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DMP Photo Booth 1.0

Well, the day has come and gone. DMP Photo Booth’s final test on June 21st went off without issue, and DMP Photo Booth has left Beta and is now considered “production ready”. The initial 1.0 release can be found on GitHub.

The significance of June 21st is the very reason DMP Photo Booth was created; the 21st is the day of my wedding. My wife wanted a photo booth for the reception. We looked into renting a photo booth, but it turns out that they run around $1,000. I turned to open source. Some quick googling turned up some options, but they were all personal projects or out of date. Sure I could get somebody else’s project working, but what’s the fun in that? I decided that we didn’t need to rent one, or download one, I could build it!

In late 2013, I set to work in earnest. I had a couple of months of downtime in school, and since I’m not currently working it was the perfect time. I decided I had three main objectives for this project: get some arduino experience, get some GTK+ experience, and do this all as portably as possible. I had initially decided to mostly ignore GLib and focus on GTK, but slowly I grew to appreciate GLib for what it is: the standard library that C never had. First I used GModule to handle shared libraries in a portable manner. Next I decided to use GLib primitives to keep from having to deal with cross-platform type wonkiness. Next, having grown tired of dealing with return codes, I refactored the project to use GLib’s exception replacement: GError.

Lessons Learned

It’s not all roses and puppies though. There are certainly things I’d do differently. DMP Photo Booth is developed in an Object Oriented style, passing opaque structs with “method” functions that operate on them. Each component of the program are organized into their own source file with file scoped globals scattered throughout. Said globals are protected by mutexes to create a semblance of thread safety. That said, threading issues have been a major thorn in my side. Long story short: I regret this design choice. While I still feel that this is the correct way to structure C code, and that if globals are required, this is the correct way to handle them; I feel that I should have made more of an effort to limit side effects. Recently, I’ve spent some time doing functional programming, and if I could do it again I’d try to write in a more functional style. Fortunately for me, this is something that a little refactoring could help with.

Additionally, one thing I thought would be a major help is something that began to be a major thorn in my side: NetBeans. As the size of the project grew, NetBeans got slower and slower. It seemed that I spent more time fiddling with IDE settings than actually coding. Even worse is that the IDE-generated makefile is so convoluted that it’s extremely difficult to modify by hand in a satisfying way. I’ve always coded with and IDE so I wouldn’t have even considered not using one, but then I spent some time with Haskell. One of Haskell’s “problems” is that it doesn’t have good IDE support. It doesn’t seem like any IDE really handles it well, so most people use Emacs. Personally, I haven’t really warmed up to Emacs, but GEdit has syntax highlighting for Haskell and a built-in terminal for GHCI. GEdit also has syntax highlighting for C. Next time, I will seriously consider using a lighter-weight text editor for a C project. All this said, I think NetBeans for Java remains the way to go.

What’s Next

Like any program, version 1.0 is just one of many versions. There certainly remains a lot of work to do with DMP Photo Booth. Some major items you are likely to see whenever I get around to working on DMP Photo Booth some more:

Options Dialog

I think anybody who has seen it will agree: the options dialog in DMP Photo Booth is bad. It’s poorly organized, and kind of wonky. Personally, I modify settings using the .rc file, which is telling. This is certainly a high-priority improvement.

Functional Refactor

Like I said above, the code could use a pass to limit side effects. Funtions need to have their side effects limited, and globals need to be eliminated unless absolutely necessary. However, C is not a functional language. While one could argue that function pointers enable functional programming in C, this is a very pedantic argument. I won’t be going crazy with functional programming techniques. There will be no Monads, or for loops being turned into mappings of function pointers.

Optional Module API

An idea I’ve had on the back burner for a while is an optional module API. This would be used for very specific quality-of-life things. For instance, a module could provide a GTK widget to be shown in the options dialog. Any module that doesn’t want to implement any or all of the optional API can just ignore it. The module loading function will gracefully handle the dlsym failure, just treating it as it is: declining to implement the API. I have no plans to change the current existing API, so all you module developers can rest easy!

User Interface Module

It occurred to me that it might be good to have a UI module. This would provide the UI, and wouldn’t be tied to the trigger/printer/camera module start/stop system. This module would be loaded at startup and unloaded on shutdown. This would allow the Photo Booth to use different widget toolkits: QT, Curses, Cocoa, WinForms, or whatever else. Under this scheme, the current GTK+ interface would be abstracted into the reference UI Module.

