Types of Images
Images are either:
- Continuous tones as in photographs, original drawings or real life images as seen by the eye.
- Halftone separations – made up of black dots on transparent acetate sheets that are evenly spaced but vary in size. One separation for each colour.
Halftone seperations (in shades of grey) control the colour seperations
- Printed halftones – colour images in a magazine, book or newspaper are achieved by laying at least 4 such films of halftone dots down with a certain colour ink. The black dots get transferred to thin metal sheets (called plates). Each separation is etched onto a different sheet; ink flows over the sheet and adheres to the dot etchings; the plate rolls over the paper and the ink is transferred to the paper. The printed halftone is then the result of the ink images each being transferred to the paper on top of each other. These separations are printed at angles so as to prevent raster of dots interfering with each other and causing an effect called moiré:
- Moire – Raster dots interferining with one another
This interference is minimised by printing each seperation at different angles
- Printouts by Inkjet and colour laser printers have an image created by spraying or melting an array of CYMK dots. The colours are always 100%. Shades are achieved by the eye being fooled by the relative sizes of halftone dot and white background. (There is an exception here in the 2 extra colours of light magenta and light cyan found in 6-colour Photo Inkjet printers, but the principle is the same.)
Pixel image (top) vs Halftone image (below it)
- Bitmaps are digital pictures stored on computers and disks that consist of picture elements (pixels). These appear as tiny squares of colour (or grey) shades. Like halftones, they are regularly arrayed, but unlike halftones they have a constant size but contain colour information. (The opacity can vary.) Each image contains a rectangular array or map of square pixels whose colour information is made up of a certain number of bits – hence the name – Bitmap.
Photoshop accepts bitmap input from scanners and digital cameras and existing digital image, and can create Halftone separations, printed halftones and web-suitable bitmap images.
More on Pixels and Colour Depth:
A Definition of Greyscale:
Grayscale is a range of shades of gray without apparent color. The darkest possible shade is black, which is the total absence of transmitted or reflected light. The lightest possible shade is white, the total transmission or reflection of light at all visible wavelengths. Intermediate shades of gray are represented by equal brightness levels of the three primary colors (red, green and blue) for transmitted light, or equal amounts of the three primary pigments (cyan, magenta and yellow) for reflected light.
Definition lifted directly from “whatis.com”
Note: Greyscale is the correct spelling in International English, Grayscale is correct in American English.
Each picture element or “pixel” is part of a mosaic of many thousands or millions of pixels and each individual pixel’s brightness, hue and saturation is defined numerically.
A Greyscale image has no colour values – just luminosity. Greyscale images are made up of 256 shades of grey – known as an 8-bit image because 8 binary bits are needed to show 256 different possibilities.
A 1-bit greyscale image:
A 1-bit greyscale image contains only white or black pixels. This is called a “Line art” or “Lith” image.
A 4-bit greyscale image:
A 4-bit image contains 16 levels of grey with the 2 extremes being white and black.
An 8-bit greyscale image:
An 8-bit image contains 256 levels of grey with the 2 extremes being white and black.
Channels
An RGB colour image is made up of 3 sets of 8-bit greyscale images – one for Red, one for Blue and one for Green. They are called colour channels. Each channel is simply a set of 256 levels of either Red or Green or Blue. This results in a 24-bit image. Each channel is often viewed separately in Photoshop. It then looks like a greyscale image and indicates the areas of lots of colour (transparent) or very little colour (dark).
Advanced photoshoppers often work on just one channel to clean up or modify an image.
A single pixel in an RGB image contains:
A value between 0 and 255 for the Red channel – (8-bits)
A value between 0 and 255 for the Green channel – (8-bits)
A value between 0 and 255 for the Blue channel – (8-bits)
Total = 3 Values = 24-bits
Some images have another channel containing another 8-bit greyscale image. This channel (called an Alpha channel) is used to control the transparency of an image. Any black on this channel results in a transparent area, while anything white makes the image opaque. Anything grey in the alpha channel provides semi-transparency. This would then be a 32-bit image since there are now 4 channels controlling the appearance of the image.
Total = 4 Values = 32-bits
Confusions Caused by the Advertising Industry…
PPI – Pixels Per Inch – The resolution of scanners. A good flatbed scanner should be capable of a resolution of 1200 PPI – (not dots per inch because input devices produce pixels not dots). If two figures are quoted (as in 1200×2400), then they refer to the x-direction sampling rate – (determined by the number of sensors in the CCD imaging array); and the y-direction sampling rate – (determined by the precision of the stepper motor).
CRT Monitor resolution is also expressed as PPI – the PC monitor is usually around 96ppi.
