PolarGridHelper: Allow zero radials or zero circles

[JeremieBourque1]

Prevents from getting NaN values when 0 radials or 0 circles are used.

Related issue: none

Description

Previously, if a PolarGridHelper is created with 0 radials or 0 circles, we get the following error:

THREE.BufferGeometry.computeBoundingSphere(): Computed radius is NaN. The "position" attribute is likely to have NaN values. 

The NaN values come from the for-loops doing an iteration for 0 wich causes a 0/0 division resulting in a NaN.

By changing the looping condition to a "less than", no iterations are done so no NaN is created, allowing for a polar grid with no radials or no circles without getting an error.

[LeviPesin]

Now the maximum value of v would be less than 2*Pi, which will result in radials not making full circle. Is this what we want?

[JeremieBourque1]

We don't need v to go completely around the circle because that will create two lines at 0 degrees instead of only one.

[Mugen87]

I've added the change to my local branch and experimented with it.

With this change, adding 0 for both radials and circles results in an empty geometry. Only setting one of the parameters to 0 results in a malformed polar grid (only radial lines or circles).

Right now, it seems not allowing 0 is the better approach.

[JeremieBourque1]

@Mugen87 In my use case, I want to be able to set 0 radials and have only circles because otherwise it ends up cluttering the ground plane too much. As for 0 circles, I don't have that use case but I could see someone potentially only needing the radials.

The way I see it, it would give the user a bit more freedom without the need of writing their own helper just for a slight modification.

[WestLangley]

The production code has the following shortcomings:

1. a circle of radius 0 is rendered unnecessarily,
2. the first and last radial are coincident -- and sometimes rendered in different colors, and
3. a single radial is rendered if radials is 1.

This PR currently fixes (1) and (2).

Instead, I would rename radials to sectors, and (optionally) circles to rings. sectors and rings must be at least 1.

class PolarGridHelper extends LineSegments {

	constructor( radius = 10, sectors = 16, rings = 8, divisions = 64, color1 = 0x444444, color2 = 0x888888 ) {

		color1 = new Color( color1 );
		color2 = new Color( color2 );

		const vertices = [];
		const colors = [];

		// create the sectors

		if ( sectors > 1 ) {

			for ( let i = 0; i < sectors; i ++ ) {

				const v = ( i / sectors ) * ( Math.PI * 2 );

				const x = Math.sin( v ) * radius;
				const z = Math.cos( v ) * radius;

				vertices.push( 0, 0, 0 );
				vertices.push( x, 0, z );

				const color = ( i & 1 ) ? color1 : color2;

				colors.push( color.r, color.g, color.b );
				colors.push( color.r, color.g, color.b );

			}

		}

		// create the rings

		for ( let i = 0; i < rings; i ++ ) {

			const color = ( i & 1 ) ? color1 : color2;

			const r = radius - ( radius / rings * i );

			for ( let j = 0; j < divisions; j ++ ) {

				// first vertex

				let v = ( j / divisions ) * ( Math.PI * 2 );

				let x = Math.sin( v ) * r;
				let z = Math.cos( v ) * r;

				vertices.push( x, 0, z );
				colors.push( color.r, color.g, color.b );

				// second vertex

				v = ( ( j + 1 ) / divisions ) * ( Math.PI * 2 );

				x = Math.sin( v ) * r;
				z = Math.cos( v ) * r;

				vertices.push( x, 0, z );
				colors.push( color.r, color.g, color.b );

			}

		}

		const geometry = new BufferGeometry();
		geometry.setAttribute( 'position', new Float32BufferAttribute( vertices, 3 ) );
		geometry.setAttribute( 'color', new Float32BufferAttribute( colors, 3 ) );

		const material = new LineBasicMaterial( { vertexColors: true, toneMapped: false } );

		super( geometry, material );

		this.type = 'PolarGridHelper';

	}

}

[JeremieBourque1]

@WestLangley I have done the modifications you suggested.

[mrdoob]

Thanks!