If you have not fully understood the colouring of your images, you have go through the chapter on Basic Concepts.
Vector images, such as those created in Adobe Illustrator, are made up of mathematically defined lines and curves called vectors.
Raster images, such as those created in Adobe Photoshop, consist of a grid, or raster, of small squares, know as pixels.
Several types of resolution are important when working with digitized images.
Image resolution refers to the spacing of pixels in the images and is measured in pixels per inch(ppi). If an image has a resolution of 72 ppi, this will have 5184 pixels in a square inch.
Pixel depth(bit resolution) is a measurement of the number of bits of stored information per pixel. Bit resolution determines how much color information is available for each pixel. Greater pixel depth means more available colours and more accurate color representation in the digital image. E.g. A pixel with a bit depth of 8 has 2**8, or 256 possible values(colours); and a pixel with a bit depth of 24 has 2**24 or 16 million, possible values (colours).
Monitor resolution defines the number of dots or pixels per unit length of output. It is measured in dots per inch (dpi). The device resolution for a Macintosh monitor is typically 72 dpi.
Screen frequency, also known as screen ruling, refers to the number of halftone cells per inch in the halftone screen used to print grayscale image or colour separation. Screen frequency is measured in lines per inch.
Output resolution refers to the number of dots per inch (dpi) that output device, such as a laser printer reduces.
Determining the scan resolution depends on the capability of your output device. If the image is used on screen, the best is 96 ppi for PC compatibles, or 72 to 120 pixels per inch for a Macintosh monitor.
Scanned file size is determined by the scan resolution, for example, the size of an image with a resolution of 200 ppi is four times greater than an image of identical dimensions and a resolution of 100 ppi.
Screen frequency is the resolution of the screen you will use when you print your final image.
As a general rule, the image resolution should be twice the screen frequency to achieve the best printed results.
To estimate scan resolution:
The following image file formats are allowed: Photoshop 2.5 and 3.0 native format, Adobe Illustrator, Amiga IFF and HAM, BMP, Compuserve GIF, EPS, JPEG, Kodak PhotoCD, MacPaint, PCX, PICT file(raster only), PIXAR, PixelPaint, Raw, Scitex CT, Targa and TIFF. Among the above file formats, some of them are plug-in modules.
Exporting images from Photoshop:
Photoshop allows you to export to the following image formats: Photoshop
2.5 and 3.0 native format (with 2.5 compatibility mode enabled). Amiga
IFF and HAM, BMP, Compuserve GIF, EPS, JPEG, MacPaint, PCX, PICT File (raster
only), PIXAR, PixelPaint, Raw, Scitex CT, Targa, and TIFF.
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 The marquee tool lets
you select rectangular or elliptical areas by dragging over an area of
the image. By default, the marquee is dragged from its corner.
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It is important to keep this principle in mind because it demonstrates that colour mapping is a balancing act. It seems that the first nine operations you have performed may make an image look progressively better, but the tenth may send it to a decline. The tips is, as always true, save your work on disk before launching into the colour mapping process.
Photoshop's colour correction functions fall into three categories: those that produce immediate and useful effects, such as Invert and Threshold; those requiring significantly more work but are nonetheless designed to be understood by novices, such as the Brightness/Contrast and Colour Balance commands; and those that are more complicated but provide better control and better functionality, such as Hue/Saturation, Replace Colour, Levels and Curves.
By moving the Hue slide bar in the Hue/Saturation dialog box, the colours will be shifted around the colour wheel. It is as if the pixels were playing a colourful game of musical chairs, except that none of the chairs disappear. For example, if you select the Master radio button and enter a value of +60 degrees, all pixels will stand up, march one sixth of the way around the colour wheel and sit down. A pixel that was red becomes yellow, a pixel that was yellow become green and so on...
By applying the saturation values to specific colour options, you can selectively fade and fortify colours. The Saturation option is especially useful for toning down images captured with low quality scanners that exaggerate certain colours.
