Photoshop

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Vary optimization based on a mask

You can vary optimizations settings across an image (called weighted optimization) using masks to produces higher-quality results in critical image areas without greatly increasing file size. You can set weight optimization for the following settings:

  • Quality for JPEG images.

  • Dithering for GIF, PNG‑8, and WBMP images.

  • Lossy for GIF images.

  • Generated color table for GIF and PNG‑8, favoring colors in specific image areas.

To apply weighted optimization, you first create an alpha channel mask, or use the automatically generate mask for type layers and shape layers.

The white areas of the mask (which include type or shapes on shape layers) indicate the image areas that will have the highest level of image quality. The black areas are the least important parts of the image and will have lower JPEG quality, or the higher dithering and lossiness for GIF images. The level of optimization in gray areas of the mask varies depending on the gray level. For generated color tables, the white areas of the mask indicate important image areas, so more colors in the white areas are kept.

  1. In the Save For Web & Devices dialog box, click the Mask button  next to an option. For example, Quality for JPEG images or Lossy for GIF images.
  2. In the dialog box that appears, choose the mask that you want to apply. All the image text layers, all the image shape layers, or an alpha channel. You can choose more than one mask.

    A thumbnail of the mask appears in the lower right of the dialog box.

  3. Use the sliders to modify quality, lossiness, or dithering, or enter minimum or maximum values for the setting in the text boxes.

    The white slider affects the white areas of the mask and the black slider affects the black areas of the mask. Areas of the mask that are gray are partially affected so that the transition between areas is less abrupt.

    Note: For the Lossy setting, a higher number actually means greater compression, as opposed to greater quality (as in most other settings).