseg_atlas?

A `ggseg_atlas` is an S3 object that bundles five pieces of information into one
handle. Let's print the bundled Desikan-Killiany atlas to see what that looks
like:

```{r}
#| label: print-dk
dk()
```
The print method gives you a quick overview: the atlas name, type, how many
regions it has, which hemispheres are present, what views the 2D geometry
provides, and whether palette and rendering data are available. Below the summary
you see the core table, the single source of truth for region identity.

## Anatomy of an atlas

The five slots are accessed with `$`:

```{r}
#| label: atlas-slots
dk()$atlas
dk()$type
```

`$atlas` is a short name (used to look up atlases by string) and `$type` is one
of `"cortical"`, `"subcortical"`, or `"tract"`.

The `$palette` is a named character vector mapping labels to hex colours:

```{r}
#| label: palette
head(dk()$palette)
```

`$core` is a data frame with one row per region. It always has `region` and
`label` columns, and will often include `hemi` and additional metadata like
`lobe` or `structure`:

```{r}
#| label: core
head(dk()$core)
```

Finally, `$data` is a `ggseg_atlas_data` object that holds the actual geometry.
Its contents depend on the atlas type.

```{r}
#| label: data-class
class(dk()$data)
```

## Three atlas types

ggseg.formats ships three atlases that illustrate the three types.

**Cortical** atlases like `dk` parcellate the cortical surface. Their data object
is a `ggseg_data_cortical` containing sf polygons for 2D rendering and vertex indices
for 3D:

```{r}
#| label: cortical-type
dk()$type
names(dk()$data)
```

**Subcortical** atlases like `aseg` represent deep brain structures. Their data
is a `ggseg_data_subcortical` with sf polygons and individual 3D meshes:

```{r}
#| label: subcortical-type
aseg()$type
names(aseg()$data)
```

**Tract** atlases like `tracula` represent white matter bundles. Their data is a
`ggseg_data_tract` with sf polygons and centerlines that generate tube meshes for 3D:

```{r}
#| label: tract-type
tracula()$type
names(tracula()$data)
```

In every case the sf component drives 2D plotting and the type-specific component
(vertices, meshes, or centerlines) drives 3D.

## Core: the region table

The `$core` data frame is the single source of truth for what regions an atlas
contains. Every manipulation function updates core first and then propagates
changes to geometry and palette.

The required columns are `region` (a human-readable name) and `label` (a unique
identifier that links core to geometry). Most atlases also carry `hemi`:

```{r}
#| label: core-structure
str(dk()$core)
```

Some atlases include additional metadata columns. The `dk` atlas, for instance,
has `lobe`:

```{r}
#| label: core-lobe
unique(dk()$core$lobe)
```

You can add your own metadata with `atlas_core_add()` (covered in the
manipulation vignette).

## Querying an atlas

A set of accessor functions lets you pull information out without reaching into
slots directly.

`atlas_regions()` returns the sorted unique region names:

```{r}
#| label: atlas-regions
atlas_regions(dk())
```

`atlas_labels()` returns the unique labels (the identifiers used to join
geometry):

```{r}
#| label: atlas-labels
head(atlas_labels(dk()))
```

`atlas_views()` returns the available 2D views:

```{r}
#| label: atlas-views
atlas_views(dk())
atlas_views(aseg())
atlas_views(tracula())
```

`atlas_type()` returns the type string:

```{r}
#| label: atlas-type
atlas_type(dk())
atlas_type(aseg())
atlas_type(tracula())
```

`atlas_palette()` retrieves the colour palette. You can pass the atlas object
directly or its name as a string:

```{r}
#| label: atlas-palette
head(atlas_palette(dk()))
```

## Extracting render-ready data

When you need the actual data frames that ggseg and ggseg3d consume, use the
`atlas_*()` extractors. These join core metadata and palette colours onto the raw
geometry so you get a single, ready-to-use table.

`atlas_sf()` returns an sf data frame for 2D rendering:

```{r}
#| label: atlas-sf
sf_data <- atlas_sf(dk())
sf_data
```

`atlas_vertices()` returns the vertex data for cortical 3D rendering:

```{r}
#| label: atlas-vertices
vert_data <- atlas_vertices(dk())
vert_data
```

`atlas_meshes()` returns mesh data for subcortical or tract 3D rendering:

```{r}
#| label: atlas-meshes
mesh_data <- atlas_meshes(aseg())
mesh_data
```

`as.data.frame()` is a convenience method that produces a merged sf data frame
similar to `atlas_sf()` but with atlas-level columns (`atlas`, `type`) attached:

```{r}
#| label: as-dataframe
df <- as.data.frame(dk())
names(df)
```

## Checking and converting

`is_ggseg_atlas()` tests whether an object has the right class:

```{r}
#| label: is-ggseg-atlas
is_ggseg_atlas(dk())
is_ggseg_atlas(mtcars)
```

`as_ggseg_atlas()` coerces lists with the right structure into a proper
`ggseg_atlas`:

```{r}
#| label: as-ggseg-atlas
atlas_list <- as.list(dk())
recovered <- as_ggseg_atlas(atlas_list)
is_ggseg_atlas(recovered)
```

If you have an atlas object from an older version of ggseg that stored sf data
directly in `$data` instead of using the new `ggseg_atlas_data` wrapper,
`convert_legacy_brain_atlas()` will migrate it to the unified format.
