question: public double pixelSizeAtDistance ( double distance ) { if ( this . pixelSizeFactor == 0 ) { double fovyDegrees = this . fieldOfView ; double tanfovy_2 = Math . tan ( Math . toRadians ( fovyDegrees * 0.5 ) ) ; this . pixelSizeFactor = 2 * tanfovy_2 / this . viewport . height ; } return distance * this . pixelSizeFactor ; }
target:  Returns the height of a pixel at a given distance from the eye point. This method assumes the model of a screen composed of rectangular pixels, where pixel coordinates denote infinitely thin space between pixels. The units of the returned size are in meters per pixel. <p/> The result of this method is undefined if the distance is negative.