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Source:

Text on this page was pilfered from Adobe's Help files for Illustrator and Photoshop. Some sentences were added, removed or modified. I'm sure Adobe wouldn't mind.

Computer Graphics Primer

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When creating graphics on a computer, there is a distinction between painting and drawing. Painting involves changing the colours of pixels using a painting tool. You can apply colours gradually, with soft edges and transitions, and manipulate individual pixels using powerful filter effects. However, once you apply a brush stroke, there is no simple way to select the entire brush stroke and move it to a new location in the image.

Drawing, on the other hand, involves creating shapes that are defined as geometric objects (also called vector objects). For example, if you draw a circle using the ellipse tool, the circle is defined by a specific radius, location, and colour. You can quickly select the entire circle and move it to a new location, or you can edit the outline of the circle to distort its shape.

Working with shapes provides several advantages:

• Shapes are object-oriented--you can quickly select, resize, and move a shape, and you can edit a shape's outline (called a path) and attributes (such as stroke, fill colour, and style). You can use shapes to make selections and create libraries of custom shapes with the Preset Manager.
• Shapes are resolution-independent--they maintain crisp edges when resized, printed to a PostScript printer, saved in a PDF file, or imported into a vector-based graphics application.

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About bitmap images and vector graphics

Computer graphics fall into two main categories--bitmap and vector. You can work with both types of graphics in Photoshop and ImageReady; moreover, a Photoshop file can contain both bitmap and vector data. Understanding the difference between the two categories helps as you create, edit, and import artwork.

Bitmap images

Bitmap images--technically called raster images--use a grid of colours known as pixels to represent images. Each pixel is assigned a specific location and colour value. For example, a bicycle tire in a bitmap image is made up of a mosaic of pixels in that location. When working with bitmap images, you edit pixels rather than objects or shapes.

Bitmap images are the most common electronic medium for continuous-tone images, such as photographs or digital paintings, because they can represent subtle gradations of shades and colour. Bitmap images are resolution-dependent--that is, they contain a fixed number of pixels. As a result, they can lose detail and appear jagged if they are scaled on-screen or if they are printed at a lower resolution than they were created for.

Vector graphics

Vector graphics are made up of lines and curves defined by mathematical objects called vectors. Vectors describe an image according to its geometric characteristics. For example, a bicycle tire in a vector graphic is made up of a mathematical definition of a circle drawn with a certain radius, set at a specific location, and filled with a specific colour. You can move, resize, or change the colour of the tire without losing the quality of the graphic.

Vector graphics are resolution-independent--that is, they can be scaled to any size and printed at any resolution without losing detail or clarity. As a result, vector graphics are the best choice for representing bold graphics that must retain crisp lines when scaled to various sizes--for example, logos.

Because computer monitors represent images by displaying them on a grid, both vector and bitmap data is displayed as pixels on-screen.

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Learning About Pixels

A pixel is the smallest discernible element in an image. Each pixel displays one colour. A pixel’s colour and brightness range is determined by its bit depth.

Pixels are grouped together to create the illusion of an image. On colour displays, three colour elements (one red, one green, and one blue) combine to form a pixel. As the number of pixels increases, the image's detail becomes sharper, more clearly representing the original subject. Therefore, the higher the pixel count, the more likely the displayed image will look like the original subject.

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Dimensions

The number of pixels along the height and width of a bitmap image is called the pixel dimensions of an image.

The number of pixels per inch (ppi) printed on a page determines the image resolution.

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Image resolution

The number of pixels displayed per unit of printed length in an image, usually measured in pixels per inch (ppi). In Photoshop, you can change the resolution of an image.

In Photoshop, image resolution and pixel dimensions are dependent. The amount of detail in an image depends on its pixel dimensions, while the image resolution controls how much space the pixels are printed over. For example, you can modify an image's resolution without changing the actual pixel data in the image--all you change is the printed size of the image. However, if you want to maintain the same output dimensions, changing the image's resolution requires a change in the total number of pixels.

