GS-PS: Transform, Save for Web

Review

• Type
• Quick Masks
• Levels
• Barbara's Rectangle Question - select > paint bucket

Transform 

• To resize an object or selection, choose Transform from the Edit menu. You can transform using scale, skew, distort, perspective, rotate 180°, rotate 90° clockwise, rotate 90° counter-clockwise, flip horizontal and flip vertical.

Save for Web

• Photoshop's Save for Web command is specially designed for saving Web images, which you generally want to keep as small in file size as possible. Save for Web allows you to adjust various settings and save your images out to the following Web-friendly formats:

• JPEG
• GIF
• PNG
• WBMP - this format consists of only black and white pixels and are used for mobile devices such as cell phones. Chances are you won't be working with this format.

• Each of these formats have their own unique settings which can effect file size and the appearance of the image. In the Save for Web dialog, box, you can preview up to four different settings and compare the results before committing to the Save. 
• The Save for Web (SfW) shortcut is Alt-Shift-Ctrl-S on Windows or Option-Shift-Command-S on Mac. The dialog box is completely resizable, so it can be as large as your entire monitor area if necessary. 
• Across the top of the Save for Web dialog are four tabs giving you the choice of four different views: Original, Optimized, 2-up, and 4-up. You can easily switch between any of the four views by selecting the appropriate tab in the dialog. The Original and Optimized tabs let you flip back and forth between showing the original image and the Web optimized version which will change in appearance according to your settings. The 2-Up tab lets you see the original and optimized version side-by-side, and the 4-Up tab compares 4 versions. As you adjust settings, you can click on each of the squares to see that version. 
• While you're in the 4-Up view, change each of the four views to a different preset using the preset menu at the top right of the SfW dialog. Use one of these presets for each of the 4 views:

• Original
• PNG-8 128 Dither
• GIF 64 No Dither
• JPEG Low

• By default, Photoshop shows you the time estimate based on a 28.8 Kbps connection speed. This is probably the lowest speed used today. If you want an estimate closer to what the average user will experience, you can change this to 56.6 Kbps from the preview menu. You access the preview menu by clicking the tiny arrow at the top right of the dialog, just above the 4 preview squares. Choose the options for "Size/Download Time (56.6 Kbps Modem/ISDN)"
• As you move your cursor over the previews, the bottom portion of the dialog box displays the color values of the underlying pixels in the preview. 
• Along the left edge of the Save for Web dialog, you find a small toolbar with 6 buttons. Here's a description of the three tools on the left edge of the dialog box:

• Hand Tool - Use it to pan around the image previews. The Hand tool only functions in the outlined view when you are in 2-Up or 4-Up mode, but all views will pan to show the same area after you release the mouse button. The same spacebar shortcut for the hand tool where here just as it does in the main Photoshop workspace.
• Slice Select Tool - Used for creating slices when you need to cut an image up for your Web page layout.
• Zoom Tool - Use it to zoom in on a preview. You can also use the zoom drop down box at the bottom left of the Save for Web dialog. When you zoom in and out, all your views change to the same zoom level.
• Eyedropper Tool - Use it to select individual colors from your image. The eyedropper color is displayed in the color swatch just below the tool button. Like the hand tool, it can only be used in the Outlined view when in 2-Up or 4-Up mode.
• Eyedropper Color - Shows the color you last selected with the Eyedropper tool. 

