three.js/examples/js/particles_general/Atlas.js

/**
* @author Mark Kellogg - http://www.github.com/mkkellogg
*/

THREE.Atlas = function( texture, createFirstFullFrame ) {

	this.texture = texture;
	this.imageCount = 0;
	this.imageDescriptors = [];

	if ( createFirstFullFrame ) {

		this.addImageDescriptor( 0, 1, 1, 0 );

	}

};

THREE.Atlas.ImageDescriptor = function( left, top, right, bottom ) {

	this.left = left;
	this.top = top;
	this.right = right;
	this.bottom = bottom;

};

THREE.Atlas.prototype.addImageDescriptor = function( left, top, right, bottom ) {

	this.imageDescriptors[ this.imageCount ] = new THREE.Atlas.ImageDescriptor( left, top, right, bottom );
	this.imageCount ++;

};

THREE.Atlas.prototype.getImageDescriptor = function( index ) {

	return this.imageDescriptors[ index ];

};

THREE.Atlas.prototype.getTexture = function() {

	return this.texture;

};

THREE.Atlas.createGridAtlas = function( texture, left, top, right, bottom, xCount, yCount, reverseX, reverseY ) {

	var atlas = new THREE.Atlas( texture );

	var width = right - left;
	var height = top - bottom;

	var xBlockSize = width / xCount;
	var yBlockSize = height / yCount;

	var xInc = 1;
	var yInc = 1;

	var xStart = 0;
	var yStart = 0;

	var xFinish = xCount;
	var yFinish = yCount;

	if ( reverseX ) {

		xInc = - 1;
		xStart = xCount - 1;
		xFinish = - 1;

	}

	if ( reverseY ) {

		yInc = - 1;
		yStart = yCount - 1;
		yFinish = - 1;

	}

	for ( var y = yStart; y != yFinish; y += yInc ) {

		for ( var x = xStart; x != xFinish; x += xInc ) {

			var currentLeft = left + xBlockSize * x;
			var currentTop = bottom + yBlockSize * ( y + 1 );
			var currentRight = currentLeft + xBlockSize;
			var currentBottom = currentTop - yBlockSize;

			atlas.addImageDescriptor( currentLeft, currentTop, currentRight, currentBottom );

		}

	}

	return atlas;

};