Kstars

Stars


Design document for the implementation of star catalog support in KStars.

Class Inheritance

  • StarComponent - Manages all stars
    • struct starName - Holds a star's names (defined within StarComponent)
  • DeepStarComponent - Manages unnamed stars (instances managed by StarComponent)
  • StarBlock - A block of many (usually 100) stars
  • StarBlockList - A list of StarBlocks; machinery to load data dynamically from catalog files
  • StarBlockFactory - Maintains a cache of recently used StarBlocks
  • BinFileHelper - A generic helper class used to deal with binary data files (See ../binfilehelper.h)
  • StarData - holds 32 bits of data describing a star (See ../skyobjects/stardata.h)
  • DeepStarData - holds 16 bits of data describing a (faint) star (See ../skyobjects/deepstardata.h)

Categorization of Stars

Stars could be either named or unnamed. Named stars and unnamed stars are treated differently in KStars. Stars having names are handled by the methods defined in class StarComponent. Unnamed stars are handled by the methods defined in class DeepStarComponent, which are invoked via the methods of StarComponent.

A second categorization is "static" and "dynamic" stars. The StarObjects for "static" stars are always kept in memory. They are loaded into memory when KStars initializes. "Dynamic" stars are loaded into memory from the disk when required. When the sky map is slewed to a new region, or zoomed to a greater depth, dynamic stars are loaded "on-demand" into memory from the disk and drawn. Named stars are always static. Unnamed stars could be either static or dynamic. Naturally, StarComponent deals only with static stars whereas DeepStarComponent deals with both static and dynamic stars.

A third categorization is "shallow" and "deep" stars. Shallow stars are the brighter stars, which have enough catalog data to require a 32-byte data structure (StarData) to hold them. Deep stars are the fainter ones, data for which can be fit into a 16-byte structure (DeepStarData). All named stars and all static stars are shallow. All deep stars are dynamic and unnamed. StarComponent manages only shallow stars. DeepStarComponent manages both shallow and deep stars.

Stars are indexed using multi-level Hierarchical Triangular Mesh to speed up drawing and searching for stars in the skymap.

Stars are stored in a custom Binary Format Binary Format to speed up reading and loading of data into memory.

Catalogs

As of this writing, KStars supports the following catalogs:

  1. Default named stars catalog ("namedstars.dat" in the data directory) [Together with default unnamed stars catalog, makes up all stars to mag 8]
    • Named
    • Static
    • Shallow
    • Managed by StarComponent's methods
    • Names of stars stored in "starnames.dat" in the data directory, in order of appearance in namedstars.dat
  2. Default unnamed stars catalog ("unnamedstars.dat" in the data directory) [Together with default named stars catalog, makes up all stars to mag 8]
  3. Tycho-2 catalog (Downloadable add-on) Prerequisite: Default catalogs (namedstars.dat, unnamedstars.dat and starnames.dat)
  4. USNO NOMAD (part) 100 million star catalog (Downloadable add-on) Prerequisite: Tycho-2 catalog

Named Stars

Named Stars are dealt with by StarComponent. Their case is very simple because they are all static stars. The star names are stored in a separate file (starnames.dat), in which the names are maintained in the same order in which stars appear in the named star catalog (namedstars.dat). Both files are opened while reading (See StarComponent::loadStaticData()) and data is read off from the two files sequentially, side-by-side.

Each record in starnames.dat is 40 bytes long, containing a long name (32 bytes) and a Bayer designation (8 bytes).

Each record in namedstars.dat is 32 bytes long, and holds a direct memory dump of struct StarData, made on a little-endian system. (On big endian systems, byte order inversion is done at runtime)

Named stars are read off the disk at initialization time by StarComponent::loadStaticStars() and are drawn by the loops in StarComponent::draw(). All of them reside in the memory throughout KStars' life.

Unnamed Stars

Much of the machinery listed out above go into maintenance of unnamed stars.

Unnamed stars are dealt with by DeepStarComponent.