• The image size settings allow you to resize an image to specific pixel dimensions or to a percentage of the original size. This can be handy if you forget to scale down an image before entering the SfW dialog. Or, if you can't quite squeeze the file size down to what you need using the optimization settings alone, you can often reduce the size even more by reducing the image size. 
• At the bottom of the Save for Web dialog you'll see a browser icon and a drop down menu button. Clicking the browser icon opens a page in your default Web browser with a preview of the optimized image along with the format, dimensions, size, settings, and the HTML code to display the image. Clicking the arrow next to the browser icon allows you to choose any of your configured browsers or another browser. Choose "Edit List..." is you want to configure a browser to appear in the list 
• One important thing to know about Save for Web is that your image will always be exported with a resolution of 72 ppi, and it will strip out the Exif information in your digital camera photos. So if you need to retain the print size and resolution or Exif information, you shouldn't use Save for Web. Of course, you can always keep a working copy of your original file in Photoshop (PSD) or another format, and then save a Web version using the Save for Web command. This is a good practice to get into in general because you never know when you'll need to go back to that original.

Sharpening Images

• Use Filter>Sharpen>Unsharp Mask to sharpen an image. In the dialog box that appears, keep the preview box checked. This allows you to see the effects of different settings.

• Amount -- How much sharpening is applied. Consider using a value between 50% and 150% for typical images.
• Radius -- The number of pixels affected around edges. Consider using a value between 0.2 and 2.0.
• Threshold -- Defines what brightness difference qualifies as an edge (0 is default, sharpens all pixels). Consider using a value somewhere between 1and 5 (for Threshold, the higher the value, the less pronounced the sharpening effect).
• Note: Sharpening is more an art than a science, and is somewhat subjective.  Keep the Preview box checked in the Unsharp Mask dialog box .

Replace Color

• The Replace Color dialog box contains options for adjusting the hue, saturation, and lightness components of the selection
• Select the Rectangular Marquee tool, and draw a selection border around the an image. Don't worry about making a perfect selection, but be sure to include all of the image
• Choose Image > Adjustments > Replace Color.
• The Replace Color dialog box opens, and by default, the Selection area displays a black representation of the current selection.
• Notice the three eyedropper tools in the Replace Color dialog box. One selects a color; the second adds a color to the sample; the third removes a color from the sample. 
• A. Eyedropper tool B. Add To Sample eyedropper C. Subtract From Sample eyedropper  
• Using the Eyedropper tool, click anywhere in the object in the image window to sample that color.
• Then, use the Add To Sample eyedropper to sample other areas of the object until the entire object  is selected and highlighted in the mask display in the Replace Color dialog box.
• Drag the Fuzziness slider up to 45 to increase the tolerance level slightly.
• Fuzziness controls the degree to which related colors are included in the mask.
• If the mask display includes any areas that are not part of the object, get rid of those now: Select the Subtract From Sample eyedropper and click those areas in either the image window or in the Replace Color mask display to remove those stray pixels. (It's OK if a few remain in the selection.)
• In the Replacement area of the Replace Color dialog box, drag the Hue slider
• As you change the values, the color of the object changes in hue, saturation, and lightness, and the object changes color
• Click OK to apply the changes. 
• Choose Select > Deselect, and then choose File > Save.

Core Concepts for Photoshop

• create a new (duplicate) layer before you get started by dragging the background layer to the New Layer icon.  You can then poke out the eyeballs of the background layer and get to work
• Always create Adjustment Layers, don't adjust the image itself
• Print/export from the .psd file, if possible, don't convert to another file format unless necessary.  And, if you have to, do it last
• Create "Comps" as you go for states that you like - layers/layer options/edits... frozen
• If you have selected an area (using the selection tools), adjustments you make affect only the selected area. Otherwise most adjustments affect all the pixels in the image. 
• If your image has multiple layers, adjustments you make generally affect only the current layer. (The exception to this is adjustment layers) 
  

