| Description: The pixel_proc_file_name attribute gives the name of the file that documents the CRISM onboard compression options selected. Onboard compression converts a 14- bit DN with a value of 0-16383 to a 12-bit- 0-4095 value or 8-bit 0-255 value for downlink. Corrections are done on a line by line basis. The pixel processing file is a 4- column, 480-row text file. The four attributes in each row are the row number, the gain correction performed, the offset correction performed, and the 12 to 8 bit lookup table used if lossy compression is performed. Both gain and offset corrections are always performed to convert 14-bit to 12-bit values prior to downlink. First the offset is subtracted from the 14-bit value. Then the difference is multiplied by the gain to shorten the result to a 12-bit value. If lossy compression is being performed (if compression_type = 8_BIT), then the 12- to 8-bit lookup table value gives the table in the file named by lookup_table_file_name that was used to convert the 12-bit value to an 8- bit value. |