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Chapter 22. Save for Web and Image Optim... > Photoshop at Work: How to Optimize F...

Photoshop at Work: How to Optimize Files with Alpha Masks

Variable compression allows you to alter image compression and quality in different areas of the same image. You can focus more detail around a central character or image while allowing details to erode in unimportant areas, such as the background or flat areas of color. The result is an optimized image that delivers higher quality and lower bandwidth. To optimize files with alpha masks, follow these steps:

To create an alpha mask in Photoshop, open Photoshop and select File, Open. From the Open dialog box, select the file you want to optimize (see Figure 22.37).

Figure 22.37. Open the original image.

Select Window, Channels to launch the Channels palette. Select New Channel from the Channels palette menu. Modify any parameters as desired in the New Channel dialog box (including mask color and area definition options) and click OK to create the new channel.

Initially, the new channel will turn the image black. To view the channel and the image as a composite, go to the Channels palette and turn on visibility for all channels, including the new channel you just created.

In the final compressed image, white areas of the alpha channel will carry the most detail; black areas will have the least detail. With this in mind, paint the channel to emphasize image detail (see Figure 22.38). When you're finished, turn off visibility for the alpha channel in the Channels palette.

Figure 22.38. Paint the Alpha Mask.

Choose File, Save for Web to launch the Save for Web dialog box. In the Save for Web dialog box, select JPEG from the Optimize File Format menu. Click the icon next to the Quality control to launch the Modify Quality Setting dialog box.

As you paint, you must have only the alpha channel active; it will display in your open document as your selected mask color (red is the default). Also, remember to name your alpha channel and save the PSD file.

From the pop-up menu, select the channel created in step 2. With the Preview box checked, adjust the slider to specify the minimum or maximum quality setting to be applied, based on the selected channel (see Figure 22.39). Remember that the pure white channel areas correspond to the maximum settings, and the black areas correspond to the minimum settings. Click OK.

Figure 22.39. Apply JPEG quality adjustment through mask.

In the Save for Web dialog box, select GIF from the Optimize File Format menu and set the GIF settings as desired. You can use channels to modify the Dither, Lossy, and Colors settings by clicking the icon next to the associated control to open a dialog box and then by following the process in the preceding step.



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