GS-PS: Type, Quick Masks, and Levels

Review

• Color Balance
• Background/Foreground Color
• Color Picker

Adding Type

• To add type to images use the Type tool ("T" in the Toolbox).
• To enter a single line of text, click on the image with the type tool. Then type. A type layer is automatically created.
• To create a paragraph (column) of text, click and drag with the type tool, then start typing. When you enter the text, it will wrap at the point where you stopped dragging.
• To reposition the text while you are using the Type tool, press the Ctrl key.The Type will change into the move tool so you can move the text box.
• When you are finished typing, click the check mark in the options bar. To cancel, click the X mark in the options bar.
• To adjust the color and format of your type, select the type and then change the settings in the options bar (at the top of the Photoshop desktop), or use the Character or Paragraph palettes (to display these palettes, click on the text palettes button ( ), which appears in the Options bar whenever the Type tool is selected).
• When you use the Type tool, a type layer is automatically created. As long as your type is on a Type layer it is editable. If you convert it to an image layer by rasterizing it (Layer>Rasterize), you will no longer be able to edit it. 

Quick Mask Mode 

• A mask lets you select an area of your document that is (or isn't) affected when you make color changes, use patterns, adjust colors or set gradients.
• If you want to colorize a part of an image, isolate an area using any of the selection tools
• Then, switch to Quick Mask mode. Photoshop fills everything but your selection with a 50% red screen to indicate that it is protected.
• Using the Brush tool, anywhere you paint with white is added to your selection. Anything you paint with black is removed from your selection and covered with the 50 percent red screen.
• When you switch back the Standard Editing Mode, your selection, plus any additions you made in Quick Mask mode, are still selected.

Levels

• The Levels command is an excellent tool for adjusting contrast. 
• Create an Adjustment Layer, and choose Levels
• The Levels dialog box displays a histogram of the brightness values of the pixels in your image. There are two sets of sliders (triangles): input levels and output levels. For most editing tasks, use the input levels sliders (the three triangles just below the histogram). 
• To increase contrast (create more dark and light pixels): Move the left and right sliders toward the middle. 
• To adjust the midtones: Move the middle input levels slider to the left to lighten midtones, and move the middle slider to the right to darken the midtones.
• Keep the Preview box checked in order to see the effects of moving the sliders on your image.
• To agree to the adjustment, click OK. 
• To cancel it, click Cancel.

• In the above example, there are few (if any) pixels that are white or even light gray. By moving the right slider towards the middle, we can lighten image to correct the exposure. Because the Preview box in the layers palette is checked, we can see the result on our image as we adjust the sliders.

• In the above example, the Levels dialog box Channel popup menu is set to the default of RGB, which means the histogram represents a combination of all of the channels (Red, Green, and Blue light) in the image. You can use Levels on an individual channel, but this distorts the color balance of the image, so in most cases should be avoided 
• The Levels command is often a good way to get a more even distribution of brightness of pixels. But this is not always desirable. For example, a properly exposed photograph of something dark such as a coal mine would be dark, and an image of a snow scene would be white. In such cases the histogram would be lopsided.  In such a case there may be no need to make an adjustment in the Levels dialog box.

GS-PS: Color Balance, Foreground/Background Colors, and Color Picker

Review

• Feathering Selections
• Cropping
• Adjustment Layers
• Brightness/Contrast 

Adjustment Layers

• Adjustment layers are special layers that you can use along with several image editing commands, including Levels, Photo Filters, Hue/Saturation and others. Adjustment layers allow you to make non-destructive (editable) corrections to your images. For example, if you create a Levels adjustment layer, you can go back to the Levels dialog box later and change settings at any time. 
• To create an adjustment layer,  click on the adjustment layer button   at the bottom of the Layers palette, then choose one of the menu items (Levels, Curves, Color Balance, etc.).  Alternatively, you can choose Layer > New Adjustment Layer, and you will see the same menu of items. 
• The advantage of this over an ordinary Levels command is that the adjustment layer is fully editable. At any time, you can click the adjustment layer thumbnail  in the layers palette to edit the layer. When you do, the dialog box will reappear and you can change the settings. 
• Unlike ordinary layers, adjustment layers require minimal memory and do not significantly increase the size of the file.

