Colour Adjustment - Part 2

In Part 1 we spoke about the general nature of colour and colour perception. In Part 2 we look at the numbers associated with colour, particularly in Adobe Photoshop.

Numbers are a key part of digital manipulation. No two people would be able to mix a “turquoise” in separate rooms and come up with the same colour. However, if they were told to make a specific RGB colour and each displayed it on a calibrated monitor from the same colour space then the colours would be identical. It is the ability to adjust colours numerically that provides the power behind programs such as Photoshop. RGB is universal in image manipulation and in essence you define a colour by specifying how much Red, Green and Blue are to be put into the mix. CMYK only appears in the top end software and is universally used for the ink on paper printing industry (eg newspapers).

It is worth taking the trouble to understand the numbers associated with digital imaging. They crop up all over the place and in the main they describe colours or changes to colours. They fall mainly into two categories - percentages and 8-bit.

Percentages are easy. If a value of 68% cyan is given it means that 68% of cyan ink will cover the area in question, the remaining being either other inks or white, clear paper. In some palettes you can flip between percent and 8-bit by clicking the double facing arrows at the bottom (eg in the Curves Palette).

8-bit is a little more complicated as it is a binary number. In Photoshop, which is an 8-bit system, you can have values of zero to 255 because 8 bits gives you 28 possibilities. In colour terms Red zero is no red content at all, 255 is as red as you can get and 128 red is half way up the red scale. Combining 8-bits of red, green and blue gives 24 bits, which is where the 16million colours of Photoshop comes from (224 is 16,777,216 or 16m for cash). Although a 36-bit scanner can theoretically create zillions of different colours, the eye can only cope with about 11 million.

Relationships The numbers in Digital are always related although sometimes they appear to have a mind of their own! Thus 50 RGB points translates to 80% on the percentage scale. The same is true of the more complex colour numbers - a given set of RGB values will always convert to the same set of CMYK values providing that things like the colour space are constant.

The Simple Dialogue Boxes We start with the simpler numbers. The Adjustments drop down in Photoshop (7.0) lives under Image. Once there you see the following main options:

Levels

Auto Levels

Auto Contrast

Auto Color

Curves

Color Balance

Brightness/Contrast

Hue/Saturation

The emboldened ones are almost universal to all software although they may have slightly different names and they certainly live in different places in the drop down menus. Taking each one in turn and for brevity we only discuss Photoshop. The adjustments are made either with sliders, joy sticks or by typing in numbers from the keyboard.

Brightness and Contrast Values on the slider run from -100 to +100

Brightness +30 adds 30 RGB points to all values. Any that are thus lifted beyond 255 are left at 255

Contrast +30 contrast adds up to 30 points pro rata to all values above 128, pegging at 255. It also reduces all values below 128 pro rata by up to 30 points pegging at zero. Thus a grey of 128 remains unchanged, a 225 highlight becomes 255 and a 30 shadow becomes zero. -30 contrast nominally subtracts 30 points from the highlights end and adds 30 to the shadow end. Thus a 255 becomes 225; a 30 becomes zero; a 128 remains unchanged as the curve is pivoted about this point. For colours the Hue angle remains unchanged across the whole spectrum.

Hue and Saturation Hue values slide between -180° and + 180°; Saturation and Lightness run -100 to +100

Hue +30° adds 30° to the hue value of all colours. Saturation and Brightness remain unchanged.

Saturation +100 pushes saturation of "saturated" colours to 100%. Greys are left unchanged. Hue angles are preserved. Some Brightness values changed.

Brightness The Brightness is annotated Lightness in the dialogue box. It does what it says ie: +50% adds 50% to any shadow (0%) values and adjusts the mid and upper tones pro rata. +100% makes the entire image white, -100% makes the entire image black. Hue and Saturation values remain unchanged.

Colour Balance This is one of the harder number sets to get to grips with unless you are a traditional colour printer. The dialogue box is divided into sliders, which enable you to add one colour or simultaneously subtract its complementary colour. The values allegedly correspond quite closely to the CC Values on the colour enlarger. Colour printers talk about adding 05 cyan (5 on the Photoshop slider scale) or subtracting 25 magenta (25 on the Photoshop scale) and so if you are used to using these adjustments in wet printing you will have a feel for them. If not, you are more or less on your own and left to judge by eye, thus relying on your visual perception and the accuracy of your screen calibration.

