File Formats

The Imspector MSR File Format

The Imspector ‘.msr’ file format is a native binary format and now api exists to read or write it in its entirety. As window positions, data dependencies, hardware configuration and settings measurement state and configuration settings are saved in each document reading the entirety of this information is hardly desirable for other programs. The file format evolves with Imspector but old files will always remain readable with newer versions of Imspector or a conversion tool will be made available.

The data and directly associated meta-data contained in any Imspector file is organized according to the specification of the OBF (*.obf) file format and Imspector also reads OBF files in an intuitive way. There are bindings for OBF to both c++, Matlab and Java and we are working on Labview and Python bindings to allow reading of Imspector data into most applications used in scientific data analysis and to allow writing to an Imspector-readable format from Labview.

The C++ API for OBF will be rather stable as the file format evolves. The format itself is forward and backwards compatible between all versions released.


The fact that OBF is compatible in both directions helps when reading a newer version .msr file with an old Imspector copy. While window positions and some analysis related data is omitted, all physical data and the associated meta-data remains readable. Just (temporally) rename the file to .obf and open it in Imspector.


Some older versions of Imspector do not write OBF files and used the DBL (*.dbl) format as an exchange file format. This format is also documented below for completeness.

The OBF File Format

While it is recommended that you use the ANSI-C or C++ interface provided by the OmasIo dll to read and write OBF files the file structure is relatively simple and it should be straightforward to implement a reader or writer for OBF files. A native OBF file starts with the following binary file header. Please note that all structures saved in binary format into an OBF file have packsize 1 and that all binary data is stored in little endian byte order (i.e. the byte order used on x86, x64, Itanium and Alpha platforms)

typedef struct _OBF_FILE_HEADER
     //! Must be "OMAS_BF\n" followed by 0xff 0xff
     char magic_header[10];
     //! The actual format version of the file.
     omas_UINT32 format_version;
     //! The position of the first stack header in the file
     omas_UINT64 first_stack_pos;
     //! Length of following UTF8 description (bytes).
     omas_UINT32 descr_len;

The header is immediately followed by descr_len bytes of a UTF-8 string containing the file description which is preferably in xml format such that it can be parsed into a OPropstruct by the function omas_xml2prop() described in a later chapter.

From format version 2 on the file description string is followed by the meta data position

omas_UINT64 meta_data_position_ ;

The header members are

The magic header identifies compatible OBF files and no read routine should go on if it does not match.
The format version is used by newer routines to read older lacking features but the file format is designed to be upwards and downwards compatible. That is, newer features and additional information is to be added such that the old routines should be able read as much information from files of a newer version as possible. This document describes format version 1.
Location of the first stack header in the file. Passing this to a seek() function should position the file pointer to the beginning of a stack header (see below)
Length of the file description string following this header immediately in bytes (not in characters). The string is defined to be in UTF-8 encoding.

OBF is an embeddable file format. i.e. a more feature-rich file format may choose to be readable as OBF by having this file header at its beginning, writing any additional information between the header and any stacks as long as the first stack header is found where specified by first_stack_pos. Each stack starts with a binary header of the following structure:

typedef struct _OBF_STACK_HEADER
   //! Magic header. As long as this matches the compiled-in magic string
   //! the format is forward and backward compatible.
   //! For this version of the header the magic string is "OMAS_BF_STACK\n"
   //! followed by 0xff 0xff
   char magic_header[16]{};
   //! The version of the file format for backwards compatibility.
   omas_UINT32 format_version{};
   //! The rank of the stack.
   omas_UINT32 rank{};
   //! The number of pixels along the axes
   //! The physical length of the stack axes
   double len[OMAS_BF_MAX_DIMENSIONS]{};
   //! The physical offset of the stack
   double off[OMAS_BF_MAX_DIMENSIONS]{};
   //! The data type of the stack on disk.
   omas_DT dt{};
   //! The type of compression. Currently 1 for zip and 0 for none
   omas_UINT32 compression_type{};
   //! The compression level 0-9
   omas_UINT32 compression_level{};
   //! The length of the utf-8 name of the stack in bytes
   omas_UINT32 name_len{};
   //! The length of the utf-8 description in bytes. It should be a valid xml
   //! description. However if it is not, a reading routine should still read
   //! the stack and save the description as metadata containing a single
   //! string.
   omas_UINT32 descr_len{};
   //! Do not touch! A value of 0 will indicate data-less stack to pre-versions
   omas_UINT64 reserved{};
   //! The length of the data on the disk. For version 6 stacks this is just the offset from the
   //! end of the header plus description to the stack footer which needs to be read before reading
    //! the stack.
   omas_UINT64 data_len_disk{};
   //! The next stack position in the file
   omas_UINT64 next_stack_pos{};


