2. To enter values for the width and height, click in either the Width or Height text
`box, depending on which dimension is most important to you. Enter the desired
`value for that dimension. (You can enter decimal values, for example, 3.625, for
`the width and height.)
`
`The Adobe Photoshop program automatically enters a value for the other
`dimension.
`
`3. To enter a value for the image resolution, click in the Resolution text box, and
`enter the value you want for the resolution.
`
`4. To resample and resize the image, click OK.
`
`To determine a suggested resolution for resampling an image:
`
`1. Choose Resample from the Image menu.
`
`The Resample dialog box appears.
`2. Click Auto.
`
`The Auto Resolution dialog box appears.
`
`Hutu Resolution...
`
`”We"
`
`0 High
`
`Screen: El [lines/inch)
`
`Duality Required:
`C' Draft
`6) Medium
`
`3. Use the pop-up menus beside the Printer and Screen text boxes to choose the size
`units you want for the resampled image.
`
`4. Enter the resolution in the Printer text box at which you plan to output the image.
`
`5. Press the Tab key or click in the Screen text box. Enter the ruling for the screen
`you will use when you output the image.
`
`6. Click the quality of the Output you want.
`7. Click OK.
`
`The suggested image resolution appears in the Resample dialog box.
`
`8. To resarnple and resize the image, click OK.
`
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`

`Adjusting the size of the printed output
`You can control the size and resolution of your printed output using the Size/Ruler
`option in the Page Setup diaiog box, which is accessed through the File menu. The
`SizefRuler option allows you to change the size and resolution of a printout
`without affecting the image in memory and without resampling the image. You
`could use this option to print out an image at a marginally smaller size. This would
`save you the time involved in resampling down the image. You could also use this
`option to print an enlarged image of a screen dump. If you resampled up the
`image, the jagged edges would be removed from the image, and you might want
`to preserve these jagged edges in the ouput.
`
`When you use the Size/Ruler option, the image’s size and resolution are adiusted
`in tandem to maintain the image’s dimensions in terms of pixels. You should use
`the Resample command to adiust the images’s size and the amount of pixel
`information in the image independently.
`
`For example, you can use the SizefRuler option to print a BOO—dpi resolution
`image at twice the size, the resulting output will have a resolution of 150 dpi. The
`units of measure you specify for the Size/Rulers option also appiy to the rulers.
`
`To change the size of the image when output, you specify the height and width,
`or the resolution at which you want the image to be output. The SizelRulers
`option maintains the proportions of the image (the height-to-width ratio) as it
`adjusts the image size.
`
`If you enter a value either for the height or the width, the Adobe Photoshop
`program automatically enters the value for the other dimension that will
`maintain the image’s proportions, as well as the appropriate resolution for the
`specified size. On the other hand, you can enter the desired output resolution,
`and the size values required to achieve the specified resolution will be automati-
`cally entered.
`
`To change the output size:
`
`1. Choose Page Setup from the File menu.
`
`
`
`I:upies:|:|
`
`Pages: ©HII OFrom: |:]To: |:|[—m
`
`Cover Page: G) No C) First Page 0 Last Page
`Paper Source: 6) Paper Cassette C) Manual Feed
`
`El Correct for Printing Colors
`
`@ ColorfGragscale C) Black Er IJJhile
`
`I] hint Wheaten Steer (ling;
`I] taint “theater; {Intranet fruit;
`El Print Using Color PostSeript
`
`Encoding:
`0 HSCII
`© Binary
`
`Chapter 14: Resizing Images
`
`209
`
`235
`235
`
`

`

`. Click the Sizei' Rulers button.
`
`The Image Size/Ruler Units dialog box appears.
`
`. To select the size units, press down on the shadowed box beside the text box for
`the option you want.
`
`A pop-up menu listing the units options appears.
`
`. Holding down the mouse button, drag until the units option you want is high-
`lighted, then release the mouse button.
`
`. To enter values for the width and height, click either the Width or Height text
`box, depending on which dimension is most important to you. Enter a value for
`the desired dimension. (You can enter decimal values, for example, 3.625, for the
`width and height.)
`
`The Adobe Photoshop program automatically enters a value for the other dimen-
`sion and the appropriate resolution for the specified size.
`
`. To enter a value for the image resolution, click in the Resolution text box. Enter
`a value for the resolution.
`
`. Click OK.
`
`When you output the image, the image will be printed at the specified size and
`resolution.
`
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`

