Adjust edge start and end so they touch the box border

Modifies edge coordinate data so that connecting lines/arrows terminate at the boundaries of node boxes rather than at their centers. This prevents overlap between edge ends and node labels, improving visual clarity in DAG (Directed Acyclic Graph) plots.

Usage

adjust_edges_by_box(edges_df, nodes_df)

Arguments

edges_df

A data frame containing edge information with at minimum:

from: Character or integer identifier of the source node
to: Character or integer identifier of the target node
curvature: Numeric indicator; 1 for curved edges, 0 for straight

Additional columns are preserved in the output.

nodes_df

A data frame containing node information with required columns:

node_id: Unique identifier matching edges$from and edges$to
x: X-coordinate of node center
y: Y-coordinate of node center
half_w: Half-width of the node bounding box
half_h: Half-height of the node bounding box

These values typically come from compute_node_boxes.

Value

A data frame with the same columns as `edges_df` plus four new columns:

xstart_adj: Adjusted X-coordinate for edge start point
ystart_adj: Adjusted Y-coordinate for edge start point
xend_adj: Adjusted X-coordinate for edge end point
yend_adj: Adjusted Y-coordinate for edge end point
is_horizontal: Logical flag indicating nearly-horizontal lines
is_vertical: Logical flag indicating nearly-vertical lines

Rows are reordered with regular straight lines first, followed by horizontal, then curved edges.

Details

The function performs several adjustments:

Joins node position and box dimension data to each edge
Uses geometric intersection to calculate precise edge-to-box connection points
Applies special offsets for horizontal lines (reduces overlap)
Applies different offsets for curved edges to improve visual aesthetics

This is a critical preprocessing step before rendering DAG plots, as it ensures arrows visually connect to the perimeter of node boxes without penetrating them. The function leverages line_rect_intersection for accurate intersection calculations.

Algorithm Details

**General Case**: Uses `line_rect_intersection` to find where the line from node center to node center intersects the perimeter of each node's bounding box.

**Horizontal Lines**: For efficiency and visual consistency, applies a simplified offset (0.5 * half-width) instead of full intersection calculation. This keeps horizontal arrows visibly separated from both nodes.

**Curved Lines**: Applies custom offsets (quarter width, full height) to ensure curved edges emerge from appropriate positions on node boundaries.

Dependencies

This function requires:

dplyr for data manipulation (left_join, mutate, dplyr::filter, bind_rows)
line_rect_intersection for geometric calculations