The header is immediately followed by name_len bytes of a UTF-8 string containing the stack name and descr_len bytes containing the UTF-8 encoded stack description, which is, again, preferably in xml format such that it can be parsed into a OPropstruct by omas_xml2prop(). For format versions smaller than 6, or if num_chunk_positions in the footer is zero, binary data follows immediately after the description and takes up exactly data_len_disk bytes. It may be compressed using zlib (see below).

newline The header members are

The magic header identifies compatible OBF stacks and a read routine should stop reading the file when it is not found at the specified position.
The format version for backwards compatibility. This allows versions being set per stack if necessary. E.g. the stack footer is present only in version 1, not in version 0 and a write routine may choose to write stacks as version 0 omitting the footer and other stacks (in the same file) as version 1 including the footer. IMPORTANT: A reader for version n is allowed to read the stack of higher version as long as it found the magic_header and as long as it seeks to the end of the footer using its size member before reading the variably sized components.
The number of used dimensions. The following members are valid only up to e.g. res[rank-1].
res (only the first rank members are valid)
The number res[i]. is the number of pixels the stack has along the i’th dimension.
len (only the first rank members are valid)
The physical length along each used dimension. Units may be given as part of the dimension labels in the footer. The physical center position of the k*th* along the i*th* axis is given by off[i] + (.5 + k)*len[i]/res[i] where k runs from 0! to \lstinline!res[i] - 1.
off (only the first rank members are valid)
The physical offset. May be used to specify relative positions of stack volumes inside a larger measurement space.
dt (see OmasTypes.h for the actual values)

The binary data type as stored on disk. The {bf Omas} binary types are explained in detail later together with their helper routines. For the file format the constants are:

#define OMAS_DT_AUTO    0x00000000 // Automatically determine the data type
#define OMAS_DT_UINT8   0x00000001 // An unsigned byte
#define OMAS_DT_SINT8   0x00000002 // A signed char
#define OMAS_DT_UINT16  0x00000004 // A 16 bit word value
#define OMAS_DT_SINT16  0x00000008 // A 16 bit signed integer
#define OMAS_DT_UINT32  0x00000010 // A 32 bit unsigned integer
#define OMAS_DT_SINT32  0x00000020 // A 32 bit signed integer
#define OMAS_DT_REAL32  0x00000040 // A 32 bit floating point value (float, )
#define OMAS_DT_REAL64  0x00000080 // A 64 bit floating point value (double, )
#define OMAS_DT_RGB     0x00000400 // Byte RGB, 3 samples per pixel
#define OMAS_DT_RGB4    0x00000800 // Byte RGB, 4 samples per pixel.
#define OMAS_DT_UINT64  0x00001000 // A 64 bit unsigned integer
#define OMAS_DT_SINT64  0x00002000 // A 64 bit signed integer
#define OMAS_DT_BOOL    0x00010000 // A c++ boolean

Each of the numeric types has a complex counterpart by setting the following bit in dt:

#define OMAS_DT_COMPLEX 0x40000000 // Is set, if this is a complex array.

So a stack containing std::complex<float> values would have

The type of compression used. Currently only the values {bf 0} (no compression) and {bf 1} (ZIP compression) are supported.
The compression level used. This is whatever the library allows. For ZIP compression the levels are 0 to 9 from fastest to strongest.
The length in bytes of the UTF-8 encoded stack name following this header immediately.
The length in bytes of the UTF-8 encoded stack description following the name.
Out of use.
Pointer to the location of the next stack header.