Color Balance

• Colors are divided into channels with a slider for each. By adjusting the sliders you can control the color balance between Cyan and Red, between Magenta and Green, and between Yellow and Blue. The dialog further breaks things down loosely into Shadows, Midtones and Highlights. By selecting one of these under Tone Balance and then adjusting the sliders under Color Balance you can change the overall color of the image you are working on.
• At the very bottom of the dialog is a checkbox for "Preserve Luminosity." Keeping it checked is a good idea as this prevents the image from changing brightness (luminosity) as you adjust the color balance.
• Color Balance is generally best for trying to tweak the overall color of an image rather than correcting issues with parts of an image although you can use it with a layer mask or selection if you so choose. Be forewarned that the range of the color balance sliders is deceiving. It takes only a small change on the slider to change the appearance of an image. Don't overdo it or you will end up with strange results. It's all too easy to oversaturate the color you are trying to enhance.

Foreground and Background Colors

• The foreground color (the box shown in black in the toolbar screenshot to the left) is used by the current drawing tool – it's the active tool. The background color is used to fill in erased areas and to create gradient fills.
• To set the foreground and background to 100 percent black and white, click the small boxes next to the foreground and background color boxes in the Tool Palette. To switch the foreground and background colors, click the small arrows next to the foreground and background color boxes.
• You can choose new foreground and background tools by clicking once in either the foreground or background box to bring up the Color Picker.

Color Picker

• You can select a new foreground or background color by:

• Selecting it from the color box in the middle, sliding the arrows in the narrow color bar next to it to get a new color range. The color you select is displayed in comparison to the old color in two boxes to the right of the color boxes.
• Setting the HSB values (angle from 0 to 360 for hue, percentages of saturation and brightness).
• Setting the Lab values (Lightness values on the (a) green to magenta and (b) blue to yellow axes.)
• Setting the RGB values (values from 0 to 255 of red, green and blue).
• Setting the CMYK values (percentages of cyan, magenta, yellow and black). CMYK is the color mode used for print graphics.
• Set the hexadecimal value.

• At the bottom of the Color Picker you can limit your palette to only Web-safe colors by checking the box.
• Notice the two squares just left of the cancel button. The color shown on top is the currently selected color, the color on the bottom is the color you are replacing. If you see a triangle with an exclamation mark, it means you have selected a color that is out of gamut for CMYK. Below the out of gamut alert, Photoshop displays the closest color that is within the CMYK color gamut. If you see a small cube displayed, it indicates that the color selection is not "Web-safe." Below the Web safe alert, Photoshop displays a tiny swatch of the closest Web-safe color match. Clicking on either of the closest match swatches will change the selected color to the closest match.

GS-PS: Selecting Follow-up, Adjustment Layers and Type

Review

• Rectangle/Oval Selection Tools
• Lasso Selection Tool
• Polygon Selection Tool
• Magnetic Selection Tool
• Magic Wand Selection Tool

General Selection techniques

• To hide the selection outline, press Ctrl-H. To display the outline again, press Ctrl-H again.
• To invert a selection, choose Select > Inverse. Everything that was selected becomes masked, and everything that was masked now becomes the selection.
• To deselect, click outside of the selection, or choose Select > None.
• Shift + Rectangle/Oval Selection Tools create Square/Circle selections
• The choices on the Option Bar affects the sensitivity and behavior of many of the Selection Tools
• You can also save a selection (Select > Save Selection) for later use (Select > Load Selection).
• To move a selection outline, put the selection cursor on the edge of the selected area and drag.
• To move the contents of a selection, choose the move tool and drag the selection.
• To clone a selection, press Alt while using the move tool.
• To add to an existing selection, press Shift
• To subtract from an existing selection, press Alt
• To add to an existing selection based on color, choose Select > Similar.

 Cropping

• Choose the Crop tool () in the toolbox. Then click and drag across the image. An outline will appear that shows you the area to be cropped.

