Generating half-tone representations

DETAILED DESCRIPTION OF AN EMBODIMENT The apparatus shown in FIG.
1 may form part of an otherwise conventional half-tone representation generating apparatus such as our Datrax system.
The apparatus comprises a store 1 defining a square map of 3600 locations, a central part of which is shown in FIG.
2A.
This map defines a half-tone dot area in terms of high resolution elemental areas arranged in shells with the areas of each shell being associated with colour density component values expressed in this example as parts in 240.
Of course other representations, such as dot percentages, of colour component density could be used.
The map is addressed by X and Y address generators 2, 3 which track the position of a laser spot on a record medium as the spot scans through each dot area on the medium.
The store 1 is connected to an adder circuit 4 such that the inverse of the addressed value from the store 1 (-PV) is applied to the adder 4.
In addition, a picture signal (PS) from a store (not shown) is applied to the other input of the adder 4.
The picture signal defines a colour density value for the colour component corresponding to the separation being generated in terms of a part in 240 and for the particular pixel of the original picture which is being recorded.
Typically each pixel will correspond to one quarter of a half-tone dot area.
The output signal from the adder circuit 4 is fed to a control signal generator 5, to be explained below, which generates a two state control signal (CS) on a line 6 to a beam modulator 7.
A laser 8 generates a coherent laser beam which is fed to the modulator 7.
When the control signal (CS) is binary zero, the modulator 7 is caused to deflect the incoming laser beam in the direction indicated by the arrow 9 in FIG.
1.
When the control signal is a binary 1 the beam modulator 7 causes the laser beam to impinge on a rotatable mirror 10 where it is reflected onto a record medium mounted on a support 11.
The mirror 10 rotates to cause the beam to scan across the surface of the record medium on the support 11 while the support 11 tracks parallel with the mirror 10 to enable the beam to scan the full width of the record medium