When printed, an image with a high resolution contains more, and therefore smaller, pixels than an image with a low resolution. For example, a 1-by-1-inch image with a resolution of 72 ppi contains a total of 5184 pixels (72 pixels wide x 72 pixels high = 5184). The same 1-by-1-inch image with a resolution of 300 ppi contains a total of 90,000 pixels. Higher-resolution images usually reproduce more detail and subtler colour transitions than lower-resolution images. However, increasing the resolution of a low-resolution image only spreads the original pixel information across a greater number of pixels; it rarely improves image quality.

Using too low a resolution for a printed image results in pixelation--output with large, coarse-looking pixels. Using too high a resolution (pixels smaller than the output device can produce) increases the file size and slows the printing of the image; furthermore, the device will be unable to reproduce the extra detail provided by the higher resolution image.

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Resampling

Resampling means changing the image dimensions (and therefore display size) of an image. When you downsample (decrease the number of pixels), information is deleted from the image. When you resample up (increase the number of pixels, or upsample), new pixels are added.

Keep in mind that resampling results in poorer image quality. For example, when you resample an image to larger image dimensions, the image loses some detail and sharpness. Applying the Unsharp Mask filter to a resampled image can help refocus the image details.

You can avoid the need for resampling by scanning or creating the image at a sufficiently high resolution. If you want to preview the effects of changing pixel dimensions on-screen or to print proofs at different resolutions, resample a duplicate of your file.

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Learning About Bit Depth

Bit depth describes the number of tonal values or shades of a colour each channel in a pixel is capable of displaying. Increasing the bit depth of colour channels in an image's pixels exponentially increases the number of colours each pixel can express.

Bit depth—also called pixel depth or colour depth—measures how much colour information is available for displaying or printing each pixel in an image. Greater bit depth (more bits of information per pixel) means more available colours and more accurate colour representation in the digital image. For example, a pixel with a bit depth of 1 has two possible values: black and white. A pixel with a bit depth of 8 has 2⁸, or 256, possible values. And a pixel with a bit depth of 24 has 2²⁴, or roughly 16 million, possible values. Common values for bit depth range from 1 to 64 bits per pixel.

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File size

The digital size of an image, measured in kilobytes (K), megabytes (MB), or gigabytes (GB). File size is proportional to the pixel dimensions of the image. Images with more pixels may produce more detail at a given printed size, but they require more disk space to store and may be slower to edit and print. For instance, a 1-by-1-inch, 200-ppi image contains four times as many pixels as a 1-by-1-inch, 100-ppi image and so has four times the file size. Image resolution thus becomes a compromise between image quality (capturing all the data you need) and file size.

1 kilobyte = 8 192 bits
1 megabyte = 8 388 608 bits

Another factor that affects file size is file format--due to varying compression methods used by GIF, JPEG, and PNG file formats, file sizes can vary considerably for the same pixel dimensions. Similarly, colour bit-depth and the number of layers and channels in an image affect file size.

Photoshop supports a maximum file size of 2 GB and maximum pixel dimensions of 30,000 by 30,000 pixels per image. This restriction places limits on the print size and resolution available to an image.

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About layers

Layers allow you to work on one element of an image without disturbing the others. Think of layers as sheets of acetate stacked one on top of the other. Where there is no image on a layer, you can see through to the layers below. You can change the composition of an image by changing the order and attributes of layers. In addition, special features such as adjustment layers, fill layers, and layer styles let you create sophisticated effects.

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About channels

Channels are grayscale images that store different types of information:

• Colour information channels are created automatically when you open a new image. The image's colour mode determines the number of colour channels created. For example, an RGB image has four default channels: one for each of the red, green, and blue colours plus a composite channel used for editing the image.
• You can create alpha channels to store selections as 8-bit grayscale images. You use alpha channels to create and store masks, which let you manipulate, isolate, and protect specific parts of an image. In addition to supporting alpha channels from Photoshop, you can save, load, and delete selections as alpha channels in ImageReady 7.0.
• You can create spot colour channels to specify additional plates for printing with spot colour inks.