(At a Dot pitch of 0.26, the monitor shows 3.77 pixels per vertical millimetre, which converts to 96 pixels per inch.
LCD Monitors have what is called a “native resolution” and that is the number of pixels wide by the number of pixels high that matches the matrix within the screen. Setting the graphics card to produce any resolution other than a linear 1:1 ratio to the native resolution results in poor quality images.
MP – Mega Pixels – the resolution of a digital camera is quoted by the total number of pixels produced by the CCD. This is usually a larger figure than the actual number of pixels available in the image that it produces.
LPI – Lines Per Inch – The number of halftone lines or cells per printed inch. Otherwise known as screen ruling (nothing to do with a monitor!). The origins of this term go back to the days when a continuous toned image was broken up into dots by a finely etched crisscross screen. The luminosity of the shade determined how big a dot was formed, but there were only a fixed number of dots per inch determined by the distance between the lines.
DPI – Dots Per Inch – The resolution of a printer. The maximum number of dots (of any colour) per linear inch.
CYMK and Halftoning
Commercial printers use a combination of Cyan, Magenta and Yellow inks (the complimentary colours to RGB) in a “Subtractive” process – adding pigment subtracts from white producing ever darker tones.
Colours painted over each other subtract from each other.
In theory, if each of the pigments are printed over each other, the result will be black. In practice, the black is not deep enough for decent contrast so a fourth ink – black is used to produce the very darkest tones and give it a rich “snap”.
C=Cyan
Y=Yellow
M=Magenta
K=blacK (“K” so as not to confuse matters with the additive colour “B” for blue)
In the printing trade, this is commonly called standard 4-colour offset litho printing.
The full colour image is made up of Cyan, Magenta, Yellow and Black seperations.
And guess what? Your basic inkjet printer also has 4 ink pigments: CYMK,
and your colour laser printer has 4 colour cartridges: CYMK
The Actual Colour Notation for the 4 Subtractive Colours:
Technical colour notation
The colour values are given as an “apparent” percentage. See the halftoning section below why this apparent is in quotes. (In fact, in the printing world it’s even more complicated because small traces of another of the 4 basic subtractive colours is often added in. For example, in actual printing practice yellow often consists of 100% yellow and 2% cyan. It gets wildly complicated. But let’s not go there now… “Down boy, keep it simple, Dave!”
There is a story about the colour Magenta and how it got its name:
More than 120 years ago a printing company was experimenting with the idea of printing in colour. They used Cyan, Yellow and Black which are all found in nature, but in order for it to work, they had to concoct a brand new colour which does not actually exist in nature (or more correctly, the chemists did not recognise the colour as anything they had seen before – the closest colour was fuchsia). So they were casting around for a catchy name for this new bluey-reddish colour. One of the scientists happened to be passing at the time that the discussion was raging, and he stopped the discussion in it’s tracks. He had been a soldier in the Crimean war and in one particular battle on the slopes of a hill called Magenta so many soldiers were killed that a large amount of human blood flowed into a nearby stream. The blood mixed with some chemical in the water and the stream ran that particular colour. So the old scientist suggested that the new colour be named Magenta in memory of those that die in battle.
Dave’s Note: I don’t know if this story is true – Google doesn’t seem to know about it, but I vividly recall being told that story by my first (and only) boss – the proprietor of an offset-litho printing firm in Frankfurt/M Germany.
Halftoning
Subtractive colour (Printing) is very different to Additive (Monitor). One big difference is that a certain primary subtractive colour is always at full intensity (i.e., not diluted) while the additive primaries can vary in intensity in 256 steps.
In subtractive printing, the four inks are laid down in patterns of tiny dots which allows some of the white paper to show through, thereby allowing pale tones to be created. The more white space surrounding a dot of colour, the paler the colour appears. Since the printing process cannot alter the opacity of the colours, (in contrast to images produced by the RGB sources like the guns in a monitor’s tube), CMYK inks themselves can only be laid down at at full intensity, but apparant (it’s a trick on the eye) percentage opacity is tetermined by the ratio of the dot and the amount of white space surrounding it. This apparent opacity is quoted as a percentage as shown in the table above. Other colours and darks are created by dithering. A dithered pattern is used as shown below. The dots are so small that the eye mixes them to give the impression of a continuous range of tones.
A dithered pattern of dots makes a picture
Inkjet PHOTO printers often have 2 extra colours (pale yellow and pale magenta) to reduce the visible graininess in pastel shades caused by the wide spaces between the dots.
And now… Enough theory for a while. It’s time to get stuck into some practical work…
