Photoshop's Image Modes

Bitmap Mode

Bitmap images are made up of pixels that can have only two values: black or white. Bitmap images are thus called 1 bit images. They take the least amount of memory or disk space. This mode is very suitable for the Line art for it preserves the sharpness of the lines. One thing to be remembered is many image editing options are inactive in Bitmap mode.

To convert an image to Bitmap mode, the image mode must be set to Grayscale mode. In Grayscale mode, the Bitmap mode option is active in Mode menu.

Convert To Bitmap Mode

1. Choose Mode from the Image menu and then choose Bitmap to display the Bitmap dialog box .

2. Enter the Output resolution. By default, the current resolution will be displayed for output resolution. The output resolution will be the resolution of the resulting image.

3. You can select one of the five options for the Bitmap conversion method.

a) 50 % Threshold converts all gray pixels at or above 128 (50% black) to white and all gray values below 128 to black.

b) Pattern Dither is used to generate a dithered pattern of the black and white pixels to represent the grayscale image.

c) In Diffusion Dither the diffusion pattern creates the illusion of gray values using only black and white pixels.

d) Halftone Screen imitates the process of applying a halftone screen to a grayscale image. Halftone dots break down the gray levels of the image into dots of varying sizes. The dots of various sizes are used to form the resulting image. Thus a single solid ink color is required to print the grayscale image.

e) Custom Pattern is used to apply another image as the pattern to create the bitmap image. You create a special pattern for example wood grain pattern and apply them to the bitmap image.

Grayscale Mode

Any Grayscale image is represented by up to 256 shades of gray. The pixels brightness value is described by 8 bits of data contained in a pixel. 0 represents the black color and 255 represents white. The pixels are also represented as percentage of black where 0 % is 'White and 100 % is black. Bitmap, RGB, Lab or CMYK mode images can be converted to Grayscale mode. Using the luminosity values of the color images, the Grayscale image is created. Remaining all color information is discarded. While converting Grayscale image to RGB or CMYK the grayscale values are converted to their nearest comparable values in the color space. Let us take an example to understand this., 230 Red, 230 Green and 230 Blue represent 10 % grayscale and similarly 45 % Cyan, 32 % Magenta, 32 % Yellow, 10 % Black represents 50 % grayscale.

Converting Color Image To Grayscale

1. Open any RGB, CMYK, Lab or Multichannel image.

2. Choose Mode form the Image menu and then select Grayscale.

3. A dialog box appears asking if you want to discard the color information. Click OK to discard it. Note that once you have discarded color information it won't get restored when the Grayscale image is converted to RGB mode.

RGB Mode

The. RGB mode color images are represented by using the various degree of luminosity intensity of each Red, Green, Blue color channel. The number of levels of each color channel in RGB is 0 to 255. 8 bits of data per pixel per color are used in RGB mode. When all the three color's values are equal they give different shades of Gray. For example when all three values are zero the pixel displays black and when all three are 255 the pixel displays white. This RGB mode is widely used mode. Moreover the RGB color gamut is much larger than the CMYK color gamut and hence provides more flexibility in the usage of colors. Converting Color Image To RGB

1. Open any image.
2. Choose Mode from the Image menu and then select RGB Color.

CMYK Mode

In CMYK mode, each pixel of the color image contains a value, which is represented by the percentage Cyan, Magenta, Yellow and Black colors. Light colors contain more percentage of colors whereas the dark colors contain higher percentage of color. This mode is mostly used for four color printing purpose.

Converting an image to CMYK mode requires many complicated steps, which are beyond the scope of this book. But the good news is that Photoshop takes care of all these steps.

Converting To CMYK

1. Open any Grayscale, RGB color or Lab color image.

2. Choose Mode from the Image menu and then choose CMYK Color.

LAB Mode

There are three, color channels in Lab mode. And each of them contains 8 bits of data for each pixel. "L" represents the lightness component of the image. Each pixel has L value lying between 0 and 100. The range for "a" value (green to red) and the "b" value (blue to yellow) is from +120 to 120. The interesting fact about Lab color model is that the color specifications are device independent. Thus Lab mode is the best mode to use when transferring images from one system to another. Practically Lab color modes are hardest to understand for you need to do a lot of experimenting with lightness and color values.

While converting an image to Lab mode you can adjust the "L" value without affecting the color. Then the image can be converted back to their original mode without affecting the colors which is otherwise not possible provided you do not change the "a" and "b" values.

Converting To LAB Mode

1. Open any Grayscale, RGB, CMYK, Duotone, Indexed Color or Multichannel image. Bitmap images cannot be converted to Lab Mode.

