setheadermax

• purpose
• usage
• examples
• comments
• error messages
• see also

Purpose:

This program will reset the global maximum in the header of a 16 bit file to increase the number of significant figures retained if the file is loaded by an 8 bit version of the AIR package.

It will also optionally identify the current header file type (type 1, type 2, or type 3) and the current header global maximum.

Warning: If you have moved data from a big-endian machine to a little-endian machine (e.g., from a Sun to a PC) or vice versa, applying this program to the header file of such data will result in a failure to byte-swap the image data when loading it into AIR and other programs that use the header information to detect the need to byte-swap. You may be able to work around this problem by using the AIR program reorient to make a copy of the data (which will be fully byte-swapped during copying) and then applying this program the resulting header file.

Usage:

setheadermax filename [max]

where the following definitions apply:

filename

name of the 16 bit header file to be reviewed or altered

max

the maximum 16 bit pixel value represented in the file

Examples:

• Used without a maximum value, the program will indicate the current file type and the current global maximum. Type 1 files with global maxima less than 32767 will always be misidentified as type 2 files. As long as the global maximum truly is less than 32767, this misidentification is irrelevant because the binary representation of numbers between 0 and 32767 is identical for type 1 and type 2 data (assuming that your machine uses 2's complement representation of negative values).

• This will set the header maximum value in file mri.hdr to 18000. When file mri1 is loaded by an 8 bit version of any AIR program, the value 18000 will be mapped to an 8 bit pixel value of 255 and all lower values will be mapped proportionately. If there are actual values greater than 18000, they will be incorrrectly mapped.

Comments:

• This program will only modify the headers of 16 bit files
• You should never assign a global maximum in the header that is smaller than the true global maximum of the file. Doing so will result in values greater than the header global maximum being mapped incorrectly to values less than the header global maximum.
• It is possible to interconvert type 1 and type 2 files by changing whether the global maximum is greater or less than 32767. This has no effect on files with all values in the range 0 to 32767 since binary representation of both file types is identical in this range (assuming 2's complement representation of negative integers). So long as you follow the rule of never assigning a global maximum in the header that is smaller than the true global maximum of the file, you should not be able to generate any errors.
• If your display package uses the header information, changing the header maximum may alter the way that files are displayed. Specifically, lowering the global maximum may make all of the pixels appear proportionately brighter.
• The only effect that changing the global maximum has with regard to the AIR package is that is alters the way that 16 bit files are converted to when AIR has been compiled for 8 bit internal representation. If you are using a 16 bit version of AIR, this program is irrelevant.
• See the warning above regarding the use of this program on data that has been moved from big-endian to little-endian machines or vice versa.

Error messages: (alphabetical by case)

setheadermax only modifies files with 16 bits/pixel; this file has ____ bits/pixel

• This program only works for 16 bit files

value ____ is out of possible range for all header types

• supply a value in the appropriate range

See also:

• makeaheader
• scanheader