Login Screen Configuration on DICE

SL7 - LightDM


Once again it was not possible to adequately control the default upstream GDM (v3) display manager on this platform (additionally and perhaps more importantly there were severe user interface issues as reported in the SL7DICEEnvironment), so it was replaced by LightDM and controlled with the new lcfg-lightdm component.

Greeter Appearance

LightDM doesn't support themes as such, but can be configured to use any given image as a background and this can be set using the lightdm.greeterbg resource, which can be any file on disk. To provide fully dynamic theme support DICE SL7 employs the lcfg-webpic component to generate background images from an HTML source file. Convenient customisation, including background image and status / platform text, can be performed using the webpic resources (see the lcfg-webpic manual page for full details) and if this is not sufficient the component can be configured to be rendered from any HTML source, local or remote (though there are clear security issues which must be considered before using remote template sources!).

The lcfg-webpic component can have its output file resolution set manually, and the output image will be rendered to this size. However on DICE most desktops do not have a static monitor configuration defined in LCFG and a mismatch in aspect ratio might render some content unreadable or clipped so it turned out to be better to detect this at launch time. The getres utility fulfils this purpose minimally. It's run by the webpic component.

PCs fulfilling a specific function, (e.g. lab desktops, Exam PCs or Virtual DICE) would be expected to alter the webpic resources head, head_sub and footN resources (and one of the background resources). Login screens can be previewed by visiting the files referred to by the webpic.html_tmpl and webpic.image_file resources, typically in /var/lcfg/conf/webpic[file link — copy and paste to view].


The lightdm component must be restarted (or user logged out by other means) to see the changes made by the webpic component.

The DICE LightDM configuration calls /etc/X11/xinit/Xsession and optionally the scripts defined in lightdm.sessionsetup and lightdm.sessioncleanup.



It was not possible to adequately control the default upstream GDM (v2) display manager on this platform, so it was replaced by KDM and the lcfg-kdm component.

Greeter Appearance

It's themed using the kdm.theme resource, which refers to a directory in /usr/share/kde4/apps/kdm/themes/. On DICE SL6 we provide different themes for each "platform" i.e. sl6, sl6_64, DIYDICE and VirtualDICE, and each theme requires its own files, which we provide by RPM.

The themes are based on a slightly tricky freedesktop standard format involving an XML theme file and SVG background. Attempts were made to make a dynamic theme, but this would require replacement of the theme file and templating using the file component (at least). Most DICE machines include standard theme files in the dice-kdm-theme package, (sourced in the LCFG subversion repository).

For the purposes of indicating exam lockdown, the dice-kdm-exam-theme package is additionally installed on eligible machines. The DICE examlock component generates a (very crudely-generated using mkimg.py) PNG file containing component variables, included in the background.


The theme can be changed by altering the theme variable and restarting the component. This will also end local user X sessions so handle with care.

Login scripts are controlled using the appropriate kdm resources setup, startup and reset and documented in the lcfg-kdm manual.

Backported SL7 tools

The SL7 webpic component and associated utilities (getres, etc.) have been backported to SL6 for consistency, but are not yet in widespread use. The tools function in exactly the same way, providing a configurable background which is displayed by a custom KDM theme. The adaptations necessary to use it are maintained in <dice/options/webpic.h> and inclusion of the header is the only change needed to covert an SL6 profile to use webpic (it inserts the background image, webpic.image_file, into a fixed location belonging to an associated DICE-webpic KDM theme). It will also be seamlessly compatible with the current lab exam configuration, as this works by altering the theme on demand.

Past versions

We've used both GDM and XDM in the past.

-- GrahamDutton - 18 Feb 2015

Topic revision: r6 - 31 Jan 2017 - 11:15:12 - GrahamDutton
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