Curves
Clive R. Haynes FRPS
In our
exploration of 'methods of control' within Photoshop we've arrived at 'Curves'.
Let's recap.
Threshold
offers one slider for control
Brightness & Contrast
offers two sliders for control
Levels offers five
sliders for control
What
can 'Curves' offer???
Curves potentially offers 256 'effective sliders'
control - Wow!
Quite obviously it's impractical to actually offer 256 separate 'sliders', as individual sliders (as in the previous dialogue boxes we've explored).
'Curves' represents
the 'Input' (source) and the 'Output' (resultant image) for the image
along two axis of a graph.
Sometimes, it can be helpful to consider the 'Out' as the amount of ink
used by the printer)
Upon opening the 'Curves dialogue box, the 'Curve' is a straight line.
If we take any point as a reference along the gradient at the bottom (Input/source)
and extend that reference point upwards to the diagonal line, then read
across to the left gradient (Output) the two levels of tone will be the
same along their individual gradient scales. What this means is that the
relationship between Input and Output is unaltered (linear) and a 'straight',
unaltered (output) image will result.
What 'Curves' allow us to do is to alter the relationship between
Input and Output at any point along the length of the straight line -
by altering the the position and/or shape of this line (including bending
it thus making it into a 'curve'). This means that we can alter the input
and Output relationship in a nonlinear way. This may make it appear complicated
but it's not really.
Let's begin by looking at the opening state of the 'Curves' dialogue box.
A technical aside
(for the curious)
(You can skip this paragraph if you wish and you'll be none the worse
off)
So, where are the 256 effective 'sliders' you ask?
As there are so many points of adjustment, the individual 'points' become
so close together that a straight line is displayed and that's exactly
what the diagonal line across the 'Curves' box represents.
Why 256 and not, say, 300 or 400?
When using Photoshop in 8 bit mode, there are 256 levels of adjustment
for the 'Luminosity' (the underlying black & white / monochrome) component
of the image so we have control over each of the 256 'steps' or 'tones'
from absolute black through a multitude of grey steps to peak white.
However, unless we request the changes we make to only affect the
monochrome content, the composite image - that includes the colour
component (RGB), is adjusted over the range of 250 steps - more of this
later. For now only accept this as a 'passing reference'.
By the way, when operating as 16 bit, the number of 'steps' increases dramatically - totaling 33,000!
Tech bit over ....Continue.....
OK - so by adjusting 'Curves', what can we do?
We can begin by making some simple changes to the shape of the straight
line - making it into a 'curve'.
By changing the shape of the line we alter the relationship between Input
and Output (Image lightness and tone compared to Printer Ink).
Let's take a look at some examples.
Look at the dark grey tones of the Input (base gradient), follow the line up to the 'curve', then across to the 'Output' on the left and they have become light grey tones as represented on the Output (side gradient). The image becomes lighter and will print this area with less ink and as the curve is no longer straight, other areas will also be affected and lightened. The amount of change diminishes towards the ends of the 'curve' and as the two 'ends' are fixed the black and white points remain unaltered.