Colour film is exposed in the FR80 by passing the 'white' light from the cathode ray tube into a combined 16mm/35mm camera having a built-in secondary, or subtractive, filter set. Each filter in this system removes one of the components red, blue or green from the white light incident upon it. To the eye the filters thus appear blue-green, red-green or blue-red respectively. These colours are labelled cyan, yellow and magenta. They form the complementary set to red, blue and green.
Colour film is more sensitive to variations in light level and exposure time than is black-and-white film and so the specifications for coloured output must be more precise and, to some extent, constrained.
In order to produce secondary colours on film, filters are inserted into the light path singly; to produce primaries requires the insertion of pairs of filters with a consequent increase in the attenuation of the light. For example:
Colour Filters Red Magenta and Yellow Blue Cyan and Magenta Green Yellow and Cyan
Obtaining similarly 'bright' images on film, whatever their colour or form, requires adjustment of the effective source's (CRT) brightness at each change; for example, closely spaced parallel lines appear less bright when their spacing is increased, there being light scattering processes at work both in the tube-face and in the film. The scattered light quite clearly increases the effective film speed by a 'pre-fogging' process (the level of brightness at any point in the image depends on the light intensity at that point and the intensity of neighbouring points).
Although the FR80 has a number of parameters that can be varied to achieve different intensities, in this paper the brightness of a line is defined by the intensity selected before the line is drawn and the number of times (hits) the line is drawn.
Experiments are still continuing on the FR80 to optimise the filter set. Consequently, it is possible that the suggestions made in this paper will change. Eventually it is hoped to define a complete range of colours rather than just the primary and secondary colours defined here.
The following suggestions are made assuming that the film stock being used is HS Ektachrome with E-4 processing (this is the standard used at the moment).
Users are recommended to draw all lines at the maximum intensity of 255. If an area of colour is required, this should be obtained by drawing parallel lines 20 raster positions apart from centre to centre.
Equal subjective brightness can then be obtained by drawing lines the number of times (hits) defined in the following table:
SPROGS/SMOG GROATS Colour Number of Hits Colour Number Colour Number Area Single Line 6 2 Red 3 5 3 4 Green 3 6 5 6 Blue 3 8 1 7 Cyan 1 2 4 3 Magenta 1 3 2 5 Yellow 1 1 8 1 White (Clear) 1 2
On this basis, the film (HS Ektachrome) should be correctly exposed and the colours appear saturated with acceptably slight halation. It may be necessary to have extra hits if a bright white area is required.
Due to the characteristics of the film, no easily expressible, colour-independent, relationship exists between intensity as set and repeated hits. Variations of intensity can be used to provide small adjustments in brightness (for shadow effects perhaps). It is worth noting that the above table provides subjectively equally 'bright' results for primary and secondary colours as well as white (clear).
As can be seen from the table above, the cost of producing the. primary colours is expensive on the FR80 as each line has to be drawn a number of times. Often, the user is displaying a simple line drawing and the secondary colours together with white are an adequate set. In this case, it is possible to produce a set of colours having equal subjective brightness on the processed film with only a single hit for each line drawn but varying the intensity according to the colour required.
Two possible sets are given in the table below. Their resulting 'absolute' brightnesses are lower than for the set recommended above.
Colour Intensity for One Hit Cyan 128 64 Magenta 256 128 Yellow 32 16 White 128 64
Owing to the logarithmic film characteristic, the second set is not 'half as bright' as the first.
The following set of routines have been added to both SMOG and SPROGS to aid the production of colour output.
COLOR(COL,HITS,XINT)
This routine defines the colour, intensity and number of hits for all lines and characters drawn until its next call or an equivalent colour select routine.
The colour selected is defined as follows:
COL Colour 1.0 Cyan 2.0 Yellow 3.0 Green 4.0 Magenta 5.0 Blue 6.0 Red 7.0 Clear 8.0 Clear
The value of HITS should be between 1.0 and 15.0 and XINT should be in the range 1.0 to 256.0.