`Section 4: Advanced Image Editing
`and Color Correction
`
`
`
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`

`
`
`From a scan of a single 35mm color slide of teacups and a pot. the designer corrected
`the color in the image. She selected objects with a feathered edge and pasted them
`onto a dark background, then copied and flipped the selection horizontally, creating a
`mirror image. The designer posterized this copy and pasted it with an opacity of 40%
`on top of a blend created in the background, and used paste controls to allow iust part
`of the underlying coior to show through. She saved both selections of the cups as
`separate alpha channels; after proofing the output, she accessed the alpha channels as
`masks to make final color adjustments.
`
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`

`A
`
`Chapter 15: Converting Images and Working with Channels
`
`The Adobe Photoshop program treats images as a series of channels. The use of
`channels makes it possible to manipulate images in sophisticated ways. With
`channels, you can adjust the color in one channel of an image, but not another;
`analyze the color in two separate images by comparing and adiusting their respec-
`tive channels; and easily add detail to an image, such as text, and just as easily
`adjust the detail or remove it later.
`
`This chapter discusses channels, image types, and how images can be converted
`from one type to another. The chapter also discusses how to work with channels,
`including how to add channels to an image, delete channels, split channels into
`separate documents, merge channels, and perform calculations on channels to
`compare images.
`
`How to make color corrections to images is discussed in Chapter 16, "Making
`Color Corrections.”
`
`About channels and image types
`The Adobe Photoshop program supports a variety of image types, including bit-
`mappecl, gray~scale, RGB, indexed color, CMYK, HSL, and HSB images.
`
`in the Adobe Photoshop program, at least one channel comprises each image. For
`example, an RGB image has at least three channels, one for the red information,
`one for the green information, and one for the blue information. A channel is
`similar to a plate in the printing process.
`
`In addition, channels can be added to an image as a means of easily seeing a se-
`lection as a mask, making changes to the mask and storing it, then reusing the
`mask without having to reselect a selection.
`
`Additional channels are called alpha channels. An alpha channel is a separate, 8-
`bit gray-scale image within a document that you can use for editing and storing
`masks and for channel calculation commands. You can create an alpha channel
`using the Selection —> Alpha command in the Select menu.
`
`A document can have up to 16 channels. Bitmapped, gray-scale and indexed
`color all are single-channel images; RGB, HSL, and H513 have three channels;
`CMYK has four channels, and a multichannel image is a gray-scale image with
`more than one Channel. Other distinctions between image types are shown in the
`chart following.
`
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`

`

`
`Bits/pixel
`per
`
`
`
`
`ChannelNo. ChannelsMode Definition/when to use:
`
`
`
`Bitmapped images contain only 1
`
`bit of color éblack or white) per pixel.
`Since few e iting options are avail-
`
`able, it‘s best to convert the image
`to gray-scale mode, then back to
`
`Bitmap for export it necessary.
`
`Gray Scale
`This mode is used for editing and
`painting single channel gray-scale
`Images. Gray-scale images contain
`up to 256 levels of gray and can be
`converted to or from color images.
`If a second channel is added to a
`
`
`
`
`
`
`
`RGB Color
`
`3
`
`){1 x2
`
`”0
`{rob}
`
`
`
`
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`240
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`
`
`
`gap-scale image, it becomes a
`u tichannel image.
`
`
`
`Indexed Color images contain up
`
`
`to 256 colors/grays stored in an
`
`editable lookup table. Limited image
`
`editing is available. Indexed Color
`mode 15 used when editing of the
`
`
`table is desired, or when a limited
`
`
`color palette is required for export.
`
`
`24
`
` Generally, this is the mode of choice
`
`
`for color image editing and painting.
`Up to 16.7 million colors can be
`
`dis layed and edited on screen. Up
`
`
`
`to 3 additional alpha channels can
`be added. RGB images should be
`
`converted to CMYK mode to print
`
`
`color separations.
`
`CMYK Color
` You generally convert to CMYK
`
`mode from RGB to print color
`
`separations. Individual channels
`
`can be edited, but a composite
`
`full color image cannot be viewed.
`
`HSL Color
`The hue, saturation, and luminance
`
`in a color image can be edited as
`. hue
`_
`I saturation
`
`separate channels in HSL mode. HSL
`I luminance
`channels appear as gray-scale, but
`
`retain all color information when
`converted back to a color mode.
`
`HSB Color
`The hue, saturation, and brightness
`
`I hue
`in a color image can be edited as
`I saturation
`separate channels in HSL mode. HSL
`
`I brightness
`channels appear as gray-scale, but
`retain all color information when
`
`converted back to a color mode.
`
`Multichannel
`Adding a channel to a gray-scale
`
`
`image, or deleting a channel from
`an RGB, CMYK, HSL, or HSB image
`
`creates a Multichannel ima e. Color
`irna es converted to Multic annel
`
`mo e become gray-scale images.
`
`
`