• Adjust the area to be cropped by using handles on the edges of the image. 
• Note: If you move the cursor just outside of one of the corner handles, it turns into a curved cursor, which then allows you change the alignment of the selected area. This is useful, for example, to correct a scan of a picture that was in a crooked position on the  scanner.
• Press Enter to crop or Escape to cancel

Feathering Selections

Feathering gives a selection a softer edge. You can feather a selection by entering a value in the Feather checkbox in the options bar. Or you can feather an existing selection by choosing Select > Feather.

Adjustment Layers

• Adjustment layers are special layers that you can use along with several image editing commands, including Levels, Photo Filters, Hue/Saturation and others. Adjustment layers allow you to make non-destructive (editable) corrections to your images. For example, if you create a Levels adjustment layer, you can go back to the Levels dialog box later and change settings at any time. 
• To create an adjustment layer,  click on the adjustment layer button   at the bottom of the Layers palette, then choose one of the menu items (Levels, Curves, Color Balance, etc.).  Alternatively, you can choose Layer > New Adjustment Layer, and you will see the same menu of items. 
• The advantage of this over an ordinary Levels command is that the adjustment layer is fully editable. At any time, you can click the adjustment layer thumbnail  in the layers palette to edit the layer. When you do, the dialog box will reappear and you can change the settings. 
• Unlike ordinary layers, adjustment layers require minimal memory and do not significantly increase the size of the file.

Brightness/Contrast

GS-PS: Selecting

Review of Layers

• creating
• transparency
• visibility
• locking
• ordering
• deleting
• copying

Selecting Parts of Your Image

• In some cases, you may wish to isolate part of an image for editing, or to combine it with other images. In Photoshop, this is called a selection.  When part of an image has been selected, the image editing tools and commands work the same way, but only the selected part of the image is affected. 
• When you make a selection, the part of the image that is protected (that is, not selected) is sometimes referred to as a mask.  Just as you might use masking tape to protect part of a wall when you are painting a room, in Photoshop you are masking the part of the image that you want to protect when you have selected part of an image for editing. 
• When you make a selection, a selection outline appears to show you the selected area. In the following example, a rectangular area has been selected:

• Note: In some cases, pixels may be partially selected (see feathering, below). In such a case, the selection outline is only approximate because there is no sharp boundary between selected and masked areas.

Using Selection Tools

• To make a selection, click a selection tool icon in the toolbox
• Take a look at the Options bar for the tool you are using. (If the options are not visible, choose Window>Options.) You may want to adjust the options for the tool (for example, to make the selection feathered). 
• In some cases, tools may be hidden "behind" other tools. Click and hold the tool to see the hidden tools.

Selecting Rectangles, Ellipses, and Rows: the Marquee tools

• The rectangular marquee tool ()   is probably the most frequently used selection tool. It allows you to select rectangular areas. Click and drag diagonally with it to make a selection.  
• The elliptical and row marquee tools are less frequently used. They are hidden "behind" the Marquee tool. 
• To select an elliptical area or a single row of pixels, type M to cycle through the marquee tools. Then drag (with Elliptical tool) or click (with Single Row tools) to make a selection.

Selecting Irregularly Shaped Areas: the Lasso Tools

• To create freeform selections, you can click and drag with the Lasso tool (). But the lasso tool may not give you enough control. For more control, you may find the polygon lasso and magnetic lasso tools easier to use. 
• The Polygon Lasso tool () is often quite useful for selecting irregularly shaped areas. To use it, click on the image, then move to another place and click again to create a segment. Click multiple times to create a selection around an irregular area in your image.  Double-click to finish the selection. Note: If you are in the middle of selecting and you make a mistake, press the backspace key to go back one segment, or click the Esc key to undo the selection entirely. 
• Without pressing the mouse button, move the magnetic lasso tool around an area to select it. The tool will look for edges. You can adjust its sensitivity to edges in the options bar.

