liabru/matter-js · warning

Bodies.trapezoid: slope parameter must be < 1.

Error message

Bodies.trapezoid: slope parameter must be < 1.

What it means

Bodies.trapezoid computes the top roof width as (1 - slope*2) * width, which becomes zero or negative for slope >= 1. The library warns when the slope parameter is >= 1, then continues, producing a degenerate (zero/negative-width) trapezoid.

Source

Thrown at src/factory/Bodies.js:73

    /**
     * Creates a new rigid body model with a trapezoid hull. 
     * The `slope` is parameterised as a fraction of `width` and must be < 1 to form a valid trapezoid. 
     * The options parameter is an object that specifies any properties you wish to override the defaults.
     * See the properties section of the `Matter.Body` module for detailed information on what you can pass via the `options` object.
     * @method trapezoid
     * @param {number} x
     * @param {number} y
     * @param {number} width
     * @param {number} height
     * @param {number} slope Must be a number < 1.
     * @param {object} [options]
     * @return {body} A new trapezoid body
     */
    Bodies.trapezoid = function(x, y, width, height, slope, options) {
        options = options || {};

        if (slope >= 1) {
            Common.warn('Bodies.trapezoid: slope parameter must be < 1.');
        }

        slope *= 0.5;
        var roof = (1 - (slope * 2)) * width;
        
        var x1 = width * slope,
            x2 = x1 + roof,
            x3 = x2 + x1,
            verticesPath;

        if (slope < 0.5) {
            verticesPath = 'L 0 0 L ' + x1 + ' ' + (-height) + ' L ' + x2 + ' ' + (-height) + ' L ' + x3 + ' 0';
        } else {
            verticesPath = 'L 0 0 L ' + x2 + ' ' + (-height) + ' L ' + x3 + ' 0';
        }

        var trapezoid = { 
            label: 'Trapezoid Body',

View on GitHub (pinned to acb99b6f87)

Solutions

1. Pass a slope strictly less than 1 (e.g. 0.5) — it is the per-side taper fraction of width.
2. If you intended an angle, convert it: slope = Math.tan(angle/2) style fraction, or clamp: slope = Math.min(slope, 0.99).
3. For steep/pointed shapes, use Bodies.polygon or Bodies.fromVertices instead of trapezoid.

Example fix

// before
Bodies.trapezoid(x, y, 100, 60, 1.2);
// after
Bodies.trapezoid(x, y, 100, 60, Math.min(1.2, 0.9));

if (typeof slope !== 'number' || !(slope < 1)) {
  throw new RangeError('Bodies.trapezoid slope must be a number < 1');
}

function isValidSlope(s) {
  return typeof s === 'number' && Number.isFinite(s) && s < 1;
}

When it happens

Trigger: Calling Bodies.trapezoid(x, y, width, height, slope, options) with slope >= 1 — e.g. slope of 1, 1.5, or negative-derived values outside the expected (0..1) range.

Common situations: Confusing this slope (a fraction of width per side, expected < 1) with an angle in radians/degrees or a ratio like height/width; computing slope dynamically from dimensions so it can exceed 1 for tall narrow shapes.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

AI-assisted analysis of liabru/matter-js@acb99b6f87 (2026-09-02). Data as JSON: /api/errors/e8e8cef02cfef125. Report an issue: GitHub.