A three dimensional (3D) mixed reality system combines a real 3D image or video, captured by a 3D camera for example, with a virtual 3D image rendered by a computer or other machine to render a 3D mixed-reality image or video. A 3D camera can acquire two separate images (a left and a right) of a common scene, and superimpose the two separate images to create a real image with a 3D depth effect. The 3D mixed-reality system can determine a distance to a zero disparity plane for the real 3D image, determine one or more parameters for a projection matrix based on the distance to the zero disparity plane, render a virtual 3D object based on the projection matrix, combine the real image and the virtual 3D object to generate a mixed-reality 3D image.