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Original  Double Overlay Masking
Because:
  • Cyan pigments contain a large amount of Magenta and Yellow,
  • Magenta pigments contain a large amount of Yellow and to a lesser degree Cyan,
  • Yellow pigments contain Magenta and Cyan

- complex techniques are applied to adjust colour strengths, compensating for pigment errors where possible. A very successful method used in the days of traditional colour camera operating was Double Overlay Masking.

(The film negatives and masks were made with continuous tone film, just like the negatives photographers use for black and white prints. The cameras were very large floor mounted machines often capable of using film up to 60" x 40".)

Through each of the R, G and B filters in turn a negative is produced for each of the Secondary colours. A Black (K) negative is produced using multiple exposures with all of the RGB filters leaving an image only where there are neutral greys and blacks (little light transmitted through each filter in common areas).
Masking steps
From the 3 colour negatives masks are made by exposing a film in contact with the negatives. Various standard exposures are used depending on the amount of colour that has to be reduced (corrected) from the negatives.
Transposed masks from negatives
These faint positive masks are then placed onto the negatives of the other colours, so increasing the density where the pigments would already contain colour the same as that of the masks source colour.

Therefore by placing a magenta mask on the cyan negative, the cyan would be reduced in areas that will also appear on the magenta plate (blues) - because the magenta pigment includes some cyan (the pigment errors seen on the previous page). A yellow mask is also placed on the cyan negative, with the magenta mask, to increase the density where yellow pigment would print with the cyan pigment (greens).

Using the other combinations of masks, the steps are repeated for the magenta and yellow negatives correcting (or evening out) much of the pigment errors, when printed.

The negatives are then in turn placed in the copyboard of the camera (where the original was when making the negatives) and with the lights behind them, high contrast film is exposed through a special screen that produces the varying halftone dot sizes. The final films are called Screened Positives and are used to make the printing plate.

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Todays software that converts the RGB scanned image into CMYK attempts to achieve this by referring to look-up tables, and must include variable controls to suit different stocks (i.e. papers - metal - cloth), different inks (i.e. Screen Printing inks) etc..

(Under Colour Removal or Grey Colour Replacement will be explained in the UCR/GCR page.)

Unsharp Masking

One sharpening filter found in most Painting programs is the Unsharp Masking filter. Traditionally the masks described above were exposed by a diffused light source instead of a pinpoint light source. Because the light directed towards a negative and used to expose a contact mask (refer above) came from different directions, the light undercut any contrasting edges of picture detail in the negative and created detail images larger than the original (thousandths of an inch).

This minute difference created a sort of halo affect where the detail was of the highest contrast. If applied correctly the resulting edges were barely detectable, but added to the contrast between tones and colours. The results appeared to enhance the image sharpness when viewing the final print of all process colours.

• Especially in glossy magazines, look for edges where operators have applied too much unsharp masking - probably when using modern high end drum scanners.

The Unsharp Masking filter available in painting programs tries to reproduce this effect.

General sharpening filters in painting programs look at every pixel in a bitmap image, and compares each to their surrounding pixels. The more the contrast/colour value difference found in the averaged comparison, the more each pixel is changed, increasing the difference.

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