Technology ICT Option: Photoshop. Digital Imaging - Photoshop A digital image is a computer representation of a photograph It is composed of a grid of

Technology

ICTOption: Photoshop

Digital Imaging - Photoshop

A digital image is a computer representation of a photographIt is composed of a grid of tiny squares called pixels (picture elements)Each pixel has a position on the grid and a colour valueThese pixels can be edited (changing the pixel colours) individually on in groups The illustration below shows an image of a car and a section of it zoomed to show the individual pixelsThe shape of the pixels is clear as is their positions on the grid making up the digital photograph


The resolution of an image is the number of pixels it containsThis is normally given as dots (pixels) per inch (dpi) or dots per centimetre (dpcm)The resolution of an image effects its quality - higher resolution - better quality, but this comes at a price as an increase in resolution means an increase in file sizeA doubling of the resolution produces a quadrupling of the file size

The chart below shows photographs taken at 150dpi and 300dpiThe resultant file sizes are shown when the photograph is taken as black and white, grayscale and colour


Colour models Image editing software uses colour models to represent the production of colour on the screen and on paper. The main models in use are:

• The RGB Model - used on computer monitors and projectors. Values ranging from 0 to 255 can be assigned to each of the primary colours red, green and blue. These colours are known as the additive primaries because if they are combined at full value (255), they produce white. If they are combined at values of 0, the resulting colour is black. All other colours are achieved by varying the values assigned to the three primaries


• The CMYK model - used for printing. Based on the colours cyan, magenta and yellow. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to produce black. For this reason these colours are called subtractive colours. Because all printing inks contain impurities, these three inks actually produce a dark brown and must be combined with black (K - Keystone black) ink to produce a true black . As it uses percentages for each colour, the values only go to 100 and not 255 as in RGB

The subtractive (CMY) and additive (RGB) colours are complementary colours. Each pair of subtractive colours creates an additive colour, and vice versa


Graphics formats Digital images come in two main types:

• Bitmap images - made up of a grid of pixels. Similar to a mosaic, each pixel has an address and a colour value. As the number of pixels is fixed (resolution dependant), these images become jagged when enlarged. Photographs are held as bitmaps because they can display the vast number of colours necessary for photo realism. Paint software produces bitmap graphics


• Vector images - a series of geometric objects such as lines and curves. Each object will have properties such as colour, width, size, fill and position. They are resolution independent and can be scaled without any loss of resolution. Photo realism is difficult to achieve with vector graphics and because of this they are rarely used for photographs. They tend to be used in logo creation and technical drawings. Their file sizes are considerably smaller than bitmap image files


Digital file formats Image editing software can open and save files in many file formats Some of the file formats are universal and can be produced by any image editing programOthers are proprietary which means they are produced by specific image editing programs (Photoshop, GIMP etc) The most common universal file formats are:

• BMP - this is the Microsoft graphics format. Images produced using MS Paint will be BMP

• TIFF - these are high quality images. A good digital camera will offer TIFF as it’s best output. It produces images with a large file size and is used in magazines etc

• JPEG - This is the most common file type created by digital cameras. JPEG images use lossy compression. This means that quality of the image is reduces to give a smaller file size


• GIF - Gif (graphics Interchange Format) files can only display 256 colours in an image compared to 16 million in a TIFF image. Because of this, it is used mainly for logo’s, drawings and diagrams. GIF images can have transparency and this is useful when placing an image on a coloured background

• PNG - This file format (Portable Network Graphic) was designed as a replacement for GIF. It has the transparency advantage of GIF and can hold a lot more colours making it suitable for photographs


The most common proprietary file formats are:

• PSD - these files are produced in Photoshop• PSP - these files are produced in Paint Shop Pro • XCF - these files are produced in GIMP• CPT - these files are produced in Corel Photo Paint

As these file types are proprietary, they will have special features not available in universal file typesFor this reason they can usually only be opened in their own programs They must be converted into a universal file format to be used in wordprocessing etc


Photoshop - the industry standard for image editing. It has a comprehensive range of tools and is used by professionals the world over

When Photoshop is started, the Work Area is blank. This is because the application is mainly used to edit images loaded from camera’s, the Internet etc


The Toolbox The full range of tools are available from the drop down menu, but the most common tools are found in the Toolbox The tools can be broken into categories:


The default painting colours in Photoshop are Black foreground and White background The Toolbox allows these colours to be reversed, or returned to default mode by clicking the relevant controls Some of the tool icons have a black triangle in the lower-right corner. This means that there are more tools of the same kind available on a pop-out menu

Each of the tools in Photoshop has it’s own set of options. These appear on a ribbon above the Toolbox when a tool is selected. Below are all the options when the Text tool is selected


