FBDUMP

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fbdump is a simple tool that captures the contents of the visible portion of the Linux framebuffer device and writes it to the standard output as a PPM file. In other words, it takes a screenshot of anything running on the framebuffer. It currently has fairly complete support for packed-pixel framebuffer types and also works with the VGA16 framebuffer driver.

News

Background

fbdump was inspired by an even simpler program I came across ages ago called fbsnap, which I believe was written by Gerd Knorr (although no author is mentioned in the source code of that program, so I could be wrong). fbdump bears little relationship to fbsnap, however, and is generally much more useful. It supports framebuffers employing a greater variety of pixel and colour models, works (should work?) on big- and little-endian architectures, and is reasonably quick (although reading from video memory is never going to be super fast).

I first wrote fbdump many moons ago when I needed a quick and simple way to grab lots of images from the framebuffer. As most of you will probably know, although it's possible to do a raw framebuffer dump by performing a simple cat /dev/fb0 command or similar - depending on what pixel format and colour model your framebuffer uses - the results are not always particularly useful. With fbdump, on the other hand, you always get a nice, portable, 24-bit, true-colour image - since it takes care of any pixel format or colour-space conversion required (at least for formats it supports).

Why does fbdump save the image as a PPM - rather than, for instance, a PNG - you might ask? Well, a PPM is cheap to write out and doesn't require linking against any external libraries to do so. On the x86 architecture, fbdump currently compiles down to a miniscule 8K executable once stripped - and has no dependencies other than the standard C library. Having said that, I may get around to adding support for PNG at some point. It's not strictly necessary, though, because with the NetPBM package it's easy to convert PPMs into something more useful. For example, you can do something like

to output the framebuffer as a PNG. PPMs can also be read directly by most open source image processing packages, such as the Gimp and ImageMagick.

Downloads

Compiling

A configure script is provided in the source package, so to build fbdump, just follow the usual ./configure and make incantation. There are no unusual build requirements.

The only extra configure option supported is --disable-vga16fb, which will cause fbdump to be built without support for the vga16fb framebuffer driver. This option should be set appropriately, automatically by the configure script, according to your target architecture. If you have problems and are building for CPUs other than x86, then try manually disabling the vga16fb support.

Usage

fbdump currently supports the following command-line options:

If the -fb option is not provided, fbdump defaults to grabbing from the device /dev/fb0. You can also specify the framebuffer via the environment variable FRAMEBUFFER (a value supplied with the -fb switch on the command line will override this). For example:

The -vt option effectively allows grabbing from another virtual terminal other than the one that fbdump itself is running in. This is a useful feature, since it allows you to easily grab from an application which takes over the entire framebuffer, without requiring you to log-in remotely. If a valid VT number is supplied with the -vt option, then fbdump will also detach itself from the terminal it's running in (before switching VTs). This is necessary because, otherwise, the framebuffer accesses would still be directed at fbdump's VT rather than the new foreground terminal.

The -delay option is particularly useful if you are on a large and deep framebuffer and you want to switch VTs with the -vt option. A suitable delay will give the application running in the target VT enough time to redraw itself before fbdump starts grabbing.

Currently, only packed-pixel framebuffer formats and the vga16 framebuffer are supported. You will need root permission to grab from a vga16 framebuffer (it needs to access the VGA registers), but with other devices you just need permission to read from the framebuffer device (/dev/fb0 or what ever that happens to be).

fbdump understands the following packed-pixel colour models:

This should cover majority of what most people use, but I will flesh out support as time and requirements dictate. If you need support for a different pixel format, please get in touch.

fbdump should work on big- or little-endian hardware. It has been tested on a variety of x86 and PPC systems and has proved reliable. However, fbdump does make a number of assumptions that may or may not work in all cases. For example, fbdump assumes that the endian-ness of the framebuffer is the same as the endian-ness of the host CPU. In short, problems may occur on some systems. If they do, let me know.

Contact

Send bug reports and suggestions to me, Richard Drummond, at <info@rcdrummond.net>

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