Note that you may adjust the Midtones, Highlights or Shadows independently; this is the same as adjusting particular parts of the curves, more on this later on. The Preserve Luminosity check box is important. It allows changes to be made in the colour balance without an attendant jump in brightness. So if you radically change the colour of something towards red it will not result in an increase in the brightness level (ie the Luminosity is maintained!). This feature is a powerful one which is a little shy in the text book indexes (including the manual) - it does not crop up under its own name. Colur

The Remaining Dialogues For the remainder of the menu items things area little more complicated. The Levels and Curves Palettes of Photoshop provide inter-related information but it is harder to follow the presentation of them on your screen.

Look at the picture shown. This is a scan from a print with a grey scale scanned at the same time. Now examine the Levels Histogram. (A histogram is a type of graph in which the data are separated into groups and shown as bars each with a height equivalent to the number of units at that group size). A couple of things stand out. Firstly there are a lot of white pixels at the right hand end of the scale. These come almost exclusively from the white surround to the picture. Also note the series of peaks in the histogram. Each peak is actually one of the swatches of the grey tones but a couple of them are hidden by additional pixels from within the image. The largest peak on the scale comes from the green of the bus. This can be confirmed by looking at the individual histograms of the Red, Green and Blue channels (do this by clicking Image>Histogram…) - there is a strong contribution from Green and Blue but hardly any from Red.

There are 5 sliders in the Levels Palette. Three of them control input parameters, a black slider, white slider and the centre Gamma Slider. The Centre Gamma slider controls the input to output ratio of the mid-tones. If the gamma slider is pulled until a value of 0.5 is set, the picture goes darker. All values around 128 (the midtone) are halved. Values either side of 128 are reduced pro rata so the high and low values are hardly changed at all. Exactly the same result can be achieved (RH Image) in Curves by clicking on the mid-tone Curves point and changing the output value to 64. Try this for yourself. Open a file and make a duplicate of it (Image>Duplicate). Open Levels in the left-hand image and change the Gamma to 0.50. Now go to the right hand image and open the Curves Palette (Ctrl-M). Click on the exact centre of the graph at the 128 point and make sure that this shows as Input at the bottom left of the palette. Now type 64 in the Output box. If you now compare the two images they are almost identical. This test only really works for variations of gamma between about 0.8 and 1.3. If you use values of 0.5 or 1.5 then end effects start to show.

Mathematically speaking, the Histogram actually stands up on the line in the Curves graph, that is, vertically out of the screen. This is illustrated in the diagram, as are the relationships between the various numbers in the palettes. We have fudged them together with screen grabs, you cannot actually see them all together in Photoshop.

Where does this help? Knowing which values change which aspects of colour is vital to many manipulation processes. For example if two product shots have been taken under different lighting conditions and you wish to bring them together in the same picture showing the same colours, it is the hues that you should match up. Hue is the colour e.g. the greenness or blueness; saturation is the purity of the colour i.e. how vibrant it looks; brightness is the amount of light that the colour reflects.

Colour

Where do you see them? In Photoshop the Info Palette shows the colour values in two colour spaces of your choosing. When you change a colour with a slider two values are reported in the Info Palette colour before/colour after the adjustment has been made. This is a very powerful piece of information and can be used with great confidence as it tells you where your adjusted colour is headed.

The untouched Histogram is shown at A. The input levels and Gamma are at the rest positions of 0, 1.00 and 255. The corresponding Curve is shown at B. The rest (neutral) position for the curve is straight and at 45° from corner to corner. No curve points have been set on the diagonal line and so the Input/Output number boxes are greyed out.

At E the highlight portion of the curve has been dragged down 50 points (2 squares) to 200 as indicated by the number box at D. In other words all highlight values of 255 will be depressed to 200 and appear just off white. At F the shadow has been dragged 50 points. This does n Curves ot currently show in the box because the upper point, which is a filled diamond, is the active one. In a nutshell then we have kept the contrast the same because the slope of the curve is still 45° but we have reduced the tone range by 50 at each end.

These are the same effects on the image as moving the sliders in the levels palette as follows. At G the input slider has been moved from 0 to 50 by dragging the slider. At H, the output slider has been dragged down from 255 to 200.

The overall effects is identical and is illustrated by the perspective drawing, which shows the position of the histogram on the curve before and after the sliders have been moved. Remember that you cannot actually see any of this within Photoshop as you are not allowed to have more than one interactive palette operating at the same time. We have fudged it using screen grabs.

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Photo Quote: The ear tends to be lazy, craves the familiar and is shocked by the unexpected; the eye, on the other hand, tends to be impatient, craves the novel and is bored by repetition. - W. H. Auden