Color reproduction¶

When reproducing (synthesizing) colors, two systems are possible, according to the fact that the device is “passive” or reflective (in painting, printing, etc.), i.e. that it needs to be illuminated, or “active”, and emits its own light (screen, projector, etc).

Additive synthesis¶

When a system emits its own light, such as a screen or projector, it produces colors ranging from black* (a system that is turned off, emitting nothing), to white*.

In such an additive system, the most luminous color is the white: it results from the addition of the various primitives * of which the device pixels are made, in general a combination of green, red and blue.

This system works by adding rays of light to blend the colors.

A consequence to such a system is that a monochromatic color (the color of a primary* of the system, or the simulation of another monochromatic light obtained by blending these primaries*) is inevitably less intense than white, contrary to the real world where everything is possible (the intensity is independent from the tint).

An additive system, being a light generator, can work in complete darkness, in which case the color reproduction can be perfectly controlled by not being influenced by any external light (as in a movie theater).

Subtractive synthesis¶

When a color reproduction system is a reflective surface, the pigments are responsible for the alteration of the received light; each pigment absorbs part of the received rays and reflects the rest. The resulting light is therefore of a lower intensity than the one illuminating the surface: the system subtracts light rays from an available set.

Such a subtractive system produces colors by going from he color of the empty sheet or surface¹ (reflecting all received light) to almost-black* (absorbing all received light).

An important consequence is that a subtractive system depends on the light illuminating it to reproduce colors: seen under a light poor on monochromatic rays*, an exhibited painting will not render all its possible colors. It’s necessary to workaround this issue by illuminating the painting not only with a white light, but with a white light “containing” enough different monochromatic* rays, a light of higher quality².

A painting, a poster or any other object illuminated under a monochromatic light will be able to reproduce only this monochromatic* color, at different intensities, whatever the pigments which compose them are³.

The video-mapping case¶

Video-mapping is the projection of videos on volumes, the creation of large images on structures such as monuments. By projecting light, colors are synthesized in an additive manner, then returned to the viewer by the surface of the structure, like a cinema screen. But unlike in cinema, two parameters must be taken into account to anticipate the perceived color after light is reflected by the structure:

• The surrounding light also illuminates the structure; the more complete the darkness, the more detailed, contrasted and controlled the result can be. In fact, the surrounding light adds to the projection (therefore increasing its intensity and pulling the colors towards that of the surrounding light, generally a shade of white, or the artificial lights color at night).

• As the structure isn’t white, if the intensity of the projection isn’t sufficient, the lightest color will only be that of the surface; it will be very difficult, if not impossible, to obtain white if the structure is very colored. In the same way, complementary colors (farthest away in the spectrum) to those on the surface will tend to appear darker (being those primarily absorbed by the surface).