`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 wantfor the resolution.
`
`4. To resample andresize 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.
`
`© High
`
`Auto Resolution...
`
`Printer:
`
`(dots/inch)
`
`Screen: [ss (lines/inch)
`
`Quality Required:
`© Draft
`@ Medium
`
`3, Use the pop-up menusbeside the Printer and Screen text boxes to choosethe 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 orclick 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 resample andresize the image, click OK.
`
`208
`
`User Guide
`
`234
`234
`
`
`
`Adjusting the size of the printed output
`You can controlthe size and resolution of your printed output using the Size/Ruler
`option in the Page Setup dialog box, which is accessed through the File menu. The
`Size/Ruler option allows you to changethe 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 couldalso usethis
`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.
`
`Whenyou use the Size/Ruler option, the image’s size and resolution are adjusted
`in tandem to maintain the image’s dimensionsin terms of pixels. You should use
`the Resample commandto adjust the images’s size and the amount of pixel
`information in the image independently.
`
`For example, you can use the Size/Ruler option to print a 300-dpi resolution
`imageat 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 apply to the rulers.
`To changethesize of the image when output, you specify the height and width,
`or the resolution at which you want the image to be output. The Size/Rulers
`option maintains the proportions of the image (the height-to-width ratio) asit
`adjusts the imagesize.
`
`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.
`
`LaserWriter
`cones)
`
`
`
`(Cx)
`Pages: @ an Orrom[ —Jree[—] (Cancer
`
`(Correct for Printing Colors
`
`Cover Page: @No (© First Page ©) Last Page
`Paper Source: @® Paper Cassette © Manual Feed
`
`@ Color/Grayscale © Black & White
`
`(J frint selected frea ang
`(FO Print Seleested Channe) Gnle
`(Print Using Color PostScript
`
`Encoding:
`© ASCII
`® Binary
`
`Chapter14: Resizing Images
`
`209
`
`235
`235
`
`
`
`. Click the Size/Rulers button.
`
`The Image Size/Ruler Units dialog box appears.
`
`. To select the size units, press down on the shadowedboxbeside the text box for
`the option you want.
`
`A pop-up menulisting the units options appears.
`
`. Holding down the mouse button, drag until the units option you wantis 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 dimensionis 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.
`
`210
`
`User Guide
`
`236
`236
`
`
`
`Section 4: Advanced Image Editing
`
`and Color Correction
`
`237
`237
`
`
`
`
`
`From a scan of a single 35mm colorslide 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 theselection 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 just part
`of the underlying color to show through. She saved bothselections of the cups as
`separate alpha channels; after proofing the output, she accessed the alpha channels as
`masks to makefinal color adjustments.
`
`238
`238
`
`
`
`—
`
`Chapter 15: Converting Images and Working with Channels
`
`The Adobe Photoshop program treats images as a series of channels. The use of
`channels makesit 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 adjusting their respec-
`tive channels; and easily add detail to an image, such as text, and just as easily
`adjust the detail or removeit later.
`
`This chapter discusses channels, image types, and how images can be converted
`from onetype 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, includingbit-
`mapped, 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 imagehas atleast three channels, onefor 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 havingto reselect a selection.
`
`Additional channels are called alpha channels. An alpha channelis a separate, 8-
`bit gray-scale image within a document that you can usefor 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
`colorall are single-channel images; RGB, HSL, and HSB havethree channels;
`CMYK has four channels, and a multichannel imageis a gray-scale image with
`more than one channel. Otherdistinctions between image types are shownin the
`chart following.
`
`211
`
`239
`239
`
`
`
`
`Bits/pixel
`per
`Definition/when to use:
`No. Channels
`Channel
`
`
`
`
`
`Bitmapped images contain only1
`
`
`bit of colorpec or white) perpixel.
`Since few editing options are avail-
`
`able, it's best to convert the image
`to gray-scale mode, then back to
`Bitmap for export if necessary.
`
`
`
`Gray Scale
`
`
`
`
`
`RGB Color
`
`5
`
`
`
`This modeis 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
`Grayscale image, it becomes a
`ultichanne!l image.
`
` Indexed
`
`Indexed Color images contain up
`
`Color
`to 256 colors/grays stored in an
`
`
`editable lookuptable. Limited image
`editingis available. Indexed Color
`mode is used whenediting 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
`displayed and edited on screen. Up
`
`
`
`to 13 additional alpha channels can
`
`be added. RGB images should be
`converted to CMYK modeto print
`
`
`color separations.