If COL is given the value 8.0 (clear), the routine COLOR can be used to define multiple hits and intensity setting on hardcopy and black-and-white film. In fact, if colour output has not been specified, the colour is forced to be 'clear' whatever the value of COL. If the microfiche camera is selected only the intensity is set, since there is no colour or multiple hits feature for microfiche. No output is produced if one of the arguments is zero.
RASTER(X,Y)
This routine sets X and Y to the distance between raster points on the output device in terms of the units of the current region limits. It is useful for calculating line densities for shaded areas. For example, to shade an area 100 units wide and 500 units high in the current region limits with lines every 12 raster positions:
CALL RASTER(X,Y)
N = 100.0/(12.0*X)
CALL VECFAM(0.0,0.0,0.0,500.0,100.0,0.0,100.0,500.0,N)
VECFAM(X1,Y1,X2,Y2,X3,Y3,X4,Y4,N)
This vector family routine draws the two lines (X1,Y1) to (X2,Y2) and (X3,Y3) to (X4,Y4) and draws N-1 lines between these two. The N-1 lines are such as to partition the space between the two lines into N equal areas. The lines are of the form:
(X1+a(X3-X1),Yl+a(Y3-Y1)) to (X2+a(X4-X2),Y2+a(Y4-Y2)) where O<a<1
This is a generalised form of the routines HGRATX and HGRATY. The two lines can be in any direction and may intersect. For example, if (X1,Y1) = (X3,Y3), a fan of lines is produced. Of course, spacing between lines will not be constant along their length and so a coloured fan would vary in brightness.
VECFAR(DX1,DY1,DX2,DY2,DX3,DY3,N)
This routine is a relative version of the VECFAM routine. The first point is assumed to be the current point (XP,YP) and the other points are all defined relative to this one. The routine draws the two lines (XP,YP) to (XP+DX1,YP+DY1) and (XP+DX2,YP+DY2) to (XP+DX3,YP+DY3) and N-1 lines in between.
This routine does not update the current point; its value is the same on exit as it was on entry.
The routines VECFAM and VECFAR should be used wherever possible to shade solid areas.
JVECFM(IX1,IY1,IX2,IY2,IX3,IY3,IX4,IY4,N)
JVECFR(IDX1,IDY1,IDX2,IDY2,IDX3,IDY3,N)
These two routines are equivalent to VECFAM and VECFAR but have integer arguments.
To produce a picture consisting of the red line from (0,0) to (1 ,1) and the cyan line (1,1) to (2,2):
CALL COLOR(6.0,5.0,256.0)
CALL VEC(0.0,0,0,1.0,1.0)
CALL COLOR(1.0,2.0,256.0)
CALL VEC(1.0,1.0,2.0,2.0)
To produce a green square from raster position (8000,8000) to (9000,9000) and a magenta square from raster positions (9000,9000) to (10000,10000)
CALL REGION(0.0)
CALL COLOR(3.0,3.0,256.0)
CALL JVECFM(8000,8000,8000,9000,8000,9000,9000,50)
CALL COLOR(4.0,1.0,256.0)
VALL JVECFM(9000,9000,9000,10000,10000,9000,10000,10000,50)
To produce an evenly-spaced set of 11 alternating red and yellow vertical lines over the square (0,0) to (10,10):
CALL COLOR(6.0,5.0,256.0)
CALL JVECFM(0,0,0,10,10,0,10,10,5)
CALL COLOR(2,0,1.0,256.0)
CALL JVECFM(1,0,1,10,9,0,9,10,4)
It is more efficient for all the lines of one colour to be drawn first. To produce a magenta square from (1,1) to (10,10) in the units of the current region:
CALL RASTER(DX,DY)
N=9.0/(20.0*DX)
CALL COLOR(4.0,1.0,256.0)
CALL JVECFM (1,1,1,10,10,1,10,10,N)
To draw a green square from (0,0) to (1,1) and a blue square from (1,1) to (2,2):
raster length(rx,ry); dx := l.25*rx; intensity (255,1,4); intensity (255,1,6); selectcolour(0,0,0,3,0,0,0); for x := 0 step dx until 1 do vector(x,0,x,1); selectcolour(0,0,0,0,0,3,0); for x := l step dx until 2 do vector(x,1,x,2);