Colourizing images:
Photoshop
provides four colour controls in the toolbox:
In Photoshop, "filters" enable you to apply automated effects to an image.
In addition to allowing you to correct lighting and perspective fluctuations, Photoshop's filters can accomplish a great deal more. You can slightly increase the focus of an image, introduce random pixel, add depth to an image or completely rip it apart and resample it into a hunky pile of goo. Many number of special effects are made available via filters.
How and When to use filters:
Corrective Filters: the effects that you have applied are subtle enough that a user won't even notice what you have done through the corrective filters.
These filters include those that change the focus of an image (Blur, Sharpen), enhance colour transitions (High Pass), and randomize pixels (Add Noise).
Destructive Filters: found under the Filer-> Distort. Pixelate, Stylize and Render which produce effects that completely overwhelm your artwork, making the filter more important than the image.
To get best results from these filters, apply them to selected portion of an image, rather than to the entire image. And make sure to "Save" your image to disk before applying a destructive filter so that you can revert to the saved image if you found the changes are not the way that you had wanted.
General Filtering techniques:
1. Preview filter effects before you apply them.
2. Reapplying the last filter. Once is not enough (sometimes!).
Samples:
Add noise
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Blur
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Lighting
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Wave
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Wind
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Patchwork
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Color Pencil
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Facet
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1A. The Photoshop software is available in three platforms: in Unix, in PC and in Macintosh. The PC version is installed in the student laboratories in the Run Run Shaw Building and the Old Library Building.
One copy of the software is installed in the Silicon Graphics Web Force station located in the Special Facilities Room in the Run Run Shaw Building. There is also a Power Macintosh connected to a scanner with one user license in Room 104, Run Run Shaw Building.
2. What is the best memory configuration in order to run Photoshop?
2A. The best way to improve the performance of Photoshop is to increase the amount of RAM. To support both Photoshop and Windows applications, it is better to have 32MB to 256MB of RAM. But when the RAM is not enough, it will use a virtual memory scheme which allows you to use the disk space for storing data and performing computation temporarily during a work session.
3. What is resizing and what is resampling?
3A. With resizing, there is an inverse relationship between size and resolution--size goes up when the resolution goes down, and vice versa. Resampling affacts either size or resolution alone.
4. The tools do not seem to work?
4A. The tools in the toolbox may appear not to work, 1)if you are in Quick Mask mode and are trying to edit the protected part of an image; 2) if there is an active selection that is hidden; 3)if you accidentally selected an alpha channel as the target channel but you are viewing the composite RGB or CMYK channel.
5. Photoshop often displays out-of-memory messages?
5A. To designate a secondary scratch disk, from File menu, select Preference and select Plug-ins & Scratch Disks, enter new drive to Secondary Scratch Disk.
5B. To increase the amount of RAM available to Photoshop software, from File menu, select Preference and select Memory and Image Cache, enter Cache Level or Physical Memory Usage.
6. What are the new features in Adobe Photoshop V.5?
6A. There are a lot of enhancement in the new version of Photoshop.
Please refer to Help Menu, select Contents to read What's new.
Gif - CompuServe Graphic Interchange Format, supports 256 colours only. But it is commonly used to contain photographic images in the BBS and Internet. These files usually end with .gif extension.
JPEG- Joint Photographic Experts Group format, is most efficient and essential compression. It is a compression which sacrifices image quality to conserve space on disk. However, you can determine how much you would like to keep the image quality. The lower the image quality, the higher the compression. These files usually end with .jpeg or .jpg
Netscape browser can display both Gifs and JPEG format images without launching other helper applications. For file transferring, with both Gif file and JPEG file containing the same dimension of image, JPEG file size is about one third of Gif file, thus it is faster to transfer a JPEG file than a Gif file.
A tip on using images in Web Page is to use smaller stamp-size Gif file for easy preview and link this to a good quality of the original JPEG image.
Our examples are in the University's
Estates home page of The University of Hong Kong.
