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Binary labels are application-defined extensions to a VICAR image that are used to store ancilliary information about the image. They are made up of two parts: binary headers, which are extra records that appear at the beginning of the image, and binary prefixes, which are extra bytes that appear at the beginning of each image record. Binary labels are not part of the image data. In fact, an application will never even see the binary label data unless it specifically asks for it via a COND BINARY optional to x/ zvopen.
Binary headers are extra records that appear at the beginning of the file, between the standard label and the image. The number of records is specified by the NLB (Number of Lines of Binary) system label item. Binary headers are often thought of as extra ``lines'' of data, but depending on the file organization they can actually be extra lines, samples, or bands. The size of each binary header record is exactly the same as the size of each image record. The headers specified by NLB occur exactly once in the file, not once per band (BSQ organization) or once per line (BIL or BIP organizations).
Binary prefixes are extra bytes that appear at the beginning of each and every image record. The number of bytes is specified by the NBB (Number of Bytes of Binary) system label item. The image record consists of NBB bytes of binary prefix data, followed by the samples that make up one line (for BSQ organization). Other file organizations label the units differently: a record for BIL is NBB plus the samples that make up one band, while a record for BIP is NBB plus the bands that make up one sample. NBB is always specified in terms of bytes, not in terms of pixels, even if the pixels are larger than one byte.
Binary labels (both headers and prefixes) pose probably the most difficult porting challenge for application programmers. The basic problem is that the data stored in them is application-defined. The RTL has no idea what types of data are stored in the binary label, and therefore cannot automatically convert the data to the native host format as it does for image data. The application program has sole responsibility for converting the data to the native host format when it is read.
To make things worse, only a few application programs actually understand any given kind of binary label. For example, the Voyager project has a definition for what goes in the binary label of its images. The Voyager-specific processing programs know how to interpret these labels and can make use of them. The Galileo project also has a definition for its binary labels. Galileo-specific programs can interpret the Galileo binary labels. However, the two kinds of binary labels are different, and programs written for one cannot make use of the other. The Magellan project has its own binary labels in yet another format.
The variety of binary labels poses a problem for a general-purpose application. How do you deal with the binary labels? They could certainly be ignored, but then the information would be lost. This would be a frequent occurence for a program like COPY or LABEL, and the binary labels would disappear often. Therefore, many general-purpose programs simply copy the binary labels from the input to the output, thereby preserving the information. As long as the information is only copied, not used, the application does not need to know what kind of data it is.
But what happens when you start mixing machine types? Say you have a file with binary labels that was written on a VAX. You want to run COPY on the file from a Sun. COPY reads the input image, and writes the output image. The RTL automatically converts the image data from VAX to Sun format on input, and so the file gets written in Sun format. The system labels also say it is in Sun format. COPY also reads the binary labels, and writes them to the output file. The binary labels cannot be converted, as neither the RTL nor the COPY program know what data types are in the binary labels. Therefore, the binary labels get written out in the only way possible: in VAX format.
This is clearly untenable, as the system labels say the file is in Sun format, but the binary labels are still in VAX format.
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