Opaque surface or black card — blocks film area optically to create VFX space. Classic in-camera technique, now digital.
You need a specific area of the image to composite something into later—an explosion, an additional building, a character—but you don't just put a green or blue screen behind it. Instead, you use a matte: an opaque material, usually black paper or cardboard, that you place in front of the camera or mount directly onto the lens. It optically displaces a portion of the film area, so this area remains light-tight later and appears black on the negative. This is your placeholder for visual effects.
This works on the principle of matte painting, but without the painted component—pure blockage. In the classic process, you work with a hard-edged matte cut directly in front of or behind the lens. The position is critical: the closer to the lens, the harder the edge; the closer to the set, the softer the transitions due to blur. You expose the clear area normally, the matte zone remains unexposed, and it later becomes your silent negative that you incorporate into a second exposure or the compositing process.
Today, most of this is done digitally: you film the scene normally and use the matte as a masking concept in editing or in the VFX pipeline. In Nuke or After Effects, you work with geometric or organic masks—digital mattes—to exclude areas and make them available for new content. The advantage: no physical attachment on set, no exposure problems from semi-transparent edges. Digital is easier to handle for optical purity.
Classic use case: You film a scene against a natural sky but want to insert a digital sky or motion graphics later. A black matte in front of the upper part of the image optically blocks out the sky—ideal for later compositing. Unlike keying (greenscreen), you don't need lighting control or clean color separation here. The matte is either there or not there. You can remove it with pen and paper, or today: with brush strokes on the mask layer.