Alpha Bands and MrSID

All images encoded to MrSID Generation 4 (MG4) have an alpha channel or alpha band. If one is present, GeoExpress uses the alpha band in the input image. If no alpha band is present in the input image, then GeoExpress creates one.

The alpha band is always encoded losslessly, even if the rest of the image undergoes lossy compression.

MG2 and MG3 do not support alpha channels, but the transparency information in the alpha band of your source image is still preserved when encoding to MG2 and MG3. The alpha band will first be applied to the source image and then discarded from the output file.

Selecting a Method for Alpha Band Generation

To specify what method GeoExpress should use to create the alpha band, use the -alphasource or -as switch. Acceptable values are:

• "opaque" – Specifies that all pixels should be treated as valid image data. By default the Opaque method is selected.
• "auto" – Specifies that GeoExpress should query the image metadata for the transparency values and treat all pixels with matching band color values as transparent.
• "<value>" – Specifies a transparency value for each band. GeoExpress treats as transparent all pixels whose band color values match the specified values. This value can take a number of forms:

  • For 8- and 16-bit RGB and grayscale images a transparency color can be specified in RRR_GGG_BBB format.

  • For multispectral images, the values can be specified by a list in the form <bandlist>@<color>

    where <bandlist> is a comma separated list of

    • indices and/or
    • ranges denoted by a ':' or
    • all unspecified bands denoted by REMAINDER (if not specified, REMAINDER@0 is assumed)

    and <color> is a non-negative integral color value.

Examples:

1:5@0,6:8@255 

REMAINDER@0 

1:255@0 

1@127, REMAINDER@1 

For more examples and information on acceptable values, see Setting Transparency Metadata.

NOTE: For users of GeoExpress Standard Edition, the encoding of the alpha band does not additionally decrement your data cartridge. Your encode job will be metered the same with or without alpha bands.