Palettes help with the editing process. The main ones:

• Swatches - used to set the foreground and background colours

• Histogram - shows the spread of the pixels in the image (Levels). The pixels will be divided between the Highlights on the right of the graph and the Shadows on the left and the Gamma (mid-tones) in the centre of the graph. A perfect image will show as a bell curve


• Navigator - used to control the image magnification and the viewing area. Dragging the slider at the bottom of the dialog zooms the image in and out. Dragging the box around the dialog controls the area of the image shown in the workspace. Clicking an area of the image within the dialog moves the rectangle to that area

• History - the Undo History palette keeps a record of all the editing processes carried out on the image. The huge advantage of this is this system is that you can undo any past edit at any time. Normal undo has to be done in the reverse order to the edits


Layers - the Layers palette shows all the layers in the image in stacking order. The Background layer is at the bottom

• Layers are like a stack of transparent sheets• Images or paint can be placed on individual sheets and edited

without effecting images on other sheets• Layers can be dragged up or down the stack• Layer display is controlled by toggling the eye icon to the left

of each layer• To edit a layer it must be active (highlighted)


Opening images Files in all the universal formats can be opened in the workspace. Depending on the amount of memory in the computer, several images can be open in the editor at the same time To open an image file from the editor:

• Click the File menu and choose Open. The Open dialog box opens.

• Navigate to the folder containing the file• Click the file to select it and click the Open button


Inputting images Scanning digitises an image so that it can be read, displayed, edited and printed by a computer To scan an image in Photoshop:

• Click the File menu and choose Import and select the scanner

• Scanning takes place in two stages - preview and then final scan. The preview scan allows you to select the exact area of the page or photograph you want to scan


To do a preview scan:

• Click the preview button. When the scan is finished, the right hand portion of the scan window will display the preview


Before performing the final scan, the software needs to know some information:

Firstly, the exact area of the image to be scanned needs to be set. This is done by dragging the handles in the preview windowSecondly, the image type must be set. This can be colour, grayscale, black and white or custom settingsWhen all the settings are set, Click Scan. The image is scanned into Photoshop and is ready for editing


What resolution to use: The simple formula below can be used to set the scanning resolution:

Example - An original photograph is 50mm x 50mm. It is to be printed at 300dpi. The photograph is to be scaled to 100mm x 100mm in the publication. What resolution should the scanner be set to:

Using the above formula:

Answer:(300/50) x 100 = 600dpi


A Digital Camera stores pictures electronically on a small internal memory card. The pictures can then be transferred from the card to the computer. When the camera is connected to the computer, it is seen as a removable disk drive and the Removable Disk window opens:

• Select Copy Pictures to a folder and click OK


• The Scanner and camera wizard opens. Click Next

• Tick the required images or choose Select All


• Choose a folder for the images. Tick the Delete Pictures box if you want the pictures erased from the memory card

• The images are copied to the folder

Click Next in the following screen and Finish


Alternatively, you can choose Open folder to view files in the Removable Disk window to control the files as if they were of a disk drive


Selecting Just as text to be edited in a word-processor must be selected, an area of an image or an entire image must be selected if it is to be edited. The main selection tools are:

• Marquee - the rectangular and elliptical tools select a defined shape. Select the tool and drag the required area. The area is surrounded by marching ants. Hold down the Shift key while dragging to constrain the shape to a perfect square or circle


• Polygonal Lasso - click at intervals along the perimeter of the object to be selected. Press delete to undo the clicks one by one. Hover over the start point until a small circle appears and click to complete the selection

• Magnetic Lasso - this tool selects highly contrasting edges by allowing the positioning of control nodes around the edges of the image. Click your way around the edges of the object to be selected and join the last point to the first to complete


• Magic Wand Tool - this tool selects areas based on colour. Choose the tool and pick a pixel. Set the tolerance in the options. This is the variation in shade that will be included in the selection

• Path Tool - select the tool and set the option as shown. Click to create nodes around the perimeter of the object. Join the first and last points to create a path. Select the Path tab in the Layers palette and click the small arrow in the top-right corner to produce a menu. Select the Make Selection option and click OK in the dialog box to create the selection


Feather

Feathering is necessary when blending images togetherStandard selections have very hard edges, and when placed in a collage, the sharp edges ruin the realismFeathering a selection creates a soft edge for better blendingFeather values are measured in pixelsBelow is shown the options bar for the rectangular marqueeThe feather value is set to three pixels which means that a transition area of three pixels exists outside of the selection to blend the image with the area below

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Technology ICT Option: Photoshop. Digital Imaging - Photoshop A digital image is a computer representation of a photograph It is composed of a grid of