StarComponent has a QVector of DeepStarComponent objects, which are created for each unnamed-star catalog loaded. (See StarComponent::loadDeepStarCatalogs().) StarComponent takes responsibility to call methods of the DeepStarComponents when required. For instance, StarComponent::draw() calls DeepStarComponent::draw() on each catalog, after drawing the named stars. (StarComponent, on the other hand, is managed by SkyMapComposite)

Memory map

Memory map of unnamed stars:

Loading unnamed stars

As mentioned earlier, unnamed stars may be both static and dynamic, and could be both deep and shallow.

Creation of unnamed StarObjects

TODO: This is outdated. Stars are now created differently.

IMPORTANT: Unnamed stars, for fast loading, are not constructed!!! They are allocated in memory (using malloc) and data is copied directly (using memcpy) onto them. This can lead to very strange segfaults with misleading backtraces if not understood carefully.

For each record read off the data file, a StarObject instance (plainStarTemplate) is loaded with the data using StarObject::init(...) and then memcpy()'d and held in a StarBlock

Loading unnamed, static stars

The method DeepStarComponent::loadStaticStars() (invoked from StarComponent) loads all static stars in an unnamed-star catalog.

The star data files are organized trixel by trixel. The method enters each trixel, creates a StarBlock of variable size, enough to hold all the unnamed stars in the current trixel, and then reads star-by-star into memory.

Static stars always occupy the first StarBlock in a StarBlockList.

Loading unnamed, dynamic stars

Dynamic stars are loaded on-demand (during the draw() call) in StarBlockList::fillToMag().

NOTE: This can be a problem for objectNearest(). objectNearest() calls work correctly only when the sky map is centred on the required trixel(s). For extending this when required, one must call StarBlockList::fillToMag() from objectNearest()

fillToMag() is invoked by DeepStarComponent::draw(), for every trixel present on-screen, with the current magnitude limit (calculated from the zoom level) as an argument. fillToMag()'s job is to ensure that the StarBlockList is filled with stars up to that magnitude by loading more stars from disk if required.

An important point to note is that StarBlockList maintains stars sorted by magnitude - brightest first - to match the data file. StarBlocks at the "tail end" of the StarBlockList are the faintest. fillToMag() quickly checks the magnitude of the last star in the last StarBlock of the StarBlockList and starts filling into the last StarBlock.

Each StarBlockList maintains the current read offset in the data file, so that it can quickly seek to the point where it last loaded and continue loading fainter stars. When stars are removed from a StarBlockList, the read offset is decremented.

StarBlocks holding dynamic stars are always of fixed size (default=100). There could be half-filled StarBlocks. (We save allocation overhead by recycling StarBlocks using StarBlockFactory. More about this later.) StarBlockList::fillToMag() is aware of this fact and continues to fill half-filled StarBlocks when required.

Recycling of StarBlocks

StarBlockFactory maintains a Least-Recently-Used cache of "dynamic" StarBlocks (which hold dynamic stars).

When StarBlockList::fillToMag() asks for new star blocks, StarBlockFactory sees if there are unused (trixel not-on-screen) StarBlocks lying around. If there are none, it allocates a new StarBlock and hands it over to StarBlockList. But if there are unused StarBlocks lying around, StarBlockFactory recycles them.

Imagine a situation where the map is panned. A trixel that was on-screen till now might no longer displayed on the screen, but a new trixel might come into focus. In this case, StarBlockFactory detaches the StarBlocks attached to the StarBlockList of the old trixel and gives them to the StarBlockList of the new trixel. The new data read off the disk is then StarObject::init()'ed into the StarBlock.

Each draw cycle in KStars has a drawID that is incremented upon every draw. This makes the drawID almost unique, since it has a very large period. StarBlockFactory has a local copy of this drawID that is maintained in sync with KStars' drawID. When stars are drawn in StarComponent::draw(), every dynamic StarBlock that's drawn is "touched" by calling StarBlockFactory::mark*().