DMP Photo Booth: Crunch Time

Over the last few months, I’ve become distracted. As anybody who’s ever committed to one project can probably tell you: it stops being exciting. What was once your pride and joy becomes the daily grind. Things get stale. As was the case with me, I suspect that this happens for most people when development of new features ends and the focus shifts to bug fixes.

I became distracted. My mind began to wander to the next thing, which in my case ended up being Haskell. I began learning Haskell, and it was definitely educational. I learned a lot with Haskell, and I plan to stick with it so that when I list it on my resume, I don’t get destroyed on a whiteboard test. Then came The Great Matrix Affair of 2014; I got overwhelmed at school. I spent so much time studying and doing homework that I couldn’t muster up the motivation to program. Things fell by the wayside, as you can see in my blog post output for February. Luckily for me, that is done, and I have the next two months free to program!

What Remains To Be Done?

It’s been a good 6 – 8 weeks since I’ve really focused on DMP Photo Booth, so the first order of business is the figure out what needs to get done. After doing some brainstorming I’ve settled on a list:

Finish The Trigger

I’ve mostly completed the trigger, but it doesn’t work. The button is soldered wrong, and while it was magically working for a while, it has since stopped. I need to fix the wiring issue, and then drill a hole in the box to put it through. After that and maybe a quick coat of paint it will be complete.

This particular task is due by Friday. I have a series of VIP demos coming up, the first of which is Saturday morning. A few of my cousins are coming in from out of town on Saturday to do wedding stuff, and I want to show it off then. While my cousin Allen is an engineer, and can appreciate a breadboard mockup of what will Totally Become A Real Thing, it certainly won’t be impressive. My cousin Laraine will likely be less amused, but I’m sure I’ll get a pat on the head for my “hard work”. Due to this, it’s important that the trigger be done before then.

Facebook Printer Module

The reference suite of modules was planned to be: a Trigger Module using my Arduino implementation, a Printer Module using CUPS, a Camera Module using LibGPhoto2, and a Lua module for each so that modules can be created using Lua. Of these, only the Lua Printer Module remains to be done. Since creating a Lua module is a trivial task (and not terribly important to be honest), this is a very low priority item.

However, the current Printer Module requires a physical printer. This might not always be ideal, since printers are big and heavy. What if you just want to bring a laptop and a camera and have a photo booth? My fiancée is having just this sort of situation; she wants to use the photo booth at her bachelorette party, but who wants to lug a huge printer to a hotel room? To solve this, I’ve promised her a Facebook Printer Module.

The idea is that when dmp_pm_print() is called, the photo strip will be uploaded to facebook. While I know this sort of thing can be done, I haven’t really researched it much. If it turns out that you have to pay facebook money to get this sort of access, I will find a hosting service that is free. Maybe Photobucket, or Dropbox, or whatever. The important thing is that the photo strips will end up in some central location on the internet so that anybody at the party can download the strip later. Ideally, this central location would be a facebook gallery so people can tag themselvs and be all Web 2.0.

My fiancée’s bachelorette party is in May, so this project isn’t a burning priority. However, this represents the most substantial addition of new functionality remaining to be done, so I plan to work it sooner rather than later. Code will be posted on my Github as it evolves, and like DMP Photo Booth will be available under the GPLv3.

Mac Support

To this point, all my development has been done in Linux. First using Ubuntu, and now using Debian. However, most people don’t use Linux. While Linux is the main target platform for DMP Photo Booth, I have been coding as portably as possible, so it should be little effort to port the application to Mac. Over the next few months, I’ll be making sure it compiles and runs correctly my old Macbook. My Macbook is vintage 2010, as such it is only running Snow Leopard. Therefore if anybody in the audience is a Mac user, and has a current version of OSX, feel free to give it a shot as well and let me know how it goes.

Ideally, my fiancée will bringing the Macbook to her bachelorette party and not my development laptop, therefore this project is due at the same time as the Facebook Printer Module, in May.


Finally, bugs. I need to identify them. I need to squash them. And I need unit tests.