`

`,—~.
`
`You can split the various channels that comprise an image into separate docu-
`ments, and merge these documents to create another image. Some gray-scale
`scanners allow you to scan a color image through a red filter, a green filter, and a
`blue filter to generate red, green, and blue documents. Merging allows you to
`combine the channels of a color image that was scanned this way, and save the
`image as a single, color document.
`
`The Adobe Photoshop program lets you select the channel you want to work on
`(the "active" channel) as if it were a separate document. Once you select a
`channel, any changes you make are applied to that channel. The Channel sub-
`menu under the Mode menu allows you to View and activate the various
`channels in an image. Note that the RGB command in the Channel submenu
`allows you to display and work on the three channels ofan RGB image together.
`
`Besides selecting channels from the Mode menu, you can also select channels
`from the keyboard. Each channel in a multichannel image is assigned a
`Command-key equivalent. The Command-key equivalent reflects the channel’s
`position in the list of channels in the Channels submenu. For example, cyan
`is the first channel, magenta the second, yellow the third, black the fourth, and
`so on. Alpha channels added to an image are assigned the next channel number
`in the sequence, such as 5, 6, and so on.
`
`To activate the channel. simply hoid down the Command key and type in the
`channel number. For example, the cyan channel in a CMYK image is the first
`channel listed in the Channels submenu and is assigned the Command key
`equivalent Command-1. To activate the cyan channel from the keyboard, type
`Command-1.
`
`To select the active channel:
`
`. Choose Channel from the Mode menu.
`
`A list of the available channels in the image appears.
`
`. Holding down the mouse button, drag to highlight the name of a channel to
`activate it.
`
`Chapter 15: Converting Images and Working with Channels
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`

`

`3. When you have selected the desired channel, release the mouse button.
`
`Bitmap
`Gray Scale
`Indexed Color...
`JREB Eolur
`CM‘EK Color
`HSL Color
`HSB Color
`
`Multichannel New Channel
`
`
`lleie is} f. innmné
`
`
`Split Channels
`
`Merge iiiissutteés...
`
`
`
`The selected channel is displayed. The name of the channel appears in the title
`bar of the active window.
`
`Display options
`The Adobe Photoshop program lets you have several documents open at the same
`time. Each document can have a different color lookup table to hold the colors
`used in the image. If you display two or more images that use different color
`tables, such as an indexed color and a gray—scale image, the display of the colors
`in the inactive documents will be incorrect (unless your monitor uses more than
`8 bits of information per pixel.
`
`To make the colors in the inactive documents appear closer to their actual colors,
`you can use the Use system palette option in the Preferences dialog box, which is
`accessed via the Preferences command in the Edit menu. When you select this
`option, the colors in the system's color table, rather than the colors in the active
`document’s color table, are used to display colors in all the open documents. This
`means that the colors for the active document will be less accurate than if the
`
`option is unchecked.
`
`When you work on an R63 01‘ CMYK image, you can choose to display the
`channels in their respective colors or as gray scales. Because each channel is nor-
`mally printed as a halftone, gray-scale image on the file that is sent to the
`printer, you may want to View each channel in a gray—scale format, rather than
`in its color format.
`
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`