Selecting by brightness and color: Magic Wand tool (W)

• Click somewhere on the image with the magic wand tool to select adjacent portions of the image based on color brightness. The higher the tolerance entered in the Options palette, the more pixels are selected.

GS-PS: Layers

Review

• Navigator Pallet/Hand Tool
• JPEG vs. RAW vs. PSD
• Undo vs. History Pallet vs. Revert
• Image Size
• Color Modes

Before You Begin to Edit

• It is always a good idea to keep your original image somewhere secure, as a backup, and work on a copy
• Always convert to .psd first, before beginning any editing

Layers

• Images in Adobe Photoshop are made up of layers. Initially, each image has only one layer (called the background layer), but you can add more layers to the image.
• Layers are very useful for editing images and creating special effects. For example, you could make a copy of a layer and make some changes to it. If you don't like the results, you can delete the layer and still have your original layer as a backup. You can also combine images by putting different content on different layers. 
• Note: Keep in mind that layers (except for adjustment layers) require a substantial amount of memory.  Photoshop files with large numbers of layers typically are large files.
• To work with layers, display the Layers palette by choosing Layers from the Window menu. The following is an example of the layers palette along with an image.

• 
  
  
  
  
  The first column of the layers palette shows which layers are currently visible, indicate by the eye icon ( ). In the above example all the layers are visible. You can change a layer's visibility by clicking the eye icon in the first column of the layer
  
  
  

  Note: Even when a layer is visible, part of it may be transparent. For example, this happens if you select part of an area and then press the delete key. When part of a layer is transparent, the layers underneath it can show through. Photoshop represents transparent areas by a checkerboard pattern:

  
  
  

  • You can change the opacity of a layer by entering a value between 0 and 100 in the Opacity field at the top of the layers palette. For example, at an opacity of 10%, the layer would be nearly transparent; at an opacity of 90%, it would be almost completely opaque. The opacity default is 100%.

  • Editing you do within Photoshop affects the current layer. In the example above, "Layer 1" is the current layer.

  • To make a layer the current layer, click on the layer in the Layers palette.
  • To change the name of the layer, double-click on the layer's name, then type in a new name.

  
  
  

  

  
  
  To lock (prevent changes to) a layer, click one of the Lock: boxes near the top of the layers palette. From left to right, they are: lock transparent areas, lock pixels (prevent changes to non-transparent areas), lock layer position, and lock all.
  Other actions can be performed by clicking on icons at the bottom of the layers palette:
  

  

  • To link two or more layers, first select them by shift-clicking on them in the layers palette. Then click the link layers icon. When they are linked they move together, so if you reposition a layer you will simpultaneously reposition all layers linked to it.

  • To add effects to a layer (for example, shadows), click on the layer styles icon.

  • To add an adjustment layer, click the Adjustment layer icon (adjustment layers are discussed below).

  • To create a new layer, click the new layer icon at the bottom of the Layers palette, or duplicate a layer by dragging the layer to the new layer icon.

  • To delete a layer, click the trash icon, or drag the layer to the trash icon (next to the new layer icon).

GS-PS: Core Concepts for Image Quality

Review

• Menu Bar
• Status Bar
• Pallets
• Tool Bar
• Image Window
• Zoom Tool
• Hand Tool

Navigator Pallet + Hand Tool

• When using another tool, you can press the spacebar to temporarily access the hand tool. You can then drag to move around the image. When you stop pressing the spacebar,  you will return to using the other tool.
• To zoom and move around the image with the Navigator palette, choose Window>Show Navigator. The navigator palette will appear:

Use the navigator palette to zoom in or out of an image (by dragging the navigator slider) or to move around the image by dragging the view box (when you are zoomed in). The view box (red box in the above example) shows the part of the image that is currently being displayed in the main image window.

JPEG's vs. RAW vs. .PSD's

• Always try to take images in RAW format if you camera allows it.
• If you have a RAW image, you can do some editing before you import the image, then save the file as a .psd
• If you have a .jpg/.jpeg, you should immediately save it as a .psd before doing anything else.

