wiki:Grass7/RasterLib

Raster library in GRASS7

GIS library has been split into G_() (general routines) and Rast_() (raster-related routines) part. See also GRASS 7 ideas collection.

Progress:

Status: done

Replacement raster format

GRASS's long-standing raster format is overdue for a major overhaul.

Below you will find some ideas and roadmaps for future work.

The idea of this page is to collect ideas and flesh out a
specification so that when the change occurs, all the
components will be in place, pitfalls expected, and
the implementation, when it comes, quick and painless. Most
importantly it can serve to keep interested parties informed
and working together instead of in parallel forks.

Any changes to the data format will necessitate a bump in
major version number (i.e. from GRASS 6 to GRASS 7) so if
possible changes should happen in the same development cycle,
and relatively minor changes should be held back in
experimental status until a major change is committed.

Core raster format

Lead developer: Glynn Clements

  • Storage in tiles instead of by row
    • Reasoning: Glynn said on the mailing list: "In most cases, single-level tiled storage will give you close to the same performance with a lot less complexity."
      • The G3d library already implements a tile based approach with tile caching and tile file buffer for arbitrary read and write access, have a look at G3d tile layout
    • Function needed to check whether tiles are all null, or all the same value.
    • What tile size should be used? Could be user/program specified, or standard value of something like 64x64. If there isn't a fixed value then there should be utility program that can convert tiled rasters to different tile sizes.
      • The tile size in the G3d library is variable and can be set when a new map is created. The program r3.retile can be used to store raster3d maps with different tile size. Additionally a function is implemented to compute the optimal tile dimension for a given max size in bytes.
    • However, row-oriented storage has the advantage that you can easily skip entire rows when downsampling. Using e.g. 64x1 "tiles" would provide both optimisations, but at the expense of reduced compression, as you need a pointer (8 bytes) for each tile, and the compression has to be restarted for each tile.
  • Merge NULL file into main data array:
    • This approach is implemented in the G3d library

Notes about NULL file format

  • Current cell_misc/null file size in GRASS GIS 7: The size of the cell_misc/<mapname>/null file depends only upon the number of rows and columns, not the data. It's one bit (null/non-null) for each cell, rounded to an integral number of bytes per row (i.e. the number of columns is rounded up to the nearest multiple of 8).
  • Compression idea: see #2349

Directory structure

  • Centralize map components in $MAPSET/raster/$MAPNAME/* instead of many $MAPSET/cell/$MAPNAME, etc. directories. Many library functions and modules will need to be updated. The GRASS 6 vector format has already been ported to this structure.
  • demo 2-way conversion scripts: Gforge patch #372

Meta-data support

The existing raster meta-data handling is rather weak (currently stored in $MAPSET/hist/$MAPNAME). Total replacement will be the best option. See metadata support.

Recent discussions

Other suggestions:

  • Time series in GRASS support
  • Unit Of Measurements (UOM) support
  • Scale and offset support
  • Band Interleaved by Pixel (BIP) access and Band Interleaved by Line (BIL) modes

Function name changes from GRASS 6 to GRASS 7

See Grass7/RasterLib/ListOfFunctions.

Troubleshooting

  • To avoid the problem "ERROR: Input window changed while maps are open for read", essentially replace G_get_set_window() with Rast_get_window() - see e.g. r47601. Note that Rast_set_window() should be called before the map is opened.
Last modified 18 months ago Last modified on Jun 1, 2016 2:40:57 PM