2. Choose Mode from the Image menu and then select Lab Color.

Indexed Color Mode

Indexed color images use a specific color palette, which contains up to 256 colors. If there are less than or equal to 256 colors then an indexed color image can contain just the colors used in the image. This makes the file size smaller and thus makes the display faster. In fact, because of this reason they are the best color modes for the images to be used in Web. Whenever a color image is converted into Indexed Color mode, Photoshop creates a Color LookUp Table (CLUT). This CLUT is used to store the color values for the images. It may happen that your image contains more than 256 colors then Photoshop finds the closest matches and builds an indexed color table. One more facility is you can edit the color table.

Converting To Indexed Color Mode

1. Open a RGB image and choose Mode from the Image menu and then select Indexed Color. A dialog box as shown in Fig 4.16 a ppears after selecting Indexed color.

2. Choose a Palette from the scroll down window. There are 10 palettes.

a) The Exact palette is used to create the image with exactly same colors used in the RGB file. Note that there must be less than or equal to 256 colors in the image otherwise this option will be grayed out.

b) The System (Macintosh) palette uses the Macintosh default Color palette. Select this palette when you are sure that the image is going to be used in a Macintosh system only. The System (Windows) palette uses the Windows default Color palette. Select this palette when you are sure that the image is going to be used in a Windows system only.

d) The Web palette uses a palette of 216 colors, which are recognized by the Web browser application. If you are exporting an image to GIF89a format, choose Web palette when converting, to Indexed color for the best results in both Macintosh and Windows system.

e) The Uniform palette uses a uniform sampling of the colors in the RGB image's gamut of colors. The colors of the Uniform Color palette are generated on the basis of the color depth.

f) The Perceptual palette is used to create custom palettes by giving priority to colors, which are more sensitive to human eye.

g) The Selective palette creates custom palettes similar to Perceptual color table but favoring broad areas of color and the preservation of Web colors.

h) The Adaptive palette creates the color table by choosing the colors with most common occurrences. The images, which are having more weight on specific colors, are the best suited one for this palette.

1. The Custom palette is used to build your own color table of up to 256 colors.

2. The Previous option uses the Custom or Adaptive Color palette, which was generated previously.

3. The Force is for the forceful inclusion of certain colors in the color table. This option gets highlighted only when the palette selected is Perceptual, Selective or Adaptive. The Black and White option adds a pure black and a pure white to the color table; the Primaries option adds red, green, blue, cyan, magenta, yellow, black, and white; the Web option adds the 216 web safe colors and the Custom option is used to define the custom colors.

4. If you check in the Transparency check box then transparency will be preserved and if it is deselected then the transparent areas will be filled with white or matte color if matte color is chosen.

5. When a RGB image with more than 256 colors are converted into Indexed color mode the resulting image appears posterized. When dithering is used the missing colors are simulated. If you use the Exact color palette then this option will be grayed out. There are four dithering options.

i. When None is selected and the missing colors are not simulated and hence the posterization becomes apparent.

ii. Pattern is selected then a pattern of pixels is created to simulate the missing color.

iii. If Diffusion is chosen then the pixels are offset to avoid an obvious pattern when dithering. This is the most common choice of dithering. Dithering is actually the level of interference in colors while browser interprets colors. The Dithering Amount will be active for this option only.

iv. If you choose the Noise option it will slice the image for placement in an HTML table. The Noise option helps to reduce seam patterns along sliced edges.

6. If you check the Preserve Exact colors check box then the color table will be protected from dithering.

Multichannel Mode

When color images are converted into Multichannel mode, the original channels are converted into spot color channels. It uses 256 levels of gray in each multiple channel. You can convert any image containing more than one color channel to multiple channels. For example if you convert an RGB image to Multichannel mode, the three channels are created named Cyan, Magenta, and Yellow representing the Red, Green and Blue channels of the RGB file. If you convert a CMYK image to Multichannel mode, four channels get created named Cyan, Magenta, Yellow and Black to represent the cyan, magenta, yellow and black plates respectively. Multichannel mode is not active for the Grayscale, Bitmap and Indexed color images for they contain single channel only. This is best suited for specialized printing.

Duotone Mode

The Duotone mode can be used for Grayscale images only. Duotone is an image that is comprised of two colors, each color representing a portion of the image's grayscale values. For converting an image to Duotone select Duotone form the Mode menu under Image menu. A Duotone dialog box appears. There are four modes available: Monotone, Duotone, Tritone and Quadtone. Monotones are 8 bit per pixel grayscale images printed with specific ink. Duotones are grayscale images printed with two colors. Tritones are grayscale images printed with three colors. Quadtones are grayscale images printed with four colors. When creating a Duotone, the gray values of the Grayscale images are designated to the colors chosen. Thus the image remains still an 8 bit perpixel image with gray values mapped to two colors. For Tritones this mapping is with three colors and for Quadtones this mapping is with four colors.

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