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Advanced JPEG Optimizations

Sunday, March 2nd, 2014 by Servage

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Optimized JPEGs have slightly smaller file sizes and better color fidelity (although I’ve never been able to see the difference) than standard JPEGs.

For this reason, you should select the Optimized option if your image software offers it. Look for the Optimized option in Photoshop and third-party JPEG compression utilities. Fireworks does not offer the option as of this writing.

Remember to save JPEGs that are targeted for iPad Retina displays in Progressive format to circumvent a Safari function that automatically downgrades JPEGs over 2 megapixels (more than 2.1 million pixels in the image).

Blurring or smoothing the image

Because soft images compress smaller than sharp ones, Photoshop and Fireworks make it easy to blur the image slightly as part of the optimization process. In Photoshop, the tool is called Blur; in Fireworks, it’s Smoothing. Blurring makes the JPEG compression work better, resulting in a smaller file. If you don’t have these tools, you can soften the whole image yourself by applying a slight blur to the image with the Gaussian Blur filter (or similar) manually prior to export.

The downside of Blur and Smoothing filters is that they are applied evenly to the entire image. If you want to preserve detail in certain areas of the image, you can apply a blur filter just to the areas you don’t mind being blurry. When you’re done, export the JPEG as usual. The blurred areas will take full advantage of the JPEG compression, and your crisp areas will stay crisp.

Selective quality (Fireworks)

Not all image areas are created equal. You may wish to preserve detail in one area, such as a person’s face, but compress the heck out of the rest of the image. To this end, Fireworks gives us Selective Quality—a method for applying different amounts of JPEG compression within a single image: one setting for a selected area and another setting for the rest of the image.

To use the Selective Quality setting, select the areas of the image you want to preserve, then select Modify → Selective JPEG → Save Selection as JPEG Mask B. In the Optimize panel, you can set the Selective Quality for your selection or click the adjacent icon C to access the Selective JPEG dialog box D with a full set of options, such as preserving type and button quality and selecting a color for the masked area. The regular Quality setting will be used for all other areas of the image.

Your primary tool for optimizing JPEGs is the Quality (compression) setting. If your tools offer them, making the JPEG Optimized or applying Blur or Smoothing will make them smaller.

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