Correcting mistakes and History Palette

• Choose Edit-Step Backward to undo your last action.
• Choose Window-Show History to view the History palette. Then you can click on a previous history state to undo one or more of your actions.
• Choose File-Revert to return to last saved version of file.

Getting Help

To access online help, choose Help > Photoshop Help.

Image Size

• Digital images are made up of pixels (picture elements), which can be defined as colored squares. Each pixel is only one color.
• A good way to learn about image size, resolution, and print size is to experiment with an image using the Image Size dialog box.  To do this, open an image in Photoshop. Then choose Image > Image Size, and the Image Size dialog box appears.
• 
• Under Pixel Dimensions, Width and Height refer to the number of pixels in an image, which has to do with the size of display on screen.
• Under Document Size, Width and Height refer to the size of the image when printed.

Experimenting with Document Size Options

• Uncheck Resample Image and change the resolution to 300 ppi. (high quality printing)  
• Note what happens to your document size numbers
• To change the print size of an image, uncheck the "Resample Image" box.  When you turn resampling off, notice that the Pixel Dimensions are no longer editable.
• Under Document Size, there is a trade off between Width/Height and the Resolution. If you increase the width and height, resolution decreases. If you increase the resolution, width and height decrease. 
• As a general guideline, newsletters and newspapers are likely to require resolutions in the range of 150 to 200 ppi, and magazines are likely to prefer high resolution images of 250 ppi or more. 
• A resolution of more than 300 ppi may be unnecessary. But keep in mind that if you crop the image and then enlarge the cropped image to the size of the original, the resolution will be lower than the original... so an image that was 300 ppi, then cropped and enlarged, may then be much less than 300 ppi.

Color Modes

• In Photoshop, you are always working in a color mode. The default mode is RGB (Red-Green-Blue), which is also the default mode for the Web. If you are only working to create color images for the Web or for a desktop inkjet printer, you may not need to use any of the other modes. But it is useful to know about modes because this gives you a better idea of how Photoshop creates, displays and prints images.
• Each color mode defines the colors that combine to make up the color of every pixel in the image. To change color modes, choose Image > Mode.
• Color modes are made up of channels. Each channel is a grayscale image. A grayscale image is an image made up of shades of brightness (as many as 256 shades, from white to black). To view the channels, choose Window > Channels, and the Channels palette will appear. Modes with fewer channels result in smaller file sizes.
• For example, in RGB mode, the red channel is a grayscale image made up of brightness values for the color red; the green channel is made up of brightness values for the color green; and the blue channel is made up of brightness values for the color blue. Each pixel in the full color image is made up of a combination of of brightness values for red, green, and blue.
• Note: It is possible to edit an individual channel, which is useful to improve the quality of some images.

Some of the most frequently used color modes are:

RGB mode three channels (Red, Green, and Blue) used by all monitors and by the Web recommended by Adobe for editing color images

Grayscale mode One channel, 256 shades of gray

Bitmap mode black and white (no shades of gray) used for line art

Indexed mode one color channel, up to 256 color values used for GIF images destined for the Web

CMYK mode

• four channels (Cyan, Magenta, Yellow, Black)
• used by professional printing houses. If you are producing a images for a publication or poster, you may need to save it in CMYK mode.

Core Concepts for Photoshop

1.  .psd is lossless.  .jpg is lossy.  Always convert jpg's to .psd's before doing anything
2. create a new (duplicate) layer before you get started by dragging the background layer to the New Layer icon.  You can then poke out the eyeballs of the background layer and get to work
3. Always create Adjustment Layers, don't adjust the image itself
4. Print/export from the .psd file, if possible, don't convert to another file format unless necessary.  And, if you have to, do it last
5. Create "Comps" as you go for states that you like - layers/layer options/edits... frozen