LAN/WLAN Interface for Digital Cameras

Helmut Dersch
der@fh-furtwangen.de
January 2005

Abstract
Remote control of almost any digital camera can be accomplished by connecting the camera via USB to a computer system which is connected to a network. This article describes the setup, installation and operation of a system consisting of a battery powered single board computer with wlan adapter. It runs embedded Linux and adapted versions of open source software. No special skills beyond plugging in cables and basic computer experience are required. Total system cost is well below 150 EUR/$. A simple webinterface has been developed which enables wireless remote operation of this setup by other computers connected to the network. Systems ranging from Windows PCs to Palm PDAs are feasible as long as they are capable of running a HTML-browser. Download the software package from here.

Introduction
The current project is similar to a webcam which is used to publish life images on the WWW. However, the focus is on the remote control capability. We want to provide an easy to setup and operate small, standalone, possibly battery powered solution for

wireless remote control for taking images and occasional downloads.
automatic operation of the digital camera, e.g. for periodic image capture.

Most digital cameras can be remotely controlled using USB or serial connections, and many software packages have been developed to provide this functionality. It is now a matter of selecting suitable hard- and software, connecting and installing everything, and providing a user interface.

Hardware
The controlling computer has to provide USB-host support to connect to the camera; the days of RS 232 are definitely over. This excludes the otherwise perfect Palm computers or other PDAs which only support USB-device protocol. A full size labtop works, but is quite large and too expensive for this task. Of the many embedded systems available today one is particularily well suited for connecting USB devices: the Linksys NSLU2 network storage link. It has 2 fast USB ports, it runs Linux, its processor is reasonably fast (250MHz) and, most important, it is one of the cheapest computers of its kind selling for less than 80 EUR/$.

Problem is that the NSLU2 is not meant to be a general purpose computer but a device to connect to USB-harddiscs turning them into network drives. The potential of this device is far beyond this task. Many users have realized that, and there is a growing number of developers liberating the firmware and open the way for new applications. There is now a replacement firmware available for download which turns the device into an almost standard Linux computer while preserving the original functionality. All this is well explained, quite simple to install, and does not require specific Linux knowledge. Basic computer experience is helpful since setting up the device requires logging into the Linux system using telnet, issuing a couple of shell commands and copying some files.

Software
Many camera manufacturers provide excellent and powerful remote control programs for their products. However, these usually do not work under Linux, and support only specific models. Several open source packages exist, and the one supporting the largest number of camera models is the gphoto program. Unfortunately, it has only very limited remote control capabilities utilizing not much more than taking images and previews, and downloading them. No exposure adjustments or manual controls are possible. Nevertheless, gphoto was the only program which supports my own camera model so I had to choose it. Users with cameras supporting ptp have more options, e.g. the program capture or ptpcanon.

Setup
The image below shows the setup.

It consists of the following components:

Linksys Network Storage Link NSLU2.
Asus Wireless Access Point WL-330. Any Wlan adapter should work, or the device may be operated by wire (Ethernet port).
Lexar Compact Flash USB Adapter with CF Card 64 MByte. This is optional. Any mass storage device (USB-drive, USB-stick, Flash-cards with USB-adapter, or none(!)) can be attached.
Pack of rechargeable batteries (Size AA, 4xNiMH, 2300mAh), or any other 5V source.
USB controllable camera.

All components except the camera are packed into a watertight box for outdoor applications. The battery pack lasts about 60 min. The NSLU2 alone runs several hours from it, but the wlan adapter is very hungry. The battery in my camera lasts approximately the same time. Power saving on the camera should be turned off to avoid automatic shut-down after periods of inactivity. Of course, if longer operating periods are required, larger batteries should be used.

Installation of software
For novices to the NSLU2 I suggest the following procedure:

Set up the NSLU2 for normal operation as specified in the manual that came with it. Connect the wlan adapter and get the setup to work in your network. The Asus wlan device may be used as adapter (simpler to set up) either in infrastructure or ad-hoc mode, or it may be used as access-point to set up its own wlan and allow access of several computers simultaneously. No mass storage device needs to be attached to the NSLU2 at this stage. It should be accessible via http://192.168.1.77 on your local network. This will also be the base address for remote control of your camera. Change it if you wish as explained in the owners manual.
Install the replacement firmware unslung-standard-3.xx. This is not difficult and explained quite well. Then unsling as described in the documentation, and connect a mass storage device (any USB-stick/USB-flash-memory).
Software installation on the NSLU2 is usually done by using the ipkg command which fetches prebuild packages over the internet. Setting up the internet connection is not entirely trivial and I provide a route without this necessity. Just in case you succeeded with setting up the internet connection I recommend installing the following optional packages which aid in working with the camera
- inetutils - use the ftp-server to access files. This way the NSLU2 together with a hard disc can be used as image-tank.
- coreutils - shell utilities for script execution. The required ones
  are provided below.
- openssh and openssl - for logging in via ssh.
Set up gphoto and the web-interface. I have packaged all required files into one zip archive which you can download from here. Remove your USB-flashcard (or stick) from the NSLU2 and connect it to your desktop computer, installation will be done there using your PC's window manager. Unzip the file gphoto.zip. This should result in one directory gphoto. Move this directory into the directory /opt of the flashcard (or stick) so that it will be copied to this location. Now reinsert the card to the NSLU2, log in as root (as you did when you unslung) and issue these commands:
```
   cd /opt/gphoto
   ls
```
This should create the following listing:

COPYING bin doc lib remote src

Now type exactly as shown below:
```
    cp -R remote /home/httpd/html/
    chmod +x /home/httpd/html/remote/gphoto_web.cgi
```
Now the NSLU2 is setup. Open up your favorite WWW-browser on your desktop computer and type the address http://192.168.1.77/remote . In place of 192.168.1.77 you have to use the exact number you typed when you accessed the NSLU2 for the first time. You should get this page. If a camera were connected you could start taking images.
Connecting the camera. This is the easiest part. Just connect the USB-cable of your camera to the free port on you NSLU2. Turn it on and open the webinterface on your desktop PC. Take a preview image. If it doesn't show reload the page. If it still doesn't show, go to the troubleshooting section.
Tuning. I recommend to remove all unneeded plug-ins from the directory /opt/gphoto/lib/gphoto2/2.1.4 . This can be done by using the Unix command rm file or by plugging the card into your desktop computer, opening folder /opt/gphoto/lib/gphoto2/2.1.4 and using your Window manager for deletion. gphoto2 loads all plugins for each and every invocation of a command. Response time will be much faster after this clean up. As an example I just keep the file libgphoto2_canon.so for my Canon Powershot. To save space the following directories can be safely deleted from the gphoto directory after installation: Documentation, src, remote and the file COPYING. This makes the whole gphoto folder just 600 kByte large which fits into the onboard memory of the NSLU2, i.e. no storage card is required. Still, the setup is not suitable for action photography: the average delay time between trigger and event is 10 seconds after these tuning steps.

User Interface
Userinteraction is accomplished by the webinterface Remote Camera. This is a plain HTML-document using forms to call a shell-script on the NSLU2 which in turn issues the required gphoto2 commands. Each action requires parsing the script, scanning the USB-bus and connecting to the camera resulting in response times > 10s. The following actions are accessible:

Preview - Takes a preview image and displays it on the Remote Camera-page. If the image does not show up, reload the page. If this happens frequently, adjust the line sleep 5 of the cgi-script and choose a longer delay time.
Capture - Captures an image leaving it on the camera.
Load - Loads an image from the camera to the client computer. No space on the NSLU2 is occupied for this operation. You have to supply directory name and image number, which can both be determined by using the Execute command below. Please note that the image number refers to the number within the choosen directory, not the global number displayed when issuing --list-files.
Execute - execute any gphoto2 command. There are countless options which can be determined by studying the attached man-page. An often required command is --list-files which supplies a directory listing of all images on the camera. The response of the gphoto2 command appears in a new HTML-page. This is quite useful for debugging.

Technology
The following open source packages have been compiled and assembled for this system:

gphoto2 - a client program for libgphoto
libgphoto - the library providing access to digital cameras. Patches have been applied to change hardcoded search paths.
libusb - the library providing access to the USB-ports. A patch has been applied to get this library working on the NSLU2.

The patches together with build instructions are contained in the directory src in the gphoto package. If you plan to rebuild the application you have to download the openembedded environment and follow the instructions provided at the NSLU2-Linux homepage. Then, apply the patches to the mentioned source files and rebuild the three packages. Please see the Readme file in the distribution gphoto.zip for details.

The web-interface uses basic HTML-technology and the user interface is realized entirely with forms. The CGI-script is a plain shell script and requires only basic unix core utilities like sed and cut. It is therefor highly portable and could be used with minor adjustment on any unix-like system. On the client side, any HTML editor supporting forms should work. I have successfully tested Internet Explorer and Netscape Navigator on Windows, and Mozilla, Firefox, Konqueror and Lynx on Linux. I suggest to read the shell script gphoto-web.cgi and the HTML source index.html to study details of the mechanism and add your own modifications.

Troubleshooting
There are many things which can go wrong. Before troubleshooting the camera setup please
make sure that

the HTTP server works correctly and displays the main page at 192.168.1.77 (or whatever your IP is). If so, check 192.168.1.77/remote. If this does not work, please repeat the installation and make sure the folder remote has been copied correctly to /home/httpd/html/.
the program gphoto2 works correctly. You can check this as follows: Telnet into the NSLU2 and log in. Then type
```
export LD_LIBRARY_PATH=/opt/gphoto/lib
/opt/gphoto/bin/gphoto2
```
Usage information about gphoto should be displayed.

Now test your camera setup and type (your camera needs to be connected now and turned on):

/opt/gphoto/bin/gphoto2 --list-files

This should display all image files in your camera. If this does not work, your camera might not be supported, please check the gphoto homepage.

Feedback and suggestions for improvements are welcome.

Helmut Dersch
Copyright C 2005
der@fh-furtwangen.de