The control curves of a monochrome, greyscale or colour image
greatly influence its appearance when output. (Note that curve
here includes a straight line!) They determine the relationship
between the theoretical brightness of each image point and the actual
raster representation calculated from it. To understand this function
properly, you should have read the introductory chapter about raster
and greyscale images. For colour images, there is a control curve for
each primary colour - red, green and blue - and you can modify them
individually or all at the same time. For simplicity, it is easiest to
describe the single control curve for greyscale images at first. This
function allows you to increase or decrease the contrast of an image,
to produce an inverse (negative) image, or to calculate the control
curve automatically from the brightness values in the image. When you
click on the Set control curve
icon, the following dialog box
will appear:
The left side of the dialog box contains the current control curve. Each of the 256 possible levels of brightness for an image, which increase from left to right, can be recalculated to a raster value. The raster values increase from bottom to top. A neutral control curve, where each brightness level corresponds directly to the same raster level, is a straight line going diagonally from the bottom left corner to the top right. For a negative image, it slants the other way; that means that the darkest values of the original will have the brightest rasters, and vice versa. Below are some sample control curves and their effects:
Neutral display:
Negative/Inverted display:
Reduced contrast:
Increase contrast (here the brightness levels at the curve end from around 90 % will appear completely black and levels below some 10 % will not appear at all):
Raise contrast in the middle range, taper it off at the extremes:
Darken brightest areas:
Increase brightness:
You can change the control curve by clicking anywhere within the
control curve area; the control curve will be modified
correspondingly, though only at this one point. In practice this means
that a point on the control curve will be dragged
to the height
where the mouse cursor was positioned at the click. By holding down
the left mouse button and dragging the mouse cursor you can also draw
a straight, curved or any other shaped line to change whole portions
of the control curve.
On the right side of the dialog box you will see various buttons.
Below the Control curve
identifier there are four arrow buttons
for setting the picture Intensity (brightness) and the same for
Contrast. The double arrows allow large adjustments; the single arrows
are for fine-tuning. [Up arrows] increase and [Down arrows] decrease
the relevant picture parameters.
The intensity or brightness of an image depends on the amount of
white it contains. So to brighten a picture, reduce all grey values by
a fixed amount (smaller grey value = lighter). Graphically, the whole
control curve moves up. You can also lose information here due to
saturation effects since more of the picture will become pure white,
and unlike the promise of washing powder adverts, you cannot get
whiter than white
. You can also see this in the last
illustration.
As may be expected, the Contrast arrows can be used to alter the
contrast of the image – in other words to increase or decrease
the difference between various grey levels. Please note that
increasing contrast above the neutral value will produce a saturation
effect at the extremes of the curve and so you will lose information
at the top and bottom brightness ranges of the image. This is due to
the fact that if a grey value of 80 %
produces a full black,
for example, then the grey value of 90 %
can not be displayed
even darker than this. The extreme case is a vertical line at 50 %;
this will produce a photocopier effect
, where the picture only
consists of 100 % black and 100 % white. In mathematical terms, the
steeper the control line or curve, the higher the contrast.
The first two large buttons allow you to create pre-defined
control curves. Neutral
gives you the standard 1:1 diagonal
control curve as a straight line from bottom left to top right.
Optimize
distributes the grey values present evenly throughout
the graphic's tone range (not yet implemented). The next button,
Invert
, changes the direction of the control curve so that
everything white becomes black and vice versa (intermediate values are
swapped over as well), thereby producing a negative of the image.
The Clipboard
button offers additional temporary storage
for a control curve. When the button is not selected, you can copy the
displayed control curve to this clipboard (the direction into
clipboard
is marked by an inward arrow to the button). If a
control curve is already stored in the clipboard, the button is
activated (inverse video) and the arrow points outwards. In that case
a click on the button moves the control curve in store to the dialog
display.
The Load
and Save
buttons allow you to keep
pre-defined control curves on floppy or hard disk. When you click on
one of these buttons, the file selector will appear. You may then
enter a filename for the control curve or select an existing one.
Control curves have the file extender CKL
.
The Apply
button has not been implemented, and so is greyed
out.
A final word on setting the control curve: You should be aware that while altering the control curve can affect the reproduction of an image within limits, these limits are set by the quality of the available image data, as Calamus cannot produce output with more information than is contained in the image you supply.
To sum up: A properly set up scanner is worth more than any
manipulation of the control curve. So before you attempt to fine
tune
an image with the help of a control curve function, you
should try to get as optimum a contrast setting as possible of your
scanner. Also, as mentioned previously, anything but a neutral control
curve will reduce the greyscale resolution of the image. But if you
are working with a raster consisting of less than 16x16 pixels (see
also the introductory chapter about raster graphics), the effective
greyscale resolution of the output is lowered in any case.
Control curves for colour images:
With colour images there are three extra buttons in the dialog
box: Red
, Green
and Blue
. You can click on these
buttons to turn the control curve for each colour on or off, singly or
in any desired combination. On colour monitors, each control curve
will appear in the appropriate colour. All changes that can be made
with the monochrome control curve described above can also be made to
the currently displayed colour control curve(s). You can also create
the effect of colour filters by changing the Contrast or Intensity
(brightness) of one or two control curves.