When drawing a trixel, markFirst() is called on the first StarBlock in every StarBlockList and markNext() is called on subsequent ones. The freshest StarBlocks are maintained at the beginning of the linked list inside StarBlockFactory and equally-fresh StarBlocks are ordered in the same order as they appear in StarBlockList. Thus, StarBlockFactory can safely pluck out the last StarBlock in the linked list and recycle it (provided it was last touched in an older draw) without affecting the ordering of stars in StarBlockList (i.e. we ensure that we don't pluck out a StarBlock from in-between a StarBlockList. Only the last StarBlock of a StarBlockList can be released at any time). Thus, the ordering of StarBlocks in the linked-list of StarBlockFactory, is:

  • First, by drawID
  • Then, by catalog (i.e. unnamedstars.dat / Tycho-2 / USNO-NOMAD-1e8)
  • Then, by trixel (only those trixels that are on-screen are considered)
  • Finally, by magnitude

When StarBlockFactory wants to re-use a StarBlock, it first verifies that the drawID is outdated. Then, it calls StarBlockList::releaseBlock() on the parent StarBlockList and thus orphans the StarBlock. The orphaned StarBlock is handed over to the new StarBlockList which adopts it.

Thus, KStars initially allocates new StarBlocks until a saturation point is reached, where there are enough StarBlocks to keep recycling. Sagittarius has the maximum number of stars, and setting the focus on Sagittarius will usually lead to saturation (where KStars consumes 200-odd MB of memory with USNO-NOMAD-1e8), and this memory is not freed till KStars exits. A possible extension would be to free StarBlocks lazily after their drawID's get really outdated, so that the memory surge is temporary.

Star Catalog Files

Star catalogs are stored in the format described in Binary Format Binary Format section.

Star catalog files were created by first taking ASCII data and putting them into MySQL database tables and then dumped in a sorted manner into the catalog files. The tools used are in ../data/tools. The ASCII originals from public domain sources are available with Akarsh Simha and Jason Harris, or with the US Naval Observatory itself.

Stars in the catalog files are first ordered by trixel (the "Index" parameter in binary file format) and then ordered by magnitude (brightest first) within each trixel.

KStars uses two HTM levels:

  • Level 3: 512 trixels are used for the following data files:
    1. default namedstars.dat
    2. default unnnamedstar.dat: Togeather with #1, they make up all stars up to ~8th magnitude
    3. Downloadable Tycho-2 deepstars.dat, all stars up to magnitude ~12th
  • Level 6: 8192 trixels
    1. Downloadable USNO Nomad USNO-NOMAD-1e8.dat, all stars up to magnitude ~16th

Proper Motion

All star data in KStars has first order proper motion rates (called "dRA" and "dDec" or "pmRA" and "pmDec"), which are used to correct for proper motion.

This leads to a problem in the case of dynamic stars, as a star may cross over from one trixel to another trixel due to proper motion, in which case it might not be drawn despite its corrected coordinates being on-screen.

To solve this, stars are duplicated in all trixels that they will pass through in a span of +/-10,000 years from the epoch. StarObject has some commented code that can be used to track the position of a star. [Maybe this should be turned into a feature in future]

TODO: Describe details like the 25 arcminute "safety" margin.

Henry-Draper Catalog Numbers

StarObject allows for a Henry-Draper catalog number to be associated with a star. KStars allows for both static and dynamic stars to have an associated catalog number. This is currently used for the Henry-Draper number, although it could be extended to other catalogs.

Both static and dynamic stars from Tycho-2 appear in the HD catalog. To find dynamic stars by HD number, a downloadable add-on "Henry Draper Index" is available. This file contains a list of 4-byte offsets in sequential order of HD number. These offsets tell KStars where to find the star in the Tycho-2 catalog binary data file. See StarComponent::findByHDIndex() for details. See Henry Draper Indexing HDIndex section for details about the format followed by the Henry-Draper index file.

FIXME: The information above is not entirely correct. Star catalogs also include 4-byte HD parameters. Needs update.

Binary Format

Stars data are stored in binary format to speed up reading and loading data into memory. There are a number of tools in the data/tools directory that parses the original ASCII data files and convert them to a KStars compatible binary format.

The Header

Preamble

The preamble consists of the following:

A 124 byte-long string containing human-readable information about the file

A 2 byte-long endinanness indicator field, preinitialized to 'KS' (0x4B53) on the host machine

The purpose of the endianness indicator is to make the file portable across machines of different endianness. A machine on which the endianness indicator field is read as 'SK' instead of 'KS' is of different endinanness from the machine on which the file was generated, and hence byteswapping must be performed.

A 1 byte-long unsigned integer for the version of the binary format structure.