`
`
` You generally convert to CMYK
`CMYK Color
`modefrom 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
`shue
`—
`* saturation
`
`separate channels in HSL mode. HSL
`* 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
`* hue
`in a color image can be edited as
`® saturation
`separate channels in HSL mode. HSL
`
`® brightness
`channels appear as gray-scale, but
`retain all color information when
`
`converted back to a color mode.
`
`
`Multichannel
`
`Adding a channelto a gray-scale
`image, or deleting a channel from
`
`an RGB, CMYK, HSL, or HSB image
`
`creates a Multichannel image. Color
`images converted to Multichannel
`
`mode become gray-scale images.
`
`
`
`
`
`
`
`
`
`
`212
`
`User Guide
`
`240
`240
`
`
`
`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 througha redfilter, a greenfilter, anda
`bluefilter 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) asif it were a separate document. Once youselect a
`channel, any changes you makeare applied to that channel. The Channel sub-
`menu under the Mode menuallows you to view and activate the various
`channels in an image, Note that the RGB commandin the Channel submenu
`allows you to display and work on the three channels of an RGB image together.
`
`Besides selecting channels from the Mode menu, you canalso select channels
`from the keyboard. Each channel in a multichannel imageis 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 hold down the Commandkey and type in the
`channel number. For example, the cyan channel in a CMYK imageis thefirst
`channel listed in the Channels submenuandis 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
`
`213
`
`241
`241
`
`
`
`3. When you haveselected the desired channel, release the mouse button.
`
`BiEMe|D
`Gray Scale
`Indexed Color...
`“RGB Color
`CMYK Color
`HSL Color
`HSB Color
`Multichannel
`
`
`en)
`>
`
`ciapaveeinsaTaeamoaaseaaialiieta| \ oa
`
`
`New Channel Green*32
`
`Delete Channel
`Blue
`363
`
`
`
`Split Channels
`Merge {haanels..
`
`
`
`The selected channel is displayed. The name of the channel appearsin thetitle
`bar of the active window,
`
`Display options
`The Adobe Photoshopprogram lets you have several documents open at the same
`time. Each document can havea 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 documentswill be incorrect (unless your monitor uses more than
`8 bits of information per pixel.
`
`To makethe colors in the inactive documents appear closer to their actual colors,
`you can use the Use system palette option in the Preferences dialog box, whichis
`accessed via the Preferences command in the Edit menu. When youselect this
`option, the colors in the system’s color table, rather than the colors in the active
`document'scolor table, are used to display colorsin all the open documents. This
`meansthat the colors for the active document will be less accurate than if the
`
`option is unchecked.
`
`When you work on an RGB or CMYK image, you can chooseto display the
`channels in their respective colors or as gray scales. Because each channelis nor-
`mally printed as a halftone, gray-scale image on thefile that is sent to the
`printer, you may want to view each channel in a gray-scale format, rather than
`in its color format.
`
`214
`
`User Guide
`
`242
`242
`
`
`
`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.
`
`© 32 bits/pixel
`
`Preferences...
`Display:
`— Colored separations
`(] Use system palette
`&] Use DirectSetEntries
`Clipboard Export:
`© Disabled
`as ietataet
`© 4 bits/pixel
`© 8 bits/pixel
`@ 8 bits/pixel, System Palette
`© 16 bits/pixel
`
`Separation Setup...
`
`-
`
`Column Size:
`Width:
`.
`Gutter: [|
`Interpolation Method:
`© Nearest Neighbor
`oO Bilinear
`@ Bicubic
`
`2. Click the Use system palette option in the Preferences dialog box.
`
`Whenthe box beside this option is checked, the optionis active.
`
`To view each RGB or CMYK channelin color, click the Colored Separations
`option in the Preferences dialog box. When the Colored Separations check boxis
`checked, the channels are displayed in color.
`
`Converting one image type to another
`You can convert one type of image to another using the commandsin the Mode
`menu. For example, you can easily convert an RGB image to a CMYK image,
`whichis 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, RGB
`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 imageto a gray-scale image, and then toa bitmapped image.) The
`image types in the Mode menuto which you cannot convert the active document
`are dimmed. The following sections describe the options available for the differ-
`ent conversions.
