Tone Reproduction
Our perception of the world is analog, i.e. we see continuous signals, continuous tones of colors as opposed to digital or discrete.
• A conventional photograph shows a smooth gradation between the lightest and darkest tones (although on a fine enough scale the photographic image is made up of very fine grains which are themselves made up of atoms).
• Nevertheless, there has been a steady movement toward digital methods of recording and reproduction in almost every field including audio, video, photography and, most important to us, graphic reproduction.
• With advances in digital memory technology, very high resolution digital representations of information allow precise control of distortions with resulting very accurate reproduction along with the ability to manipulate the (digital) signal in many ways.
• Analog systems generally suffer from degradation during transfer from one medium to another, due to noise (random fluctuations of a signal that contain no information). Sophisticated methods have to be employed to compensate for the degradation.
• Digital systems can also suffer from noise, but because the signal is encoded in binary form, it is easier to reconstruct the original. For example, the reproduction (i.e. copying) of a computer file is (almost) always exact (built into every computer/memory/disk system today are error correcting algorithms which compensate for random defects in the media).
• Although a digital signal can never perfectly reproduce an analog signal, it may be perceived as identical to the original so long as the signal is sampled at a higher frequency than the limits of human perception. (Examples of sampling and reproducing signals by both analog and digital methods are shown in Fig. 4-1)
For reproduction of color data, it is necessary to accurately convey the full range of tones while preserving color and detail.
- A photographic original is generally regarded as continuous tone (contone or CT).
- Reproducing a photograph involves recording the tonal values of the original followed by transforming (processing) these values into a form that can be displayed or printed.
- Precise control over the tonal reproduction process must be maintained whether exact reproduction is desired or some enhancements are to be made.
- It should be possible to achieve high contrast (highlights and shadows), good tonal rendering and gradation along with high resolution.
- It is important to optimize these for a given image and for the targeted viewing medium.
- There are three ways of defining tones: as gray scale values, as optical density values or as halftone tonal values.
- These are perfectly equivalent representations of tones, so long as the corresponding resolutions are sufficiently high, i.e. each can be determined from one of the others.
Gray Scale - A photographic print or transparency shows an apparent smooth transition from lightest to darkest regions. (Indeed, the darkness depends on the number of silver atoms present, a discrete quantity, but it impossible to perceive the addition or removal of single atoms). This is similarly true for other types of color originals such as artists renditions.