edge-distances: endpoints #3157

Description of new feature

What should the new feature do? For visual features, include an image/mockup of the expected output.

Sometimes, you may want to set the control point of an edge relative to the endpoints of the edge rather than the positions (i.e. centre points) of the source and target nodes.

To support this, we could add to the existing edge-distances property. This property already modifies how control points are calculated. Here, we would add a new value, edge-distances: endpoints. When set this way, the control points, segment points, etc. are calculated relative to the endpoints of the edge.

Edge cases to consider:

• The endpoints might not be well specified already, based on the values of the other style properties.
  • Case: The endpoints are set explicitly. Everything works for edge-distances: endpoints.
  • Case: The endpoints are implicit, by default. That means for edge-distances: endpoints, they would be undefined. We need to decide what the behaviour here should be.
    • It shouldn't break things if set incorrectly, i.e. it should show something rather than just giving an error with no visual output. An edge with style not quite like you wanted is better than no edge.
    • It should have a warning.
    • Option: If the endpoints aren't set properly, then assume the default edge-distances: intersection instead.
    • Other options?
• Other edge cases?

Motivation for new feature

Describe your use case for this new feature.

For instance:

If you want an even edge like the second case, then you need the control point to be relative to the endpoints.

For reviewers

Reviewers should ensure that the following tasks are carried out for incorporated issues:

• Ensure that the reporter has adequately described their idea. If not, elicit more information about the use case. You should iteratively build a spec together.
• Ensure that the issue is a good fit for the core library. Some things are best done in extensions (e.g. UI-related features that aren't style-related). Some things are best done by app authors themselves -- instead of in Cytoscape libraries.
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the unstable branch via pull request. The corresponding pull request is cross-referenced.