JPEGs - Having Them Your Way

JPEG Optimization in Photoshop 6.0



		In general, when saving an image to a JPEG file format, the greater the amount of JPEG compression used can often result in a new image with considerable loss of detail. However, in Photoshop 6, a new option lets you selectively maintain the fine detailed areas of your image even though the file shows a significant reduction in size. Here's how it's done. 1) Open an image and select the areas where you wish to preserve a high degree of fine detail. Saving a file using a moderate (medium quality) to heavy (low quality) JPEG compression scheme usually produces overall images that suffer from both poor color rendition and lower image sharpness. This technique will preserve both of these details in the selected areas.



	2) In this example, I feathered my selections, using the *Feather Selection* command. A radius of about 5 pixels was chosen. (Experiment with this setting to get the best results for your image).



	3) Save the selection areas using the *Select>Save Selection* command. It will appear as an Alpha Channel in the Channels Palette. At this point you can choose Deselect from the Selection menu.


	4) Now, save your final image using the *Save for Web* option located in the File drop-down menu. A Dialogue Box will open. Make sure the *Settings* box is set to JPEG (either medium or low quality). To the right of the *Quality* box there is a small white circular icon. This icon activates a new feature found only in Photoshop 6.0. If you pass the cursor over it a pop-up appears and says "*Use Channel to modify quality setting*". Select this icon.



	5) Once this new icon is selected, the *Modify Quality Setting* box appears. Under *Channel, choose the Alpha Channel you just created. The White* slider controls the quality of the selected area. The *Black* slider controls the quality of the unselected areas. Experiment, moving the black and white sliders. Preview the changes to both the quality of the selected and unselected areas as well as the file size.


	6) The JPEG is previewed on the right side in this example. The original is on the left. Notice on the right how the bear's face and part of the tree trunk are fully sharp yet everything else is pixelated. That's because we chose to optimize only these selected areas in the final JPEG file. The non-selected areas lost a great amount of detail during the JPEG conversion. Of course, for this demo, we exagerated the output of the optimized JPEG file to make it better to visualize the final image. Using this technique, you can dramatically reduce file sizes yet maintain the quality of the important parts of your image.