Opening raw documents

A raw document is a plain binary file stripped of all extraneous information. It contains no compression scheme, specifies no bit depth or image size, and offers no color mode. Each byte of data indicates a brightness value on a single color channel, and that's it. Photoshop offers this function specifically so you can open images created in undocumented formats, such as those created on mainframe computers.

To open an image of unknown origin, choose File ^ Open As. Then select the desired image from the scrolling list and choose Raw (*.raW) from the Open As pop-up menu. After you press Enter, the dialog box shown in Figure 3-18 appears, featuring these options:

4 Width, Height: If you know the dimensions of the image in pixels, enter the values in these option boxes.

4 Swap: Click this button to swap the Width value with the Height value.

4 Count: Enter the number of color channels in this option box. If the document is an RGB image, enter 3; if it is a CMYK image, enter 4.

4 Interleaved: Select this value if the color values are stored sequentially by pixels. In an RGB image, the first byte represents the red value for the first pixel, the second byte represents the green value for that pixel, the third the blue value, and so on. If you turn this check box off, the first byte represents the red value for the first pixel, the second value represents the red value for the second pixel, and so on. When Photoshop finishes describing the red channel, it describes the green channel and then the blue channel.

4 Depth: Select the number of bits per color channel. Most images contain 8 bits per channel, but scientific scans from mainframe computers may contain 16.

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Figure 3-18: Photoshop requires you to specify the size of an image and the number of color channels when you open an image that does not conform to a standardized file format.

♦ Byte Order: If you specify 16 bits per channel, you must tell Photoshop whether the image comes from a Mac or a PC.

♦ Header: This value tells Photoshop how many bytes of data at the beginning of the file comprise header information it can ignore.

♦ Retain When Saving: If the Header value is greater than zero, you can instruct Photoshop to retain this data when you save the image in a different format.

♦ Guess: If you know the Width and Height values, but you don't know the number of bytes in the header — or vice versa — you can ask Photoshop for help. Fill in either the Dimensions or Header information and then click the Guess button to ask Photoshop to take a stab at the unknown value. Photoshop estimates all this information when the Raw Options dialog box first appears. Generally speaking, if it doesn't estimate correctly the first time around, you're on your own. But hey, the Guess button is worth a shot.

Tip If a raw document is a CMYK image, it opens as an RGB image with an extra mask ing channel. To display the image correctly, choose Image Mode Multichannel to free the four channels from their incorrect relationship. Then recombine them by choosing Image ^ Mode ^ CMYK Color.

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Photoshop Secrets

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