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The JPEG Wizard  

The JPEG Wizard

JPEG Compression Optimization

JPEG Compression Optimization is the process of compressing an image in such a way as to obtain the best possible tradeoff between image quality and file size. This is achieved by adjusting the Luminance and Chrominance values in the image. See Luminance and Chrominance and Subsampling.

Since JPEG compression is a "lossy" compression technique, image quality is always sacrificed for compressed file size. The JPEG Wizard provides control over the output file size using "Regional Compression" and "Overall Compression" controls.

Overall Compression is the amount of compression applied to the entire image. This is similar to typical JPEG compression applications when the source data is NOT JPEG compressed. In the case where the source data IS JPEG compressed, Overall Compression is dissimilar to typical applications: JPEG data is maintained in its compressed state and the overall compression is a conversion to consecutively higher compressed states.

Regional Compression is the process of specifying regions in an image to have lower amounts of compression (and loss) than the background of the image.

Compression Optimization Procedure

· Open an Image File.

Open an image using the File Open dialog. Note the information displayed in this dialog about the image. Double click on the image preview button to view a full size version of the image if desired. Take advantage of the Windows Explorer-like controls in this dialog to rename, delete, copy and manage files on the disk.

The opened image will be displayed in the main window of The JPEG Wizard. The two upper tabs Input and Output display the source image data and the results of the compression and other settings made within The JPEG Wizard.

The initial compression applied will be as specified in the "Edit | Preferences" dialog.

· Set Overall Compression slider.

Now things start to get subjective! The Overall Compression slider is used to set the level of the least compression for regions of interest. So move the slider down until areas of the image that should have the highest quality just begin to degrade. Then back off the Overall Compression slider a bit.

Notice that the output file size is displayed at the foot of the image window and is labeled Output. Always try to minimize this file size while taking steps to keep the quality desired in the Output view tab.

· Add Region(s)

Right click on the image and select the Add Region item, or click on the Add Region button on the toolbar, or press the Add Region button on the Compress tab.

A small rectangular box will appear at the top left of the image. This is a new region, it can be moved and resized using click-and-drag by grabbing an edge or the whole region. Move and resize it to cover area of the image that should maintain high quality. Repeat this procedure adding regions to cover the rest of the regions of interest in the image.

The right-mouse click menu available when over a region allows saving the region as a separate file.

The JPEG Wizard allows up to 15 regions; each of which can belong to one of three groups. Each group has its own slider bar for compression settings. Change the Group a region belongs to by right-clicking on the region and selecting Member of Group.

· Adjust Background Compression

The Background of the image is that area not contained in a region. For best results, and in most cases, this is the majority of the area of the image.

Move the Background compression slider downward until the background areas of the image begin to suffer unacceptable quality loss. Then back the slider up a notch.

· Adjust Group Sliders

Now move the Group sliders downward to increase the compression in Regions of Interest. Take advantage of regions belonging to different groups. Text can belong to a Group using less compression than important non-text regions, for example.


Now experiment with all the settings, make drastic changes, play around, get a feel for how it works on specific image types, change the size of regions, move regions among groups, etc. This is a very subjective process, each user has different specifications for acceptable loss.

· Save the file.

Pegasus Imaging Corporation The JPEG Wizard


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