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Can I set a value for transparency when creating a composite mosaic?

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Jeff
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No. When creating a composite mosaic of MrSID images, you cannot select a no-data value for transparency.

Details 
This behavior occurs because a composite mosaic simply writes the existing tiles to a new file.

Workaround 
In order to specify a new no-data value when creating a mosaic of MrSID images, you must create a flat mosaic rather than a composite. Creating a flattened mosaic completely re-encodes the images. Note: This will result in slower encoding times compared to a composite mosaic.

Mosaicking existing MrSIDs, whether composite or flattened, will not decrement your data cartridge.

The MG3 and MG4 formats treat transparently differently. Following are procedures for each format.

To create a flat MG3 mosaic and set its transparency no-data value: 

  1. Click the Create New Mosaic button on the toolbar. The Create New Mosaic dialog appears.
  2. Select MrSID Generation 3 from the Output Format drop-down menu and click OK. (Note: By default the “Create flat mosaic” radio button is selected.)
  3. Click the Add Tiles button to browse for and add tiles.
  4. Choose Advanced Options from the Options menu, then select the Transparency 
  5. Select the Set transparency (no-data) value checkbox, then select each band and either enter a color value in the Edit Value field or click Transparency Color Picker to browse for a color.
  6. Click OK until all dialogs are closed.

To create a flat MG4 mosaic and set its transparency no-data value: 

  1. Click the Create New Mosaic button on the toolbar. The Create New Mosaic dialog appears.
  2. Click OK. (Note: By default MrSID Generation 4 is selected as the output format and the “Create flat mosaic” radio button is selected.)
  3. Click the Add Tiles button to browse for and add tiles.
  4. Choose Encode Options from the Options menu, then select the Alpha 
  5. Select the Regenerate Alpha Band checkbox at the bottom of the tab. The Alpha Source options become available.
  6. Select the Specify Value radio button, then select each band and either enter a color value in the Edit Value field or click Transparency Color Picker to browse for a color.
  7. Click OK until all dialogs are closed.