For stacks with format_version >= 1 the binary data is immediately followed by the stack footer

//! Stack footer
typedef struct _OBF_STACK_FOOTER
   // VERSION 1, no footer before VERSION 1

   //! Size of the struct
   omas_UINT32 size{};

   //! Entries are != 0 for all axes that have a pixel position array following.
   omas_UINT32 has_col_positions[OMAS_BF_MAX_DIMENSIONS]{};
   //! Entries are != 0 for all aces that have a label following
   omas_UINT32 has_col_labels[OMAS_BF_MAX_DIMENSIONS]{};

   // VERSION 1A??

   //! Length of a free metadata string which has been superseded by the tag dictionary. It may
   //! still be there for some files of version < VERSION 4.
   //! The metadata-string immediately follows col positions and col labels.
   omas_UINT32 metadata_length{};

   // VERSION 2

   //! Si units of the value carried
   OBF_SI_UNIT si_value{};
   //! Si units of the axes

   // VERSION 3

   //! The number of flush points
   omas_UINT64 num_flush_points{};
   //! The flush block size
   omas_UINT64 flush_block_size{};

   // VERSION 4

   //! The total length of the tag dictionary following the flush positions the dictionary
   //! consists of a number of entries of the following type ending with zero unit32:
   //! <len of key [uint32]><key><len of val [uint32]><val>
   omas_UINT64 tag_dictionary_length{};

   // VERSION 5

   //! Where on disk all the meta-data ends. This is only important for formats that read a
   //! container in which OBF resides as a file format and for removing extra space in the
   //! compacting routine
   omas_UINT64 stack_end_disk{};

   //! Should always be 1, as we want forward- and backwards compatibility but should still be
   //! honored in readers as an emergency break for the future.
   //! If we break forward compatibility this will be noted here and a new number != 1 will be
   //! named in the comment
   omas_UINT32 min_format_version{};

   // VERSION 5a ("blind version increase due to bioformats")

   //! The position where the stack ends on disk. The space between stack_end_disk and
   //! stack_end_used_disk is unused and not allowed to be used by anything as it can be
   //! cleared at any time
   omas_UINT64 stack_end_used_disk{};

   // VERSION 6

   //! The total number of samples available on disk. By convention all remaining data is
   //! assumed to be zero or undefined. If this is less than the data contained of the stack
   //! it is safe to assume that the stack was truncated by ending the measurement early.
   //! If 0, the number of samples written is the one expected from the stack size.
   omas_UINT64 samples_written{ 0 };

   //! The number of chunk positions in V6 chunk wise writing
   omas_UINT64 num_chunk_positions{ 0 };


where the OBF_SI_UNIT structure is defined as follows:

//! A fraction, ideally should be reduced when writing to file
typedef struct _OBF_SI_FRACTION
   omas_SINT32 numerator;
   omas_SINT32 denominator;


//! The dimensions and scaling factor of an SI unit. For each of th base and supplemental
//! units the exponent is saved as a fraction.
//! Ordering for the exponents array is as follows:
//! exponents[0]: Meters (M)
//! exponents[1]: Kilograms (KG)
//! exponents[2]: Seconds (S)
//! exponents[3]: Amperes (A)
//! exponents[4]: Kelvin (K)
//! exponents[5]: Moles (MOL)
//! exponents[6]: Candela (CD)
//! exponents[7]: Radian (R)
//! exponents[8]: Steradian (SR)
typedef struct _OBF_SI_UNIT
   OBF_SI_FRACTION exponents[9];
   double scalefactor;


The footer contains additional meta-information that is too large to be saved as a string and/or is to be made available without the need for xml parsing. Future versions of the footer may become larger so a read routine should always read the known members and then seek to a position footer.size bytes after its beginning before starting to read the variable sized parts of the meta information. In detail: The header members are