`

`To display colors using the system color table and choose a display option:
`1. Choose Preferences from the Edit menu.
`
`The Preferences dialog box appears.
`
`Preferences...
`
`El Colored separations
`El Ilse system palette
`[Z] Use DirectSetEntries
`
`Clipboard Enport:
`O nisabled
`8; 3:33;;
`C) 4 bits/piuel
`0 a bitsr'oinel
`(it) B bits/pine], System Palette
`{316 bits/oinel
`C) 32 bits/pinel
`
`
`
`Cancel
`-
`
`Column size:
`
`.
`
`.
`Gut‘EYI E:
`Interpolation Method:
`O Nearest Neighbor
`O Bilinaar
`@ BICUDIC
`
`2. Click the Use system palette option in the Preferences dialog box.
`
`When the box beside this option is checked, the option is active.
`
`To view each RGB or CMYK channel in color, click the Colored Separations
`option in the Preferences dialog box. When the Colored Separations check box is
`checked, the channels are displayed in color.
`
`Converting one image type to another
`You can convert one type of image to another using the commands in the Mode
`menu. For example, you can easily convert an RGB image to a CMYK image,
`which is essential to producing color separations. When you perform this conver-
`sion, the red, green, and blue channels are converted to cyan, magenta, yellow,
`and black channels.
`
`You have eight image type options: Bitmap, Gray Scale, Indexed Color, RG13
`Color, CMYK Color, HSL Color, HSB Color, and Multichannel. Certain image
`types cannot be converted directly to other image types; for example, an RGB
`image cannot be converted directly to a Bitmap image type. (However, you can
`convert an RGB image to a gray-scale image, and then to a bitmapped image.) The
`image types in the Mode menu to which you cannot convert the active document
`are dimmed. The following sections describe the options available for the differ-
`ent conversions.
`
`15
`
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`
`

`

`To convert an image to a different image type:
`
`1. Open the document containing the image you want to convert.
`
`2. Choose the image type to which you want to convert the image from the Mode
`menu.
`
`Converting a gray-scale image to a bitmap-type image
`A gray-scale image is a single-channel image consisting of various levels of gray.
`Depending on the number of bits of information that were saved when the image
`was scanned, a gray-scale image can contain up to 256 levels of gray. Thus, the
`gray levels can range from 0 [black] to 255 (white).
`
`When you convert a gray—scale image to a bitmap-type image, you specify the
`resolution of the gray-scale image in the Input text box of the Gray Scale to Bitw
`map Options dialog box. You also specify the resolution you want the bitmap-
`type image to be in the Output text box. By default, the current image resolution
`is entered in the Input and Output text boxes.
`
`You also have five options for how the image will be Converted: the 50%
`Threshold method, Pattern Dither, Diffusion Dither, Halftone Screen. and
`Custom Pattern. A description of the options follows. You must select a mover
`sion option before converting a gray-scale image to a bitmapped image.
`
`To convert a gray-scale image to a bitmap-type image;
`
`1. Choose Bitmap from the Mode menu.
`
`The Gray Scale to Bitmap Options dialog box appears.
`
`Greg Scale to Bitmap...
`
`0 Eastern Pattern
`
`Input:
`
`'22
`
`Output: ?2
`
`[piuelsfincll]
`
`[niuelsfincll]
`
`Conversion Method:
`
`0 50% Threshold
`0 Pattern Dither
`© Diffusion Hither
`O Halftime Screen...
`
`2. To specify the resolution of the gray-scale image and the bitmap—type image, use
`the pop-up menus beside the text boxes to choose the size units you want, either
`pixels per inch or pixeis per centimeter.
`
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`

