Together with Workspace Manager, Backdrops and Palettes are probably most recognizable aspects of CDE, by which CDE is visually distinguished from other desktop environments.
CDE backdrops, are relatively simple XPM textures and pictures consisting usualy from 2-5 base colors: background, foreground, selectColor, topShadowColor, and bottomShadowColor. This colors are taken from the current (or custom) palette and applied to the symbolic definition of colors in XPM templates. Backdrop is then generated, tiled and applied to the root window. Every workspace can have it's own backdrop texture. In 4 colors mode of the palette theme, they are all colored in the same pattern, while in 8 colors mode, every workspace from 1-4 has it's own color variant from the current palette, in that case, workspaces from 5 to 8 are repeating colors from 1 to 4. In other words, if there are more that four desks defined, color of the fifth workspace is repeated color of the first, sixth of the second and so on.
Here, in addition to original CDE textures, there are some 100 new and custom
textures created from (free and public) textures which were convenient for
this customization. In other words, NsCDE implements more than 100 backdrops,
and with Backdrop Style Manager user can import to it's
$FVWM_USERDIR/backdrops
it's own backdrops,
or put them there with terminal or file manager.
Backdrops must have alternative extension for X Pixmaps: that is, not .xpm, but .pm.
If one wants their custom backdrop to be dynamic with palette/theme of NsCDE, one must edit them to set symbolic names of the colors described above. Examining existing backdrops is the fastest way to get idea what needs to be done. Apart from symbolic names, backdrops also have a real color defined to be compatible with XPM specification, but values of this colors can be arbitrary, since they are not used if symbolic name on the same line is set. Usually, they are in gray spectrum.
Default backdrops are set from the
$NSCDE_DATADIR/defaults/backer
until
user does not redefine/set his own with
Backdrop Style Manager. Default
palette is Broica in 8 colors variant.
Thematically generated backdrops generated by Backdrop Style manager
are installed in user's $FVWM_USERDIR/backer/DeskN-<backdropname>.pm
and defined
in $FVWM_USERDIR/Backdrops.fvwmgen
as colorsets of
TiledPixmap type. NsCDE reserved FVWM colorsets numbers for backdrops
are from 31-38 for all eight possible workspaces.
$FVWM_USERDIR/Backdrops.fvwmgen
file is read by FvwmBacker(1) FVWM module.
It is automatically generated when user makes first change with
Backdrop Style Manager.
Until then, system wide file $NSCDE_DATADIR/fvwm/Backdrops.fvwmconf
applies, which itself reads pre-generated and pre-defined backdrops from the
$NSCDE_DATADIR/defaults/backer
directory.
Default color theme is Broica.
We can consider backdrops as the source form or template files, and when processed with color values from the palette, this backdrop's final form is, ready to be set by FvwmBacker(1).
When NsCDE is used with FVWM3 and per-monitor or shared
DekstopConfiguration monitor modes, FvwmBacker(1) is not in use, because it cannot treat different
physical (and virtual) monitors separately. Functions driven by the new_desk
FvwmEvent(1) event are called in that case, which will be loaded from the cache directory
$FVWM_USERDIR/backer/bgcache
if found, or automatically
generated with the help of ImageMagick's convert(1) tool if they do not exist, and then loaded.
This mechanism automatically calculates screen width, height, X and Y coordinates, desks and backdrops
to produce cjoined backdrops as one big PNG file which is then loaded to show correct background
for desks on different monitors.