The size of this structure on disk. Read the known part of the structure and discard the footer.size - sizeof(OBF_STACK_FOOTER) bytes. This allows a reader written for a lower version to read stacks of a higher version simply omitting meta-data that has been added to the new version. In case breaking changes will be introduced, the magic header will be changed and the changes will be described in this document.
For those dimensions for which has_col_positions[i] != 0 an array of res[i] (64bit) double values is appended after the label strings (see below) which signify the position of the column along its axis. If present the len! and \lstinline!off should be ignored in favor of the position values.
For those dimensions for which has_col_labels[i] != 0 an array of res[i] label strings is appended after the column position arrays. Each label string starts in the form (omas_UINT32)n:char[n] where n is the length of the string. It is thus read out by reading a 32bit integer n and then reading n bytes forming an UTF-8 encoded string.
Immediately after the label strings a block of memory is appended which is a string in UTF-8 format which contains meta-data interpreted on a higher level in the OmasIo xml format for properties described elsewhere. This entails e.g. the stack position and orientation in a global coordinate system etc. As it becomes important some of it may find its way into the obf specification appended to the header in a binary format. While you can use this field for your own meta-data this is not encouraged. The field is intended to be filled in a standard way that OBF readers may or may not read. Custom meta-data should be saved in the file and stack description fields, preferably also in UTF-8 xml(see below). Nevertheless, readers should not throw or report a fatal error when they do not understand the data contained in this field - they should issue a warning.
For The SI units of the stack values.
The SI units of the stack axes.
For zip compressed stacks this is the number of full flush points the zlib compression has created for fast seeking. The flush point positions relative to the beginning of the zlib compressed data follow immediately after the meta data as an array omas_UINT64 flush_positions[num_flush_points]. When uncompressing only a window of the stack starting at pos the inflator may start decompressing data at the disk position flush_positions[n] where n is the largest integer with flush_block_size*n <= pos. Please note that there is no ZLIB header written at that position, so the inflator needs to be initialized in ‘raw’ format i.e. inflateInit2(h, -15) needs to be called in zlib.
The number of (uncompressed) bytes between full flush points. See above.
The number of bytes contained in the tag dictionary that follows the flush points. The tag dictionary is organized as in the way outlined above. key names names are Utf8 encoded. Most key values are xml as the description. The ‘imspector’ tag e.g. provides all imspector related meta-data.
See comment.
The minimum format version needed to successfully read this stack.
See comment.
If smaller than the expected amount of samples, the stack was truncated at this point. Remaining data is zero by default (unless you want to show uninitialized data differently).
Chunk positions following the tag dictionary when stacks have been written interleaved. Each chunk position is a pair of 64bit unsigned integers:

The footer is immediately followed by rank label strings (encoded in the same form as the column labels) which are in turn followed by the column positions, column labels, meta data and flush positions, tag dictionary and chunk positions as outlined above.

If num_chunk_positions != 0, the data is not written continously but interleaved with other stacks or other file content. It is organized as follows:

Let start_pos be the position following the stack header description. The data is then organized in chunks starting at start_pos and the file_offset positions of the chunks. The size of each chunk is the difference between its logical_offset and the logical_offset of the next chunk. The logical and file offset of the first (unlisted) chunk is zero, i.e. it starts at start_pos and its length is the logical offset of the first listed chunk. The length of the last chunk is samples_written minus its logical offset.

If several segments have the same first value it means that only the last is non-empty. Pseudo code to read all data:

struct ChunkPosition
   omas_UINT64 logical_offset;
   omas_UINT64 file_offset;

std::vector<ChunkPosition> chunk_pos = chunk_positions_read_from_footer();

std::uint64_t pos = 0;;
std::size_t idx = 0;

while(pos < footer.samples_written)
   auto bytes_to_read = footer.samples_written - pos;
   std::uint64_t seek_pos = -1;

   if (idx < footer.num_chunk_positions)
      if (pos + bytes_to_read > chunk_pos[idx].logical_offset)
         bytes_to_read = chunk_pos[idx].logical_offset - pos;
         seek_pos = chunk_pos[idx].file_offset + start_pos;

   if (bytes_to_read > 0)
      read_from_file(file, bytes_to_read);
   if (seek_pos != -1)
   pos += bytes_to_read;


Backwards and forward compatibility: As outlined above, OBF files are designed to be backwards and forward compatible. However, version 6 files where samples_written is different from the total amount of samples in the stack or where num_chunk_points is not zero break forward compatibility of old readers. Readers that do not check min_format_version will fail miserably, those that do should disregard the stack and notify the user.

