• Status: Accepted
• Date: 2026-08-13

Context

The instrument now holds three published runs rather than one, and a run is several minutes of model time that a reader moves about in. Two things follow. Something has to say which run is loaded, and something has to be sendable: the useful sentence about a galaxy collision is "look at what the tidal tail is doing at t = 120", and it is useless without a way of arriving there.

The client is a static page on GitHub Pages. There is no server to ask.

Decision

Which run is loaded and which moment the transport is at are both properties of the address, as query parameters: ?run=cluster&t=12.5. Nothing else the interface holds is.

Query parameters rather than a path. A path such as /instrument/cluster needs the server to serve the client for an address that is not a file, and a GitHub Pages project site will not: it answers with its own 404 page. The usual workaround, a copy of the client at 404.html, is a second entry point that has to be kept in step with the first, and it exists to make the address look tidy.

Query parameters rather than a fragment. A fragment would work, and it is not sent to the server at all, which is a small advantage. The client already reads ?renderer=webgl2 to pin the backend, and having two mechanisms for reading the address means two places to look when one of them is wrong.

The gallery's entries are anchors with real hrefs to those addresses, and choosing one is intercepted so that the client is not fetched again. The thing being intercepted has to work on its own: a link that only works when JavaScript has run cannot be opened in a second tab, copied from a context menu, or read by anything that reads links. popstate is answered for the same reason, so that back and forward move between runs rather than changing the address under a page that has not noticed.

The address is rewritten when the transport is moved by hand and not while a run is playing. A moment worth sending someone is one that was chosen, and following playback would produce sixty history entries a second.

Nothing else goes in. The exposure, the sprite radius, the tone curve and which registers are shown are preferences, and an address carrying them would be an address nobody could read and a link that imposed the sender's settings on the person who opened it.

What an address that names nothing gets

The collision, which is the run the repository demonstrates. An address naming a run that is not published gets the same, rather than an error: a reader following a link to a run that has since been renamed wanted, at least approximately, what the gallery holds now. A moment that is not a number, or is before the run starts, names no moment and the run opens at its beginning.

Consequences

A moment named by an address is not always reachable when the page loads. The trajectory is read in order and a moment near the end of a long run may be a minute of network away, so the seek is attempted again each time more of the run arrives, until the frame for it has been decoded.

The instrument goes to the frame nearest the moment asked for rather than to the moment itself, because a trajectory is written at a stride and the instants between two frames are instants the run did not record. The clock then shows the frame's own time rather than the one in the address, which is the instant being drawn.

The transport's step follows from the same fact and was changed to match: it moves by one trajectory frame rather than by one integrator step, so an arrow key always changes the picture.