`. Enter a value for the resolution of the gray-scale image in the Input text box.
`
`. Press the Tab key or click in the Output text box.
`
`. Enter a value for the desired resolution of the bitmap-type image.
`. Click OK.
`
`3 4 5 6
`
`Selecting a bitmapped conversion method
`
`You have a choice of five methods to use when converting a gray-scale image to
`a bitmap-type image. The options determine the quality of the new bitmap-type
`image, ranging from a high-contrast black-and-white image, to a textured or half-
`tone screen. In addition, the Halftone Screen option allows you to produce a halfv
`tone for output on non-PostScript printers; if you use this option, you also specify
`the screen frequency and angle, and dot shape.
`
`To select the bitmapped conversion method you want to use:
`
`. Choose Bitmap from the Mode menu.
`
`The Gray Scale to Bitmap options dialog box appears.
`
`2
`
`.
`
`In the Gray Scale to Bitmap dialog box, click the desired option, and click OK.
`
`The gray~scale to bitmap-type conversion options are described below.
`
`The 50% Threshold option
`
`The 50% Threshold option converts pixels with a gray value above the middle
`gray level (128) to white, and pixels below the middle gray level to black. The
`result is a high-contrast black-and-white representation of the image.
`
`The Pattern Dither option
`
`The Pattern Dither option converts an image by organizing the gray levels into
`patterns made up of black-and-white dots. The resulting image is made up of geo-
`metric configurations of black and white dots.
`
`{It
`
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`
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`

`

`The Diffusion Dither option
`
`The Diffusion Dither option uses an error-diffusion process to convert the image.
`The diffusion option starts at the pixel in the upperwleft corner of the image and
`evaluates its gray level value. If the value is above a certain value, it is Changed to
`white; if it is below the value, it is changed to black. There is generally some error
`in the conversion because the gray pixel is originally some gray value between
`black and white, and the conversion changes it to either black or white. The
`amount of error is transferred to surrounding pixels before they are converted to
`black or white. In this way, the error is diffused throughout the image. The result
`has a grainy, film-like texture.
`
`Original gray-scale image
`
`
`
`
`50% threshold conversion method
`
`Pattern dither conversion method
`
`Diffusion dither conversion method
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`

`_-.
`
`The Custom Pattern option
`
`The Custom Pattern option simulates the effect of printing a gray-scale image
`through a custom halftone screen. This method allows you to apply a screen
`texture, like a wood grainJ to an image. The screen used is the current pattern.
`You can create a pattern that represents the texture you want, then screen the
`gray-scale image through the pattern to apply the texture to the image.
`
`For instructions on how to define a pattern, see the section, "Filling selections
`with patterns” in Chapter 10, ”Using the Fill Tools.”
`
`
`
`Original gray—scale image
`
`Custom pattern conversion method
`
`Chapter 15: Converting Images and Working with Channels
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`

`

`The Halftone Screen option
`
`The Halftone Screen option simulates the effect of printing a gray-scale image
`through a halftone screen. This option can be used to produce a halftone on out-
`put devices that do not use the PostScript page description language.
`
`When you select the Halftone Screen option and click OK, the Halftone Screen
`dialog box appears. You use this dialog box to specify the frequency and angle of
`the screen, as well as the dot shape.
`
`Halftime Screen...
`
`:
`Frequency -W Cancel
`Hngle:
`WWW“)
`
`0 Cross
`
`Shape:
`6) Round
`0 Elliptical
`0 Line
`0 Square
`
`Load...
`
`The screen frequency is the screen ruling or screen frequency of the halftone
`screen. The screen frequency you use depends largely on the paper stock and type
`of press on which the image will be printed. Newspapers commonly use an 85“
`line screen, whereas magazines use higher resolution screens, such as 133-line
`and 150»1ine halftone screens.
`
`The screen angle refers to the orientation or angle of the screen. Continuous tone
`and black-and-white halftone screens commonly use 45-degree angles.
`
`You can save the haiftone screen settings you define in a document and reuse
`them with other documents. You can also save these settings as defaults to be
`used in other Adobe Photoshop application documents.
`
`Chapter 19, "Printing Halftones," discusses how to produce a halftone in greater
`detail.
`
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`

`To specify the halftone screen:
`
`. Choose Bitmap from the Mode menu.
`
`The Gray Scale to Bitmap dialog box appears.
`
`. Select the Halftone Screen option and click OK.
`
`The Halftone Screen dialog box appears.
`
`. Use the pop-up menu beside the Frequency text box to choose the size units you
`want, either lines per inch or lines per centimeter.
`
`. Double-click in the Frequency text box.
`
`. Enter the pixels per inch or pixels per centimeter (from 1.0 to 999.9) for the
`screen frequency you want to use.
`
`. To specify the screen angle, press the Tab key or click in the Angle text box. Enter
`a value from -180 to 180 degrees for the screen angle.
`
`. To select the dot shape you want, click the name of the clot shape. The available
`dot shapes are round, elliptical, line, square, and cross.
`
`. To convert the image once you have made your choices, click OK.
`
`
`
`Original gray-scale image
`
`Halftone screen conversion method
`
`
`
`Chapter 15: Converting Images and Working with Channel’s
`
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`
`