`
`Chapter 15: Converting Images and Working with Channels—215
`
`243
`243
`
`
`
`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 imageis a single-channel image consisting of various levels of gray.
`Depending on the number ofbits 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 O (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 Scaleto Bit-
`map Options dialog box. You also specify the resolution you want the bitmap-
`type imageto be in the Outputtext box. By default, the current image resolution
`is entered in the Input and Output text boxes.
`
`Youalso havefive 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 conver-
`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.
`
`Gray Scale to Bitmap...
`
`
`
`Input:|72 (pixels/inch)
`
`O fusiem Patiem
`
`Output:
`
`|e
`
`(pixels/inch)
`
`Conversion Method:
`
`© 50% Threshold
`© Pattern Dither
`@® Diffusion Dither
`© Halftone Screen...
`
`2. To specify the resolution of the gray-scale image and the bitmap-type image, use
`the pop-up menusbeside the text boxes to choose the size units you want, either
`pixels per inch or pixels per centimeter.
`
`216
`
`User Guide
`
`244
`244
`
`
`
`. Enter a value for the resolution of the gray-scale image in the Input text box.
`
`. Press the Tab key orclick in the Output text box.
`
`. Enter a value for the desired resolution of the bitmap-type image.
`. Click OK.
`
`ing a bitmapped conversion method
`You havea 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 producea half-
`tone for output on non-PostScriptprinters; if you use this option, you also specify
`the screen frequency and angle, and dot shape.
`
`To select the bitmapped conversion method you wantto use:
`
`. Choose Bitmap from the Mode menu.
`
`The Gray Scale to Bitmap options dialog box appears.
`
`.
`
`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.
`
`3 4
`
`, 5 6
`
`Select
`
`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.
`
`Chapter 15: Converting Images and Working with Channels
`
`217
`
`245
`245
`
`
`
`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 upper-left 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 changesit to either black or white. The
`amount of erroris transferred to surrounding pixels before they are converted to
`black or white. In this way, the error is diffused throughout the image. The result
`
`Original gray-scale image
`
`has a grainy, film-like texture.
`50% threshold conversion method
`
`Pattern dither conversion method
`
`Diffusion dither conversion method
`
`218
`
`User Guide
`
`246
`246
`
`
`
`—~
`
`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 grain, 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
`
`219
`
`247
`247
`
`
`
`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 producea halftone on out-
`put devices that do not use the PostScript page description language.
`
`Whenyouselect the Halftone Screen option and click OK, the Halftone Screen
`dialog box appears. You usethis dialog box to specify the frequency and angle of
`the screen, as well as the dot shape.
`
`Halftone Screen...
`:
`
`Frequency: [EMM [(ines/ineny]TT
`Angle:
`(degrees)
`Shape:
`@ Round
`© Elliptical
`© Line
`© Square
`
`Load...
`
`© Cross
`
`The screen frequencyis the screen ruling or screen frequencyof the halftone
`screen. The screen frequency you use dependslargely 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-line halftone screens.
`
`Thescreen 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 halftone 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.
`
`220
`
`User Guide
`
`248
`248
`
`
`
`To specify the halftone screen:
`
`1. Choose Bitmap from the Mode menu.
`
`The Gray Scale to Bitmap dialog box appears.
`
`2. Select the Halftone Screen option and click OK.
`
`The Halftone Screen dialog box appears.
`
`3. Use the pop-up menu beside the Frequency text box to choosethesize units you
`want, either lines per inch or lines per centimeter.
`— . Double-click in the Frequency text box.
`wn . Enter the pixels per inch or pixels per centimeter (from 1.0 to 999.9) for the
`screen frequency you wantto use.
`6. To specify the screen angle, press the Tab keyorclick in the Angle text box. Enter
`a value from -180 to 180 degrees for the screen angle.
`
`7. To select the dot shape you want, click the nameof the dot shape. The available
`dot shapesare round,elliptical, line, square, and cross.
`
`8. 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 Channels
`
`221
`
`249
`249
`
`
`
`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 methodsare 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 chooseto scale
`downthesize 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 thesize of the gray-scale image by 50
`percent, enter 2 for the size ratio. If a number greater than 1 is entered for thesize
`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 imageto a gray-scale image:
`. Choose Gray Scale from the Mode menu.
`
`The Bitmap to Gray Scale dialog box appears.