Image data is stored in channels:
White = 100% colour
Black = 0% colour

Grayscale image = one channel
RGB image = three channels
CMYK image = four channels

An image can have up to 24 channels. The file size required for a channel depends on the pixel information in the channel. The uncompressed size of a file, including alpha channels and layers, appears as the rightmost value in the status bar at the bottom of the window when Document Sizes is chosen from the pop-up menu.

Channels can be represented in colour or in grey. It is better to view of them in grey, colour distracts from the purpose of channels: to see the colour distribution with the help of the intensity of the grey.

Note:

As long as you save a file in a format supporting the image's colour mode, the colour channels are preserved. Alpha channels are preserved only when you save a file in Adobe Photoshop, PDF, PICT, Pixar, TIFF, or Raw formats. DCS 2.0 format only preserves spot channels. Saving in other formats may cause channel information to be discarded.

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Alpha Channels

They store selections. That's all they do.

An alpha channel has these properties:

• Each image (except 16-bit images) can contain up to 24 channels, including all colour and alpha channels.
• All channels are 8-bit grayscale images, capable of displaying 256 levels of grey.
• You can specify a name, colour, mask option, and opacity for each channel. (The opacity affects the preview of the channel, not the image.)
• All new channels have the same dimensions and number of pixels as the original image.
• You can edit the mask in an alpha channel using painting tools, editing tools, and filters.
• You can convert alpha channels to spot colour channels.

To create an alpha channel using current options:

1. Click the New Channel button at the bottom of the Channels palette. The new channel is named according to the sequence in which it was created.
2. Use a painting or editing tool to paint in the image. Paint with black to add to the channel; paint with white to remove from the channel; paint with a lower opacity or a colour to add to the channel with lower opacities.

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About masks

Masks let you isolate and protect areas of an image as you apply colour changes, filters, or other effects to the rest of the image. When you select part of an image, the area that is not selected is "masked" or protected from editing. You can also use masks for complex image editing such as gradually applying colour or filter effects to an image.

In addition, masks let you save and reuse time-consuming selections as alpha channels. (Alpha channels can be converted to selections and then used for image editing.) Because masks are stored as 8-bit grayscale channels, you can refine and edit them using the full array of painting and editing tools.

When a mask channel is selected in the Channels palette, foreground and background colours appear as grayscale values.

In Photoshop, you can create masks, all stored at least temporarily as grayscale channels, in the following ways:

• Quick Mask mode lets you create and view a temporary mask for an image. Temporary masks are useful when you don't want to save the mask for later use.
• Alpha channels let you save and load a selection to be used as a mask.
• Layer masks and vector masks let you produce a mix of soft and hard masking edges on the same layer. By making changes to the layer mask or the vector masks, you can apply a variety of special effects.

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Having Problems?

Sometimes you can find yourself in a kind of an electronic roadblock in Photoshop. Nothing seems to work and you can't get out of where you are. Some questions to ask:

• Am I on the right layer?
• Am I on the right channel?
• Do I have a selection hidden?

If you find that you have completely messed things up, and you need to take a step back, Photoshop gives you quite a few options. To simply undo your last step, you can type "cmd z".

If you need to step back more then one step, you can use the History Palette. By clicking on a state further back, you take your image to that state. This eliminates everything you did after that state. If you take a snapshot with the History Palette, you can also use the History Brush to revert only painted areas to that captured state.

If you got yourself in really serious trouble, you can use the "Revert" command under the "File" menu to revert the whole Photoshop file to its last saved state.

A better practice is to save copies of your file with the "Save a Copy" command, under the "File" menu. This saves a whole copy of your file on your drive in its present state. This tends to take a lot of disk space, but then disk space comes cheap these days.