The header section is therefore 127 bytes long. The header range in the binary file is from address 0x0000 to 0x007E

Field Descriptor

The field descriptor describes the various fields used in the latter half of the data region. The field descriptor consists of the following:

A 2 byte-long unsigned integer, indicating the number of fields per record

A set of entries describing the fields, amounting to the number previously mentioned, each consisting of:

  • A 10 byte-long character array, providing a human-readable name for the field
  • A 1 byte-long integer carrying the size of the field in bytes
  • A 1 byte-long unsigned integer carrying the data type (see Appendix A)
  • A 4 byte-long integer storing a scale factor to divide the contents of the associated field by, to interpret it correctly. This is used for storing fixed-point fields. (Eg: A fixed-point number with 2 decimal places would have a scale factor of 100)

The set of entries is read directly into dataElement structure and byte swapping is performed if needed.

Most of KStars star binary files use 11 fields (0x0B), each is 16 bytes long (0x10). Therefore, the field descriptor total bytes count is 2 + 11 * 16 = 178 bytes. The field descriptor range in the binary file is 0x007F to 0x0130

Index Table

The format permits an index on one parameter. This index is the trixel ID. The parameter is to be represented by 4-bytes (eg: A running index from 0 to 4,294,967,295). The index table consists of the following:

A 4 byte unsigned integer indicating the number of index entries: Usually 512 (0x0200) or 8192 (0x2000)

Index table entries, each consisting of:

  • A 4 byte-long unsigned integer parameter, which is the index: 0 to 511 for HTM level 3, and 0 to 8191 for HTM Level 6
  • A 4 byte-long unsigned integer, storing the offset of the first record with this index parameter in the file
  • A 4 byte-long unsigned integer, storing the number of records under this index parameter.

Since multiple stars can share the same trixel ID, the number of records refer to the number of the stars enclosed within the trixel.

The index table total byte count depends on the index entries:

  • HTM Level 3: Total bytes = 4 bytes + (512 indexes * 12 bytes) = 6148 bytes (0x1804) The index table range in the binary file is 0x0131 to 0x1934
  • HTM Level 6: Total bytes = 4 bytes + (8192 indexes * 12 bytes) = 98308 bytes (0x18004) The index table range in the binary file is 0x0131 to 0x18134

The Data

Expansion Fields

Certain specific information about the contents of the field may be added here. The contents could be specific to each application, and could be empty. For KStars star data files, the entries in this region are:

  • Faint magnitude limit * 100, stored as a 16-bit integer
  • HTM Level for this file, stored as a 8-bit integer. The convention followed is that "N0000" would belong to a HTMesh of level 3.
  • Maximum Stars per Trixel, stored as a 2-byte unsigned integer.

The expansion field total bytes are (2 + 1 + 2) 5 bytes long.

  • HTM Level 3: The range is from 0x1935 to (0x1935 + 0x5) = 0x1939
  • HTM Level 3: The range is from 0x18135 to (0x18135 + 0x5) = 0x18139

Data Records

This region starts at the offset pointed out in the first entry of the index table. The data consists of a series of records, the details of whose fields (assumed to be placed continuously, without any gaps) are as described in the field descriptor in the header.

  • HTM Level 3: Data starts at 0x193A
  • HTM Level 6: Data starts at 0x1813A

NOTE: KStars overrides the above defined convention for performance gains and ease of programming in the star names file, where records are stored as single blocks of the 40 bytes long StarComponent::starName structure.

Henry Draper Indexing

The index file used to store the Henry Draper catalog indexes for KStars is just a list of 4-byte long offsets (to be treated as signed 32-bit integers ) describing the position of the Henry Draper stars' records in order, in the binary data files.

If the offset is negative, it indicates that the star is in the shallow star file and not in the Tycho-2 deep star add on. The actual offset in this case is just the absolute value of the offset. The first four bytes in the index file correspond to the offset of the star HD000001 in the data files, and so on.

Credits

Most of the memory layout and machinery outline described above was the brainchild of James Bowlin and it was implemented in code by Akarsh Simha. Jason Harris helped with the catalog data and also mentored Akarsh along with James. Some of the parsing tools in the kstars/data/tools directory have code due to James.

Author
Akarsh Simha
Jasem Mutlaq
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