`

`Making gray-scale conversions
`You can convert either a bitmapped image or a multichannel image to a gray-
`scale image. When you convert any image containing several channels of color,
`Adobe Photoshop discards the color information and the extra channels in the
`original image. The conversion methods are described below.
`
`Converting a bitmap-type image to a gray-scale image
`
`You can convert a bitmap-type image to a gray-scale image that consists of one
`gray level (black). When you choose this option, you can also choose to scale
`dewn the size of the image by a specified scale factor or "size ratio.” The default
`size ratio is 1, which results in a gray-scale image that is the same size as the bit-
`map—type image. As an example, to reduce the size of the gray-scale image by 50
`percent, enter 2 for the size ratio. If a number greater than 1 is entered for the size
`ratio, multiple pixels in the bitmap-type image are averaged together to produce
`a single pixel in the gray-scale image. This allows you to create a gray-scale image
`from an image scanned on a 1-bit scanner.
`
`To convert a bitmap-type image to a gray-scale image:
`1. Choose Gray Scale from the Mode menu.
`
`The Bitmap to Gray Scale dialog box appears.
`
`Bitmap to Greg Scale...
`
`
`
`Size Ratio: E
`
`2. Enter a value for the size ratio you want.
`3. Click OK.
`
`The bitmap—type image is converted to a gray-scale image.
`
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`

`Converting a multichannel image to a gray-scale image
`
`Any image that has more than one channel can be converted to a gray-scale
`image. A multichannel image is one that contains more than one channel, such
`as an RGB or CMYK image. Reference here is not to the specific Multichannel
`image type.
`
`When the Adobe Photoshop program converts an image to a gray-scale image, it
`discards all color information and extra channels in the original image. When
`you convert from an RGB or indexed color image to a gray-scale image, the
`program converts the color pixels to levels of gray based on the color values of
`the pixels, and discards the other color information.
`
`If the RGB channel is active in an RGB image, the gray levels of the converted
`pixels represent the luminosity of the pixels. [f the red, green, or blue channel is
`active, the gray levels of the converted pixels represent the color values of the
`pixels in the active channel only.
`
`When you convert from an HSL, HSB, CMYK, or Multichannel type image to a
`gray-scale image, the program uses the active channel as the gray-scale image and
`discards the other channels.
`
`Before you complete a conversion, an alert box will appear to confirm your
`action.
`
`To couvert a multichannel image to a gray-scale image:
`
`1. Choose Gray Scale from the Mode menu.
`
`. Click OK to proceed.
`
`The image is converted to a gray-scale image.
`
`
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`Making color conversions
`You can convert an image to any of the color image types that the Adobe
`Photoshop program supports: indexed colorr RGB, HSL, HSB, CMYK, and Multi-
`channel. When you convert an RGB image to an indexed color image, you can
`specify a color table for the new image. Descriptions of the conversions follow.
`
`With the exception of converting an RGB image to an indexed color image, the
`procedures for converting an image to color are similar. You simply choose the
`image type from the Mode menu to which you will convert your image; the
`image is automatically converted.
`
`Converting a gray-scale image to an indexed color or RGB image
`
`When you convert a gray-scale image to an indexed color or RG3 image, the red,
`green, and blue values for any given pixel become the same. For example, a pixel
`that had previously been described as having a gray value of 231 is now converted
`to a pixel having a red value of 231, a green value of 231, and a blue value of 231.
`
`Converting an indexed color or RGB image to an HSL or HSB image
`
`You might want to convert an image with RGB colors to an HSL or HSB image
`type to adiust the lightness or brightness of the image without changing the hue
`or saturation. When you convert an indexed color or RGB image to an HSL image,
`the image is split into three channels called hue, saturation, and luminance;
`when you convert to an HSB image, the image is split into hue, saturation, and
`brightness channels.
`
`Brightness is defined as the additive primary color (red, green, or blue) with the
`highest color value. Luminance is defined as the highest of the RGB values plus
`the lowest of the RGB values divided by two. With HSL and H83 images, you can
`only view one channel at a time. This differs from an RGB image. You can view
`the red, green, and blue channels together at one time on an RGB image.
`
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`lllindom
`Image Select
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`
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`

