setheadermax

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

Purpose:

Usage:

where the following definitions apply:

filename

name of the 16 bit header file to be reviewed or altered (.hdr or .img optional)

max

the maximum 16 bit pixel value represented in the file

NOTE:

If no max is specified, the program will identify the current file type and global maximum

For type 1 16 bit files, legal values for max range from 1 to 65535

For type 2 16 bit files, legal values for max range from 1 to 32767

For type 3 16 bit files, legal values for max range from -32767 to 32767

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 files.

• 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.

Comments:

• This program will only modify the header 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). 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 type 2 and type 3 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 8 bit internal representation. By correctly specifying the global maximum, you can assure that you use the full range of available 8 bit values. Without rescaling to the true global maximum, you may end up with a very restricted range of 8 bit voxel values.

Error messages: (alphabetical)

____ only modifies files with 16 bits/pixel...

• According to the header file (.hdr file), the file that you have specified does not have 16 bits/pixel.
• If the header file is wrong, use scanheader to review its contents and then makeaheader to generate a corrected header.

this file's type cannot be evaluated

• The file that you have specified has a header minimum less than -32768, a header maximum greater than 65535, or the difference between the header maximum and the minimum exceeds 65535.
• Use scanheader to review the global maximum and minimum in the header.
• You should remake a corrected header using makeaheader.

Type 1 files cannot have negative maxima ...

• The header of the file specified is judged to be a type 1 file based on a current header global maximum of greater than 32767. You have requested that the global maximum be set to a negative value. Only type 3 files can have negative header global maxima.
• If this is actually a type 3 file, you need to remake the header using makeaheader. It is a good idea to review the current values first using scanheader.

Type 2 files cannot have negative maxima ...

• The header of the file specified is judged to be a type 2 file based on a current header global minimum of greater than 0. You have requested that the global maximum be set to a negative value. Only type 3 files can have negative header global maxima.
• If this is a type 3 file, you will need to remake the header using makeaheader. It is a good idea to review the current values first using scanheader.

Type 3 files cannot have maxima greater than 32767...

• The header of the file specified is judged to be a type 3 file based on a current header global minimum 0. You have requested that the global maximum be set to a value greater than 32767 which is accessible only to type 1 files).
• If this is a type 1 file, you will need to remake the header using makeaheader. It is a good idea to review the current values first using scanheader.

value ___ is out of range for all possible header types

• You have requested a header global maximum greater than 65535 which is inaccessible with 16 bit data.

WARNING: You have converted a type 1 file to a type 2 file...

• Don't panic, as long as you have followed the rule about not setting the header global maximum to a value less than the true global maximum, you can ignore this warning. The header maximum was previously greater than 32767 indicating that the file must be type 1. Now the header global maximum is less than 32768, indicating a type 2 file. If the value that you have given is truly the global maximum, then this conversion will have no effect.
• If the true global maximum is greater than this value, review the comments section and reset the global maximum back to the correct value (if the correct value is greater than 32767, you will get the next warning message when you reset the value.

WARNING: You have converted a type 2 file to a type 1 file...

• Unless you are deliberately trying to recover from a previous incorrect use of this program, you probably will want to undo what you just did. If the file really was a type 2 file to start with, raising the header global maximum will result in even fewer significant bits in your 8 bit representation than you would have with a global maximum of 32767, so set it back to this value or lower. As an additional incentive to set it back, don't forget that any numbers that were previously negative and therefore not defined and ignored have now become huge (greater than 32767) positive numbers. Setting the value back below 32767 will automatically evoke the preceding error message.

See also:

• scanheader
• makeaheader