/**
 * Transforms a position from eye to window coordinates.  The transformation
 * from eye to clip coordinates is done using {@link czm_projection}.
 * The transform from normalized device coordinates to window coordinates is
 * done using {@link czm_viewportTransformation}, which assumes a depth range
 * of <code>near = 0</code> and <code>far = 1</code>.
 * <br /><br />
 * This transform is useful when there is a need to manipulate window coordinates
 * in a vertex shader as done by {@link BillboardCollection}.
 *
 * @name czm_eyeToWindowCoordinates
 * @glslFunction
 *
 * @param {vec4} position The position in eye coordinates to transform.
 *
 * @returns {vec4} The transformed position in window coordinates.
 *
 * @see czm_modelToWindowCoordinates
 * @see czm_projection
 * @see czm_viewportTransformation
 * @see BillboardCollection
 *
 * @example
 * vec4 positionWC = czm_eyeToWindowCoordinates(positionEC);
 */
vec4 czm_eyeToWindowCoordinates(vec4 positionEC)
{
    vec4 q = czm_projection * positionEC;                        // clip coordinates
    q.xyz /= q.w;                                                // normalized device coordinates
    q.xyz = (czm_viewportTransformation * vec4(q.xyz, 1.0)).xyz; // window coordinates
    return q;
}