If you'll be printing to an inkjet printer or posting your image on the Web, you need RGB color mode. (Despite the CMYK inks that you load into your inkjet printer, the printer's software expects — and must receive — RGB color data.) If you're prepping an image for inclusion in a page layout document destined for a commercial offset press, you need CMYK. (You select the image's color mode from the ImageOMode menu.) That's the simple summary. Here's a bit more detail, presented in the order in which you're likely to need the various color modes:
i RGB: RGB is the color mode for digital photos, computer monitors, the World Wide Web, and inkjet printers. All colors are recorded as proportions of the three component colors (red, green, and blue). RGB color is recorded in the three color channels (described a bit later in this chapter). RGB is an additive color mode — that is, the more of each component color you add, the closer you get to white.
i CMYK: CMYK is used primarily for printing on a commercial offset press, but you might need it for a color laser printer or a high-end inkjet printer with which you use a RIP (raster image processor, which is a specialized bit of hardware or software that lets your inkjet pretend it's a printing press). CMYK is the color mode of magazines, books, and other mass-produced printed material (such as the example in Figure 6-2). CMYK is a subtractive color mode — that is, the less of each component color you have, the closer you are to white.
i Grayscale: When most people talk about a black-and-white photo, they really mean grayscale. The image does contain black and white but also a wide range of grays in between. You might use grayscale mode for Web-based images or for prints. Keep in mind that unless your inkjet printer is designed to reproduce grayscale images with black and gray inks (or black and light-black inks), you probably won't be happy with grayscale output. Using just one black ink doesn't reproduce the full range of grays in the image. Using the color inks adds a tint to the image You do have an alternative for grayscale images: Send them to the local photo lab for printing.
i Indexed Color: Using a color table, or list of up to 256 specific colors, Indexed Color mode is for the Web. You save GIF and perhaps PNG-8 images in Indexed Color, but only those file formats require such a limited number of colors. Things like buttons on your Web page (which need only a couple of colors) should be created as GIFs using Indexed Color mode. That keeps the file size down, reducing the amount of space the image requires on your Web server and also speeding the download time (how long it takes for the image to appear on your site-visitor's monitor).
i L*a*b: Also known as Lab and CIELAB (and pronounced either lab, as the dog or a research facility, or verbally spelled out, as el-ay-be), this is a color mode that you might use when producing certain special effects or using certain techniques in Photoshop, but it's not one in which you'll save your final artwork. The three channels in an Lab image are
• Lightness, which records the brightness of each pixel
• a, which records the color of the pixel on a green-to-red axis
• b, which records each pixel's color value on a blue-to-yellow axis
You shouldn't print Lab images on an inkjet or post them on your Web site. You might see (elsewhere!) a tip that you should convert your RGB or CMYK images to Lab mode before using one of Photoshop's Sharpen filters. Bah! Apply the Unsharp Mask filter, use the EditOFade Unsharp Mask command, and change the blending mode from Normal to Luminosity. Same result, less work, and less potential for degradation of your image.
i Duotone: Duotone (including tritone and quadtone) is a very specialized color mode, exclusively for commercial printing, that uses only two (or three or four) inks spread throughout your image. Although that might sound good for an inkjet printer, in fact, Duotone is not an acceptable color mode for inkjets. Duotone images require that specific premixed inks are poured into the presses, which isn't something that you can do to your inkjet.
i Multichannel: Like Duotone, Multichannel is a color mode restricted to commercial printing because it depends on specific premixed colors of ink that are applied to the paper. Unlike Duotone, in which the inks are generally spread across the page, Multichannel images use certain inks in certain areas. You might need Multichannel mode when creating a logo for a client.
i Bitmap: Bitmap color mode is true black and white (as you see in Figure 6-3). Each pixel is either black or white. The placement of the black and white pixels produces shading, but the image doesn't really have any gray pixels. You might use Bitmap mode to create images for some wireless devices, use on the Web, or commercial print, but that's about it.
Converting between color modes or gamuts (done with the ImageOMode menu) can reduce the quality of your image by compressing different colors into a single color value. You would not, for example, want to convert from RGB (which has a comparatively large number of colors available) to CMYK (with a more restricted color gamut) and then back to RGB. After colors are compressed by a conversion, you won't restore their original values by converting back to a wider gamut.
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