Optimizing is a trick we wish all users of GeoExpress knew about. If you have files in MrSID Generation 3 (MG3) format, and they were originally encoded as “optimizable” (it’s the default, so chances are good that they were), you can perform additional compression and a number of other operations on them without having to decode and reencode them.

Why is that useful? Not having to decode (i.e. bring the image out of wavelet space) has two advantages:

• Optimizing is much faster than encoding
• Optimize operations are free (if you’re using a data cartridge, they don’t charge against it)

Loading Images and Accessing Options
Among the six tabs on the Job list is one called Optimize (shown at right). You may have noticed this tab before and never felt like venturing into it. We urge you to gird up thy loins and use it. Images you want to optimize are staged on this tab.

After you load an image into the Optimize tab (and make sure it’s selected), you can access options in one of three ways:

• Right-click on the filename in the job list and choose Optimize Options… from the context menu.
• Choose Encode Options… from the Options menu.
• Click the More Optimize Options… button on the Properties tab.

In each case, a dialog box appears that has tabs for Input, Output, Optimize Settings, and Advanced Settings.

Basic optimization settings
On the Optimize Settings tab (shown below) you can specify additional compression by ratio or by target file size, and you can resample (reduce the dimensions) by factors of 2.

The options for compression based on encoding ratio and target file size are pretty self-explanatory. If you need to fit particular images on a CD, compress by target file size to make sure you stay under the limit. If you want your images to be uniformly compressed to a fifth of their current size, set a ratio of 20:1.

Resampling in GeoExpress means changing the resolution of an image. For example, if your image measures 1000 x 1000 and you resample by a factor of 2, the result will be a 500 x 500 image. Resampling is achieved by discarding zoom levels; thus, the number of zoom levels in the image (remember, MrSID is inherently multiresolutional) determines the factors available in the drop-down.

Advanced optimization settings
The Advanced Settings tab (shown below) gives you the following additional options, which are described further on:

• Frequency balance
• Sharpness
• One-pass or two-pass optimizer
• Optimizable
• Use temp file

Frequency Balance – This determines the emphasis given to edges and flat areas in color and grayscale images. Set frequency balance lower for images requiring precise edge definition. If precise edges are not as important as consistency in flat color areas, use a higher setting. Range of acceptable values is 0.0 – 10.0. Default is 2.

Sharpness – This determines the sharpness of boundaries between different areas of an image. Use a lower setting for images with large amounts of textured area (where color or intensity changes are occurring throughout a region, rather than just at a boundary). Use a higher setting for images with little textured area. Range of acceptable values is 0.0 – 1.0. Default is 0.0.

1-Pass or 2-Pass Optimizer – This option is only available for MrSID Generation 3 (MG3) images. The 1-pass and 2-pass optimizer parameters affect encode performance and memory usage. Although not as fast as the 1-pass optimizer, the 2-pass optimizer (used by default) requires much less memory because it splits the encode operation into two passes. This is required on very large files. Encoding with the 1-Pass Optimizer is faster but requires that the entire image be loaded into memory. If sufficient RAM is not available for the image selected, the encode job will fail.

The 2-Pass Optimizer is automatically used for Area of Interest encoding even if this checkbox has not been selected.

Note that the “Estimated Memory Usage” is only displayed when the 2-pass optimizer is used (look on the Properties tab or on the Advanced Settings tab of the Encode Options dialog box).

Optimizable – Select the Optimizable checkbox if you want the output MG3 file to be optimizable later on. By default this checkbox is selected.

Use Temp File – A temp file can be used to store necessary statistical data about the image data during processing. If your image is very large this can help alleviate the memory burden on a given job.

Let us know whether or not you find the optimization functionality useful and how you think it could be better.