2.3 Image transform in reproduction

The transform of original into some intermediate copy (transparency, plate, proof) or a resulting print includes three following basic stages:

• analysis which results in producing the image signal;

• this signal (nowadays mostly the digital numerical data) functional transforms;

• synthesis as the reverse move from signal to the image on a print, display, plate, proof or, at last, in mind of a viewer.

2.3.1 Electro-optical analysis

At this first stage the tone values of picture elements with their spatial coordinates x and y are turned into amplitude of an electric signal at the output of photoelectric transducer (PET) in function of the time or sampling rate in the line-by-line or element-by-element scanning. These amplitudes depend on the light coming onto PET from a unit original area equal in its size to the sampling element - scanning aperture. If tone value stands for the analogue of amperage, voltage, frequency or other parameter of electric current the image is considered to be “analogous”. Staying in essence analogous the image capturing devices of today are fit at their output with Analogue to Digital Converter (ADC) to turn such signal in form of a binary code, i.e. to make it “digital”.

In context of the various kinds of an image processing this stage can be named as input, image capturing, scanning, light to signal transform, etc.

2.3.2 Image signal transforms

In absence of scanning and photoelectric transform like in a photo taking at light sensitive film by a camera, the functional image data transforms are of the full format (over a whole picture area) and “global” (over a whole brightness range, spatial and color spectra). Changing the gradation, for example, in highlights and dark areas without effect on middle tones is just possible by the time consuming and costly preparation of an auxiliary images (gradation masks).

Selective correction is, to the contrary, easily performed in electronic reproduction by choice of the certain amplitude transfer function in the control circuit. There is also possible to provide the correction in desired bands of color spectra. All these advantages have made obsolete the camera based prepress technology after the middle of the last century.

Essentially new facilities have appeared in the computer aided reproduction. Tone value of an each pixel is presented here by the binary code while the whole image by such codes matrix allowing for distracting from the scan time and performing the all transforms as operations with a numerical data. It becomes available to make desired correction in any local picture area and, as especially important, to combine (layout) text and illustration on a page as well as to assemble (paginate) the larger press sheets. Digital image presentation had allowed for bringing the significant part of prepress operations out print house or repro center. They are used to be performed today on the base of desktop by publishers, customers, authors…

In relation to their effect on an image the functional transforms can be divided onto four distinctive groups:

• low spatial frequencies;

• high spatial frequencies;

• format;

• structural.

The first kind of these actions is concerned, first of all, of larger details tone and color as far as the ability of vision to their discerning is great though sharply diminishes with their dimensions reduce. In analogous electronic reproduction there was used to influence for this purpose on the amplitude of an image signal and therefore such transforms were sometimes mentioned as amplitude ones.

Second group of transforms influences on the sharpness and definition by strengthen the higher image spatial frequencies. Among these procedures are the aperture correction, unsharp masking fixing the distortion caused by spatial frequencies low pass filtration at scanning, sampling, screening stages and thereby somehow compensating the loss of image contours and fine details contrast and geometric accuracy. Such parameters processing is referred in photography to Modulation Transfer Functions (MTF) of sensitive layers, lenses and the like, as far as in electronic systems to that of separate electric circuits or a whole imaging channel.

Format transformations are concerned of the image sizing, rotation and cropping. The latter is used to cut out the particular piece of original to be placed on a page according the editorial intent. It’s also used for minor correction the dimension of graphic elements inserted in each other in different color print runs. This trapping procedure provides their slight overlap to exclude the noticeable gap there between due to the paper sheet mis-register.

At final prepress stage the especially important role belongs to the already mentioned transform of continuous tone structure of an original to the halftone one – screening. Its characteristics essentially influence the most of print copy quality parameters in relation of tone and color rendition, definition, sharpness, presence of aliasing (moiré), geometric accuracy of fine details and contours…