`Converting H53 and HSL images to RGB images
`
`Once you have adiusted an HSL or HSB image, you may want to convert it back
`to an RGB image to perform other color corrections on it. when you convert an
`HSB or HSL image to an RGB image, the three channels are converted into red,
`green, and blue channels.
`
`Converting color images to multichannel-type images
`You can convert images composed of more than one channel, such as RGB, HSL,
`and H53 images, to the Multichannel image type. When you convert an image to
`the Multichannel image type, the original channels with names assigned such as
`Red, Hue, Brightness, and so on, are renamed 1, 2, 3, and so on. The channels in
`a Multichannel image are gray-scale channels. You can View and edit each
`channel individually.
`
`In the case of an RGE image, the red, green, and blue channels are converted to
`gray levels that reflect the color values of the pixels in each channel. Likewise,
`each of the CMYK channels is converted to a graywscaie channel. If you add a new
`channel to a gray scale image or delete a channel from an RGB image, the image
`is automatically treated as a Multichannel image.
`
`Converting RGB and indexed color images to CMYK images
`
`To print a color separation, you convert an RGB or indexed color image to a
`CMYK image. The conversion splits the RGB colors into the four colors common-
`ly used for printing color separations. The channels are cyan, magenta, yellow,
`and black. This conversion process uses the Separation Setup parameters defined
`in the Preferences dialog box. For more information on the CMYK conversion
`process, see Chapter 20, ”Producing a Color Separation."
`
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`

`Image Select window
`5‘ File Edit Mode
`film!!!3
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`
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`
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`Converting an RGB image to an indexed color image
`
`At times, you might want to convert an RGB image to an indexed color image
`to edit an image’s color table or export an image to an application that only
`supports 84bit color.
`
`When you convert an RGB image to an indexed color image, a color table is built
`for the indexed color image. The color table stores the colors used in the docu-
`ment and holds the maximum number of colors that can be displayed at once.
`The resolution option you choose affects the number of colors that can be dis-
`played at one time. If you select 4 bits per pixel, 16 colors can be displayed at a
`time; if you select 8 bits per pixel, 256 colors can be displayed at one time.
`
`The term “indexed color” refers to the fact that colors in the image are mapped
`or "indexed" to the colors in the color table. Color tables are also called "color
`
`lookup tables” because when a request for a color is made, the Adobe Photoshop
`program looks up the requested color in the table. If the color is not present in
`the color table, the program must match the requested color to the closest color
`in the color table, or simulate the requested color through dithering.
`
`"Dithering” juxtaposes colors to simulate another color. This is important to keep
`in mind when you want to convert an RGB image to an indexed color image. An
`RGB image can contain millions of colors, whereas an indexed color image can
`directly reference only 256 colors. As a result, the transitions between dithered
`colors in an indexed color image are not as smooth as the transitions between
`colors in an RG13 image.
`
`To simulate the color that is not present in the color table, the Adobe PhotoshOp
`program puts together pixels with colors that are in the color table. When you
`convert an RGB image to an indexed color image, you have several options for
`how colors will be dithered. You can select the options for the conversion to an
`indexed color image in the RGB to Indexed Color Options dialog box. The
`options include the number of bits of information per pixel (the resolution), the
`type of color table, and the dithering option (if any) to be used.
`
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`To convert an RGB image to an indexed color image:
`1. Choose Indexed Color from the Mode menu.
`
`The RGB to Indexed Color dialog box appears.
`
`HEB Color to lndened Color...
`
`Resolution:
`
`0 3 bitsfpinel
`O 4 hits/pixel
`O 5 hits/pixel
`O 6 bits/pinel
`O 'u' bitsfpiHel
`Q) B bitsfpiHel
`