Reverse Front Projection

In both front projection and transmission blue-screen compositing, extreme close-ups have presented various problems. In close-up photography via transmission blue, blue spill is the principal villain encountered. In front projection, if a subject approaches very close to the camera/ projector apparatus, the projected light will record on the subject in spite of the vast difference in gain between the subject and the Scotchlite screen. Furthermore, certain rules have long existed in front projection technique regarding the spatial relationships between the camera, the subject and the screen. (See Front Projection section.) These rules are directed at preventing the fringing of the subject that results from having a soft shadow rendered at the screen, the consequence of a relatively short subject-to-camera distance versus a relatively long subject-to-screen distance. Additional problems are introduced if the subject includes highly reflective surfaces, e.g., silver lamé clothing or space helmets; and all these problems are exacerbated if the subject is backlit.

In "Blue-Max" compositing, these difficulties can be resolved by the adoption of "Reverse Front Projection." In its simplest terms, Reverse Front Projection can be described as a radical rearrangement of the basic front-pro-jection setup. In conventional front projection, in which a camera and a projector are disposed at 90 degrees to each other with a beam splitter arranged between them at 45

Front Projection Diagram
Figure 1. Diagram of reverse front projection.

degrees to both, a subject to be photographed is positioned in front of the camera/projector apparatus, and a front-projection screen on which the projector will form an image is deployed beyond the subject. The camera is thus able to record and combine both the returning projected image and the foreground subject.

In Reverse Front Projection, the camera and projector are still at 90 degrees to each other, but separated by a considerable distance, and the foreground subject is placed between a very large beam splitter (which may be plain glass, or preferably a pellicle) and the camera. The front-projection screen faces the projector instead of the camera, while the camera faces the light trap normally confronted by the projector. (See Figure 1.) The effect of this arrangement is to take the diverging projected cone of light from the projector and deliver it as a converging cone of light, having turned it 90 degrees. We then position the camera so that the nodal point of its lens coincides with the focal point at which the projected cone of light converges.

By this process, we acquire all the advantages of front-projected blue, in terms of the purity of color as well as the absence of blue spill, without having to project the blue onto the subject. We have also eliminated the fringing resulting from poor alignment of projector and camera nodal points, as there is no shadow at all cast upon the screen by tire foreground subject. Furthermore, we have eliminated the haloing resulting from the backscat-tered light that occurs when the subject is backlit. This is due to a "diode effect" produced by the arrangement of elements in Reverse Front Projection. In normal front projection, a ray of light striking the back surface of a foreground subject is reflected back to the Scotch lite screen and then returns again along the same axis, plus or minus some 2%. Therefore some of the light restrikes the subject, while some passes the subject, making its way back to the camera to produce the objectionable halo.

By contrast, the "diode effect beamsplitter" handles the situation in the following manner: a ray of light striking the rear of the foreground subject is reflected back towards the beam splitter; approximately 92% of it is passed through the beam splitter to the black velvet screen, where it is absorbed. The remaining 8% is reflected back to the Scotchlite screen, and from thence returns to the beam splitter, where again 92% is passed through and 8% is reflected towards the foreground subject. Thus, only 8% of 8%, or .64%, is made available to the camera to record as halo. To be sure, only 8% of the projected blue light is being made available to the camera also, but that is not a serious problem to the Blue-Max with its massive output. It should also be borne in mind that in conventional front projection, only a theoretical 25% of the projected light survives the journey to the camera, so we are, in fact, sacrificing approximately one and a half stops.

We sacrifice some degree of camera flexibility in using Reverse Front Projection, as the camera cannot move from the nodal point defined by the projector unless provision is made to move both the camera and projector in synchrony. In some cases, it may be easier to move the subject in relation to the camera. Zooming is certainly possible, as are all nodal-point moves for the camera, and these should cover most requirements for close-ups. Apogee has applied for patent protection on Reverse Front Projection as well as the "Blue-Max," and both are available to the industry under license.

Current backing materials include the following paints and fabrics. Paints: Paramount Ultra-Marine Blue #8580 (a tough surface paint that resists scuffing, but is more applicable to television than to film, as it lacks sufficient color saturation); 7-K Infinity Blue (for years the industry standard); Apogee Process Blue, Rosco Ultra Blue and Gothic Ultra Blue. Fabrics: "FRP 100" (flame retardant) and "Tempo," (not flame-retardant though it has superior color saturation and a felt-like texture with a thin foam-rubber backing), both available from Daizians in New York and Los Angeles, and a new material from Rosco. Besides these there is a vinyl plastic sheet material from Stewarts called Ultimatte Front Lit Blue. This material, besides providing a very clean blue, is also very durable — sturdy enough to drive vehicles on.

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