`
`
`
`Bitmap to Gray Scale...
`
`Size Ratio: |
`
`2. Enter a value for the size ratio you want.
`3. Click OK.
`
`The bitmap-type image is converted to a gray-scale image.
`
`222
`
`User Guide
`
`250
`250
`
`
`
`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 hereis not to the specific Multichannel
`image type.
`
`Whenthe 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. If the red, green, or blue channelis
`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 channelas the gray-scale image and
`discards the other channels.
`
`Before you complete a conversion, an alert box will appear to confirm your
`action.
`
`To convert 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.
`
`
`
`Chapter15: Converting Images and Working with Channels
`
`223
`
`251
`251
`
`
`
`Making color conversions
`You can convert an image to any of the color image types that the Adobe
`Photoshop program supports: indexed color, 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 conversionsfollow.
`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
`
`Whenyou convert a gray-scale image to an indexed color or RGB 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 adjust 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 imageis split into three channels called hue, saturation, and luminance;
`when you convert to an HSB image, the imageis split into hue, saturation, and
`brightness channels.
`
`Brightness is defined as the additive primary color (red, green, or blue) with the
`highest color value. Luminanceis defined as the highest of the RGB values plus
`the lowest of the RGB values divided by two. With HSL and HSB 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.
`
`224
`
`User Guide
`
`252
`252
`
`
`
`& File Edit
`
`ERPEEM image Select Window
`Bitmap
`Untitled) 0) EaaS____]a
`Gray Scale
`S|
`Indexed Color...
`[™=ay
`~ RGB Color
`CMYK Color
`ete Te
`HSB Color
`Multichannel
`
`New Channel
`Delete Channel
`
`
`Bitmap
`utited-1 (ue, 1.)Sz
`Gray Scale
`Iniesed Cotor
`RGB Color
`EMYK Calar
`“HSL Color
`SH Colar
`Multichannel
`
`Color Table
`
`Channel
`
`New Channel
`Delete Channel
`
`eae)
`Saturation’ #2
`Lightness
`:
`
`;
`
`Merge Channels...
`
`Split Channels
`
`:
`
`HSL color, hue channel showing
`
`Chapter 15: Converting Images and Working with Channels—225
`
`253
`253
`
`
`
`Converting HSB and HSL images to RGB images
`
`Once you have adjusted 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 HSB 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 renamed1, 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 RGB 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 channelsis converted to a gray-scale 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.”
`
`226
`
`User Guide
`
`254
`254
`
`
`
`S File Edit MEET image Select Window
`Eee Bitmap
`nt! lO ————=r:
`Gray Scale
`Indexed Color...
`“AGB Color
`CMYK Color
`HSL Color
`HSB Color
`Multichannel
`
`
`
`New Channel
`Delete Channet
`
`Split Channels
`Merge Chananris...
`
`"
`
`
`
`& File Eait Mire image Select Window
`Bitmap
`Gray Scale
`lniiexed Color
`RGB Color
`“CMYK Color
`HSL Color
`HSH Caler
`Multichannel
`
`New Channel
`Delete Channel
`
`Magenta #2
`Yellow 5
`Black
`364
`
`Split Channels
`|_ Merge Channels...
`——-
`
`
`
`a
`CMYKcolor, cyan channel showing
`
`Chapter 15: Converting Images and Working with Channels
`
`227
`
`255
`255
`
`
`
`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 8-bit 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 dacu-
`ment and holds the maximum number of colors that can be displayed at once.
`The resolution option you choose affects the numberof 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 requestfor a color is made, the Adobe Photoshop
`program looks up the requested color in thetable. 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 anothercolor. This is important to keep
`in mind when you wantto convert an RGB imageto 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 RGB 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.
`
`228
`
`User Guide
`
`256
`256
`
`
`
`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.
`
`RGB Color to Indexed Color...
`
`Resolution:
`
`(3 bits/pixel
`© 4 bits/pixel
`©5bits/pixel
`© 6 bits/pixel
`©? bits/pixel
`@ 8 bits/pixel
`Palette:
`
`Dither:
`
`@ Diffusion
`¥
`
`O Exact
`© System
`@ Adaptive
`
`O None
`© Pattern
`
`2. Select the options you wantby clicking the button beside the option.
`
`|edi
`
`,
`
`pe a
`
`RGB