RPi.GPIO is necessary to use external buttons as a trigger but it works just fine without. Triggering is then only possible using touch screen / mouse or key 'c'.

Hardware

Raspberry Pi (Any device able to run the software stack should work fine)
Camera supported by gPhoto. I've used a Canon EOS 500D.
Optional: External button that closes GPIO23 (pin 16) and GND.

Usage

Simply download photobooth.py or clone the repository and run it. It opens the GUI, prints the features of the connected camera, e.g.,

$ ./photobooth.py 
Abilities for camera             : Canon EOS 500D
Serial port support              : no
USB support                      : yes
Capture choices                  :
                                 : Image
                                 : Preview
Configuration support            : yes
Delete selected files on camera  : yes
Delete all files on camera       : no
File preview (thumbnail) support : yes
File upload support              : yes

and waits for you to hit the button to take pictures.

Available actions:

Press q: Exit the application
Press c: Take four pictures, arrange them in a grid and display them for some seconds.
Hit a switch that closes GPIO23 (Pin 16) and GND: Take four pictures, arrange them in a grid and display them for some seconds.
Click anywhere on the screen: Take four pictures, arrange them in a grid and display them for some seconds.

All pictures taken are stored in a subfolder of the current working directory, named YYYY-mm-dd after the current date. Existing files are not overwritten.

Installation

A brief description on how to set-up a Raspberry Pi to use this photobooth software.

Download latest Raspbian image and set-up an SD-card. You can follow these instruction.

If your display needs some additional configuration, change the file config.txt in the boot-partition to your needs. For example, I'm using a Pollin LS-7T touchscreen, for which I need to enter the following to avoid overscan:
```
hdmi_group=2
hdmi_mode=87
hdmi_cvt=1024 600 60 6 0 0 0
```
Insert the SD-card into your Raspberry Pi and fire it up. Use the raspi-config tool that is shown automatically on the first boot to configure your system (e.g., expand partition, change hostname, password, enable SSH, configure to boot into GUI, etc.).
Reboot and open a terminal. Type sudo rpi-update to install the latest software versions. Reboot.
Run sudo apt-get update and sudo apt-get upgrade to upgrade all installed software.
Install any additionally required software:

Pillow:

sudo apt-get install python-dev python-pip libjpeg8-dev
sudo pip install Pillow

gPhoto2: Unfortunately, the version in the repositories is too old to work (some USB-bugs), hence one must use Gonzalos installer script

git clone https://github.com/gonzalo/gphoto2-updater
sudo gphoto2-updater/gphoto2-updater.sh

To ensure the camera can be controlled properly via USB, remove some files:

sudo rm /usr/share/dbus-1/services/org.gtk.Private.GPhoto2VolumeMonitor.service
sudo rm /usr/share/gvfs/mounts/gphoto2.mount
sudo rm /usr/share/gvfs/remote-volume-monitors/gphoto2.monitor
sudo rm /usr/lib/gvfs/gvfs-gphoto2-volume-monitor

xinput_calibrator to calibrate touchscreens:
```
wget http://adafruit-download.s3.amazonaws.com/xinput-calibrator_0.7.5-1_armhf.deb
sudo dpkg -i -B xinput-calibrator_0.7.5-1_armhf.deb
```
Calibrate by calling xinput_calibrator and pasting the shown snippet to a new file /etc/X11/xorg.conf.d/99-calibration.conf (Create the directory if necessary).

Reboot.

Clone the Photobooth repository

git clone https://github.com/reuterbal/photobooth

and run photobooth.py

Optional but highly recommended, as it improves performance significantly: install some Python bindings for gPhoto2. For that, either Piggyphoto or gphoto2-cffi can be used. At the moment, Piggyphoto doesn't allow to disable the sensor while idle, so gphoto2-cffi is preferred.

8.1 Installing gphoto2-cffi: Install cffi
```
sudo apt-get install libffi6 libffi-dev python-cffi
```
Download and install gphoto2-cffi for gPhoto2
```
git clone https://github.com/jbaiter/gphoto2-cffi.git
cd gphoto2-cffi
python setup.py build
sudo python setup.py install
```
8.2 Install Piggyphoto: Download Piggyphoto and put the folder piggyphoto into the Photobooth-directory.
Optionally make the software run automatically on startup. To do that, you must simply add a corresponding line in the autostart file of LXDE, which can be found at ~/.config/lxsession/LXDE-pi/autostart. Assuming you cloned the Photobooth repository into /home/pi/photobooth, add the following line into the autostart-file:
```
lxterminal -e "/home/pi/photobooth/photobooth.sh set-time"
```
For this to work you must install gnome-control-center by running sudo apt-get install gnome-control-center (Unfortunately, this brings along a lot of dependencies - however, I haven't found any lightweight alternative that would allow to simply set date and time using the touch screen).
Alternatively, you can also add a Desktop shortcut. Create a file /home/pi/Desktop/Photobooth.desktop and enter the following:

[Desktop Entry]
Encoding=UTF-8
Type=Application
Name=Photobooth
Exec=lxterminal -e /home/pi/photobooth/photobooth.sh set-time

Modifications

In the beginning of photobooth.py a number of config options are available. Change them to your liking.

The GUI-class is separated from the entire functionality. I'm using Pygame because it's so simple to use. Feel free to replace it by your favorite library.

Instead of gPhoto2 you can also use OpenCV to capture pictures. This is the preferred way if you want to use a webcam and is particularly useful for debugging on a different machine. For that you must install OpenCV and its Python bindings (run sudo apt-get install python-opencv) and then change the CameraModule: edit photobooth.py and replace Camera_gphoto as CameraModule by Camera_cv as CameraModule.

License

I provide this code under AGPL v3. See LICENSE.