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Photoshop in Focus

Using an alpha channel in optimization can save precious file size and speed up downloading. Here's how:

Open a photograph to be optimized as a JPEG. Because this is a non-destructive process, you can use the original rather than a copy of the image.

Create an alpha channel that uses white in areas of your image that should retain the highest quality (typically the subject of the image) and black in less important areas (typically the background). Save the alpha channel as Optimization Mask or a similarly descriptive name.

Open the Save for Web dialog box.

Select the quality setting that you want for the image's subject (the areas that are white in your mask).

Click the mask icon to the right of the Quality field in Save for Web.

In the Modify Quality Setting dialog box (shown in Figure 24.35), select your alpha channel from the pop-up menu.

Figure 24.35. In this example, the areas of the image covered by white in the mask are optimized at Quality 80, and black areas of the mask are optimized at Quality 10. The file size savings is about 12%.

Drag the left stop of the slider to the quality setting you want for the background of your image. This is the quality setting used for black areas of the mask. Gray areas of the mask (typically the transition from foreground to background) use quality settings between the minimum and maximum values.

Reposition the dialog box as necessary to see the effect of the quality settings on your image. The active preview pane's display is automatically updated, as is the file size information at the bottom of the pane. Remember that you can also adjust the right-hand slider stop, too.

When you've reached the perfect balance between image quality and file size, click OK in the Modify Quality Setting dialog box, and then click Save in Save for Web to save the optimized file.



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