Why HoloML

This page is about what HoloML is for, and why it is made as it is: it looks like HTML, it is strict about mistakes, it leaves models to glTF, and it is small.

What it is for

The web has HTML for documents on a flat page. There are formats for 3D as well, such as glTF, USD, and X3D, but they describe models and scenes for tools, not pages for people. HoloML sits between the two: a small language for 3D web pages, which an author can write by hand, which links to other pages, and which a browser can show without a game engine.

A HoloML page describes a scene: 3D models, where the viewer starts, lights, text, links, sound, and animation. A browser that knows HoloML, such as HyperSpace 3D, shows it as a space to orbit or walk around, and a link in it opens the next page, as on the web. The example sites show the range: a showroom of cars, a block game, a shop page where a sofa changes its fabric in place, a flat to tour, a sneaker store, and an aquarium to walk through.

Why it looks like HTML

A HoloML page is written as a web page is: elements, attributes, and text; a head with a title; comments; links written <a href="…">; and addresses relative to the page. Anyone who has written HTML can read one. It is plain text, so it can be written in any editor, read by anyone who opens the file, and kept in version control. The elements name what is in the scene (model, light, label), not how to draw it, and the numbers are in everyday units: metres, degrees, and seconds.

Why it is strict

A web browser does its best with a broken HTML page, and guesses what was meant. A HoloML reader does not guess. It stops at the first mistake in the syntax, such as a name in capitals, a value without quotes, or an element that is never closed, and reports it with its line and column:

<holoml version="0.2">
  <scene>
    <model src="models/coupe.glb">
  </scene>
</holoml>

Here the model is never closed. A reader stops at line 4, column 3, where it expected </model> and found </scene>, and reports the error mismatched-end-tag. There are three reasons for this:

The author finds a mistake at once, where it is, rather than later, as a scene that looks wrong in one browser and right in another.
Every reader reads a page the same way. The conformance samples, pages with the results that any reader must give, pin down the one tree of each correct page and the one error of each broken page, so a second renderer cannot guess differently from the first.
The rules stay short enough to read: the syntax fits in one section of the specification, with a formal grammar beside it.

Strict syntax does not mean that one problem hides a page. Past the syntax, a checker reports every problem it finds, such as an unknown element or a value out of range, each with its place, and a renderer shows the rest of the scene as well as it can. A model file that cannot be loaded is left out, and the rest is shown.

Why glTF

HoloML does not describe shapes. A model names a glTF file, and the page puts the model in place, turns it, sizes it, and changes its materials. glTF 2.0 is the Khronos Group's open file format for 3D models, made for sending them to where they are shown: one file holds a model's shapes, materials, pictures, and animations, or names the files that do.

It is an open standard, so HoloML needs no model format of its own, and pages can use models that tools and collections already make. The example sites' come from Poly Haven, Kenney's kits, and the Khronos Group's sample models, among others.
HoloML's space is glTF's: metres, with y up and right-handed axes. A model arrives the right way up and at the size it was made.
A page can reach inside a model: material changes a material by its name in the file, and animation plays one of the model's own.
The heavy data stays in files that the browser loads, so the page stays short enough to read: a model is one line.

Why it is small

HoloML 0.1 has 13 elements, and 0.2 has 22. Each does one thing, and they combine: a door that opens is a group at its hinge and an animate that begins on a click, and a lamp is a light and a click action. What markup cannot say, a script can: a 0.2 page runs ordinary web JavaScript, with one object, holoml, for its scene, rather than gaining an element for each new need.

A small language is quick to learn and to write by hand, and easier to build readers for, in full and alike. Every element and attribute in the specification appears in at least one conformance sample, and HoloML's own parser needs no other package. New things come when a real page needs them: version 0.2 grew with HyperSpace 3D's example sites, each adding what it needed. Ideas such as physics, and spaces shared by several people, wait for later versions.

The specification's files, syntax, checking, and versions; and How a browser shows a page and Versions.