Support mjs generator scripts

[slimbuck]

This PR adds support for generating gaussians via an external script.

The script must have .mjs extension and export a class called Generator with the following structure:

class Generator {
    // number of gaussians to generate
    count: number;
    // names of the columns to generate
    columnNames: string[];
    // callback function to get the data for row 'index'
    getRow: (index: number, row: any) => void;
};

Parameters are passed to the generator from the command line using:

--params name=value,name=value

The (name,value) pairs are passed to a static create function on Generator which is responsible for constructing the instance.

This is an example grid generator script gen-grid.mjs:

class Generator {
    constructor(width, height, scale) {
        const ls = Math.log(scale);

        this.count = width * height;

        this.columnNames = [
            'x', 'y', 'z',
            'scale_0', 'scale_1', 'scale_2',
            'f_dc_0', 'f_dc_1', 'f_dc_2', 'opacity',
            'rot_0', 'rot_1', 'rot_2', 'rot_3'
        ];

        const SH_C0 = 0.28209479177387814;
        const invSigmoid = (opacity) => (opacity <= 0) ? -20 : (opacity >= 1) ? 20 : -Math.log(1 / opacity - 1);

        this.getRow = (index, row) =>{
            row.x = (index % width) * scale;
            row.y = 0;
            row.z = Math.floor(index / width) * scale;

            // e^x
            row.scale_0 = ls;
            row.scale_1 = ls;
            row.scale_2 = ls;

            row.f_dc_0 = (1 - 0.5) / SH_C0;
            row.f_dc_1 = (1 - 0.5) / SH_C0;
            row.f_dc_2 = (1 - 0.5) / SH_C0;
            row.opacity = invSigmoid(1.0);
            
            row.rot_0 = 0;
            row.rot_1 = 0;
            row.rot_2 = 0;
            row.rot_3 = 1;
        };
    }

    static create(params) {
        const w = parseInt(params.find(p => p.name === 'width')?.value ?? '1000', 10);
        const h = parseInt(params.find(p => p.name === 'height')?.value ?? '1000', 10);
        const s = parseFloat(params.find(p => p.name === 'scale')?.value ?? '1.0');

        console.log(`Generating grid ${w} x ${h} x ${s}`);

        return new Generator(w, h, s);
    }
};

export { Generator };

Which can be invoked as follows:

splat-transform gen-grid.mjs --params width=100,height=100,scale=0.1 scenes/grid.ply

[Copilot]

Pull Request Overview

This PR adds support for generating Gaussian splat data using external MJS scripts, allowing dynamic data generation through custom generator classes. The implementation introduces a new file reader for .mjs files that can execute JavaScript modules to generate splat data programmatically.

• Added MJS file reading capability with parameter passing support
• Extended command-line argument parsing to handle script parameters via --param flags
• Integrated the new MJS reader into the main file processing pipeline

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/readers/read-mjs.ts	Implements the MJS file reader that loads generator scripts and produces splat data
src/process.ts	Adds Param type and handles param actions in the processing pipeline
src/index.ts	Integrates MJS reading, adds parameter parsing, and updates the main file reading logic

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[willeastcott]

Why not check in a generators folder with an example or two?

[willeastcott]

Also, please update the README.