A variable compression ratio apparatus of an exhaust gas brake in an engine, may include a piston reciprocally moving in a cylinder of the engine, and an exhaust control valve that selectively communicates a combustion chamber of the cylinder with an exhaust manifold in accordance with pressure generated by the piston in the cylinder to control operating pressure of the piston. Generating the computed images includes using an approximation variable that represents some relationship between the 2D images, such as the speed of motion across two images. The instructions are coded in the instruction set of a CPU core included within the microprocessor. It will be back to its original position after the force is removed. The first and second portions are adapted for assembly in a facing arrangement in which the isolating wall is aligned with the trough.