Please note that min_format_version is the minimum format version that needs to be implemented to successfully read all the information from the file this version knows about. The footer and the data that follows are allowed to grow in future versions without this value changing. You may want to notify the user if the stack or file format have gone up as information might be available but disregarded. Make sure to use the size member of the footer to jump to the variable sized data section.


SI units While simply writing SI units as a string in a certain format would have been simpler and would have allowed to display the units directly in a simple reader (and have them written more easily after user input) this format was chosen as it allows bindings to existing units implementations i.e. in C/C++, Python and Matlab more easily.

For C/C++ OmasIo contains a simple formatter and parser for unit strings into this format.

The DBL File Format (deprecated)

The DBL format is a simple binary file containing a single up to four-dimensional data stack with some header information about physical dimensions of the sampled a volume. The header is exactly 128 bytes long

unsigned char header[128];

For historical reasons it has an mixed little endian and big endian format. The rank of the stack is not explicitly contained but the pixel number of higher dimensions are simply set to 1. The number of pixels along the four possible dimensions are given by

res[2] = header[0]*256 + header[1];
res[1] = header[2]*256 + header[3];
res[0] = header[4]*256 + header[5];
res[3] = header[6]*256 + header[7];

The physical length is

len[2] = *((float *) (header +  8);
len[1] = *((float *) (header + 12);
len[0] = *((float *) (header + 16);
len[3] = *((float *) (header + 20);

where the floats are stored in little endian format. Reading on big endian machines involves flipping bytes before casting to float. The header is followed by floating point data in little endian byte order. If header[24] == 1 it is 32bit floating point (float), otherwise it is 64bit floating point data (double).

The OmasIo API, Bindings

The OmasIo library implements the OBF and DBL file format providing both a C++ interface to OBF files. There are bindings using the C++ implementation for both Matlab and Python and in addition, a pure Java implementation of a reader is in the process of becoming part of BioFormats.


All .msr files written by Imspector conform to the OBF specification. Additional information is stored between the stacks and before the first stack but any .msr file (except for very, very old ones) should be readable by a correctly implemented OBF reader.

Vice versa, Imspector reads .obf files. Because OBF is forward and backwards compatible this comes in handy when opening .msr files from newer versions of Imspector (as .msr is not forward compatible).

Meta information data model

Strictly speaking, the OBF file format does not specify the way meta information is to be associated with the file or data stacks within and because it can be embedded into arbitrary, more complex formats it even encourages the use of methods suitable for the task at hand.

For meta information that is to be shared by several applications it is however strongly encouraged that meta-information is saved as UTF-8 text in the file or stack description, preferably formatted as xml in a way compatible to the output of the omas_export_xml() function in the OmasIo library, described in a separate section. In C++, the easiest way to do this is to write the meta information into an OProp object and actually use the omas_export_xml() function to convert it to an xml string. For Matlab and Python, toolboxes are provided that can convert a (complex) variable into a compatible xml string and back. In fact these toolboxes, too first map the data into an OProp variable and then export it to xml and vice versa.

The OProp data model is strongly based on the Matlab data model. Data is organized in arrays of arbitrary numeric complex or real data type and arbitrary rank (with the special case of a scalar, which is a 1x1 array in Matlab), cell arrays (where each cell can contain data of a different type), structs (where each member is addressed by its name and can contain arbitrary data) and arrays of structs (with identical fields). In Matlab strings are one-dimensional character arrays. Matlab string arrays therefore will always contain strings of equal length (with shorter strings simply padded by NULL bytes). The toolbox will convert these to cell arrays of strings tagged with a special flag. On the C++ side they will look like cell arrays of strings but as long as the tag is untouched they will be converted back to string arrays on the Matlab side. Please note that usually it is preferable to use cell arrays of strings on the Matlab side to start with. Also, OProp knows empty ‘cells’ (an OProp with no content) which is mapped to an empty ‘double’ array in Matlab. There are similar mapping issues with other bindings like Python. The general Ansatz is that variables converted to xml by one language binding will produce the same variable when read back directly but that there is no guarantee that this applies once a property tree has been converted back and forth between different languages.