Last updated by the Computer Centre on August 25, 2000
You move a selection by dragging
it to a new location using the move tool. When the Info palette is open,
you can track the exact distance you are moving a selection.
The lasso tool lets you make a
selection by dragging a freehand outline around an area. You can also constrain
all or part of the outline to straight-edged segments.
The magic wand tool lets you select
portions of an image based on the color similarities of adjacent pixels.
This tool can be useful for selecting a consistently colored area (for
example, a red flower) without having to trace the outline with the lasso
tool. When you use the magic wand tool, Adobe Photoshop determines whether
the adjacent pixels are within the color range, or tolerance, that you
specify.
The airbrush tool lets you apply
gradual tones (including sprays of color) to an image. It simulates the
effect produced by traditional airbrush techniques. The edges of the stroke
are more diffused than those created with the paintbrush tool. The pressure
setting for the airbrush tool determines how quickly the spray of paint
is applied.
The paintbrush tool lets you create
soft strokes of color.
The eraser tool changes pixels
in the image as you drag through them. You can choose to change the color
and transparency of the affected pixels, or to revert the affected area
to its previously saved version.
The pencil tool creates hard-edged
freehand lines and is most useful for bitmapped images.
The rubber stamp tool lets you
paint a copy, or a modified copy, of an image or color into the same image
or into another image. The Clone options of the rubber stamp tool make
a copy of, or sample, an image and paint an exact duplicate of that image.
Other rubber stamp options let you paint with a pattern or with an “impressionistic”
copy of the image. You can also restore painted areas to their last-saved
states.
The smudge tool simulates the
actions of dragging a finger through wet paint. The tool picks up color
from where the stroke begins and pushes it in the direction in which you
drag.
The focus tools include the blur
tool, which lets you blur hard edges or areas in an image to reduce detail,
and the sharpen tool, which lets you sharpen soft edges to increase clarity
or focus.
The toning tools include the dodge
and burn tools, which let you lighten or darken, respectively, specific
areas of an image, and the sponge tool, which lets you change the color
saturation of an area.
You create a path using one of
two methods: you can create a new path in the Paths palette and then begin
to draw with the pen tool, or you can draw with the pen tool first and
then save the path in the Paths palette. If you create a path in the Paths
palette first, the path you draw is saved automatically. If you begin drawing
first, the path you draw appears in the Paths palette as a temporary work
path. You must save a work path to preserve its contents during a work
session.
Adobe Photoshop lets you add bitmap
type to an image using the type tool). You can specify the leading, spacing,
type styles, and alignment of the type. Normally, large bitmap characters
appear jagged on the screen. However, if you use the Adobe Type Manager
(ATM) program installed with Adobe Photoshop, or if you use TrueType‘ fonts,
characters appear almost as smooth and as well-defined as outline type.
The line tool draws straight lines
on an image. Line tool options let you specify the width of lines, set
anti-aliasing, and create lines with arrowheads.
The gradient tool lets you create
a gradual transition between two or more colors. You can choose from the
existing gradient fills in the Gradient Tool Options palette, or you can
create and edit your own gradient fills. If you don’t select a specific
part of the image to fill, the gradient tool applies the fill to the entire
active layer.
The paint bucket tool fills adjacent
pixels that are similar in color value to the pixels you click. The paint
bucket options let you specify a mode and opacity, indicate the color range,
or tolerance, of pixels to be filled, and choose whether to fill with the
foreground color or a pattern. You can also choose to create smooth edges
for the filled selection.
The eyedropper tool lets you sample
color from an area of an image to designate a new foreground or background
color. You can sample from the active image or from another image. (When
you’re using the eyedropper, you can click in a background window without
making it the active window.)
You can view different areas of
an image using the image window scroll bars or the hand tool.
The zoom tool and the Zoom commands
let you magnify and reduce the view of an image. You can also zoom to an
exact percentage of the original view by entering values in the zoom percentage
box at the lower left of the window. The image window’s title bar displays
the magnification or reduction percentage at all times. You can magnify
up to 1600% of the original image view.