After that there are four distinct and composable methods for tiling large images in ivtools drawing editors:
on-the-fly tiling: ivtools drawtool and all other drawing editors derived from drawtool support a "-tile", "-theight", and
"-twidth" command line arguments. If
"-tile" is supplied, all binary PPM or PGM images
greater than twidth x theight in size are
automatically loaded into an internal tiled representation.
"-theight" and "-twidth" default to
512x512 if not supplied. Any tile not currently on the screen
is not loaded into memory. A "level-of-detail" pyramid is
maintained for each tile, and pixel data is only loaded into
memory at the resolutions at which it appears in the on-screen
viewer. This means the entirety of a sizeable raster (10K by
10K or greater) can be viewed at once without requiring a
gigabyte of swap. If the document into which a large image has
been imported is saved to disk, the result will be one entry for
each tile with the original pathname specified along with the
subimage coordinates of the tile.
tiling by drawing editor document: a user can manually
create an ivtools drawing editor document that tiles a large
image into pieces using the :sub argument on the raster entry.
The easiest way to get an example of this kind of tiling is by
using the "-tile" mechanism described above and
saving the document out to a file.
internal raster tiling: large binary PGM or PPM image
files can be internally tiled using the tilefile command on ivtools comdraw. The arguments to the
tilefile command can be viewed by typing "help(tilefile)" into
stdin on comdraw, but for reference they are:
"tilefile(oldpath newpath [xtilesize]
[ytilesize])". The newly generated binary PGM or PPM
file contains a comment field which is used later by ivtools
drawing editors to access the internally tiled raster.
external raster tiling: finally, the most efficient
mechanism, if you have the disk space available, is to use a
utility like "pnmcut" to manually split a large
image into smaller tiles, then in a text editor generate the
drawing editor document with a raster entry for each tile with
an appropriate :transform argument to position the raster
relative to the origin and other tiles. If the tile should be
positioned at 2048,32768 relative the origin of the large image,
the :transform argument would read ":transform
1,0,0,1,2048,32768".