After making a big show about being a good person and doing unit tests, I promptly lost the faith. Soon after implementing those first tests, I made a major change to how I handled errors in my code. Suddenly, all my tests were broken, and I was faced with the choice: rewrite them all, or delete them. After some thought I decided that my tests weren’t that great and that I’d probably change something again and break them all again. So I deleted them.

I’ve got to say, I miss those tests. There has been more than a few times where I’d refactored something and wasn’t sure if it still worked. Sure, it seems to work, but does it really? If I had unit tests in place, I could say with a greater degree of certainty that they do. Plus, “it sounds like a lot of work” is not a good reason not to do something, so it won’t be one for me.

On top of that, I’ll be identifying and squashing bugs the old fashioned way: by trying stuff. I’ve already found a few doozies, and I’m sure I’ll find more. As I find them I’m going to post them on the bug tracker for DMP Photo Booth on Github. I do this for a few reasons: 1) it will provide a public centralized repository of open issues so that I don’t lose or forget about them. 2) I would like others to post bugs here. If I post them here and show that I am fixing them, I feel it will establish confidence that it is looked at and acted upon.

This is due when DMP Photo Booth goes to version 1.0. That is planned to be on June 21, the day of my wedding. DMP Photo Booth will get a good solid night of work as the 80 or so people attending put it through its paces. Assuming all goes to plan with minimal issues, DMP Photo Booth will be declared to be out of Beta. That said, to be truly version 1.0, unit tests must be in place and “all bugs must be fixed”.


Currently, DMP Photo Booth is available in source form only. No end-user ever had to compile Microsoft Office; I don’t feel they should have to compile DMP Photo Booth.

To that end, binary distributions will be available for Linux and Mac when DMP Photo Booth goes to version 1.0.

Got My Work Cut Out For Me

It’s a long list to be sure, but I have 4 months to focus on it. However, I’ve decided that I should spend some time focusing on other things too, so that I don’t burn out. To that end, I plan to spend 1 day per week focusing on learning new technologies, and 1 day per week keeping my old skills sharp.

For new technologies, this means things like learning more Haskell, and other languages or frameworks. Whatever strikes my fancy. For old skills this means practicing with Lua and Java, and maybe even C++ if I’m feeling particularly masochistic that day. This will likely take the form of blog posts on topics relating to these, so stay tuned!

The Smallest Things…

For the last few weeks I’ve been banging my head against a problem. I need my Photo Booth application to actually take a photo. It seems like such a simple thing, but it has actually been one of the most difficult ones I’ve encountered. Like the Great PostScript Debacle and the Mystery of the GAsyncQueue Ref before it, I spent a good week banging my head against the wall. I even took a day off to write an entire Lua Camera Module just so I could shell out and call a command line utility to try to work around it.

The best part? If you’ve been following the blog, you know I’m kind of a crybaby about poor documentation. While libgphoto2 is certainly a repeat offender on this count, no amount of documentation could have prepared me for what was to come.

But first, let’s go over my now-working implementation.

Taking A Picture With Libgphoto2

To take a picture, we need 3 functions:

  • gint dmp_cm_camera_init()
  • gint dmp_cm_camera_finalize()
  • gint dmp_cm_camera_capture(gchar * location)


gint dmp_cm_camera_init() { context = gp_context_new(); gp_log_add_func(GP_LOG_ERROR, (GPLogFunc) dmp_cm_log_func, NULL); if (gp_camera_new(&camera) != GP_OK) { //error handling } if (gp_camera_init(camera, context) != GP_OK) { //error handling } return DMP_PB_SUCCESS; }

There are two main structs: Camera and GPContext. A Camera represents, shockingly, a camera attached to the system. A GPContext represents work to be done. Callback functions, data, and other things of that nature.

First we create a new context. Next we can add a log function to accept log messages from libgphoto2. In my experience, no matter what you do you will get a lot of useless garbage output from libgphoto2. For this reason, I recommend you don’t just let this spew to the console or some other user-facing output. At first, I was going to send this to the console queue, but I’ve since decided against using this feature. It is good to know about though in case you need it for troubleshooting.

After all of that is done, we need to create our camera object, and initialize libgphoto2.


gint dmp_cm_camera_finalize() { gp_camera_unref(camera); gp_context_unref(context); return DMP_PB_SUCCESS; }

Nothing particularly tricky there. We need to ensure we free our memory when we’re done, so we unref our camera and context. Having seen these two functions, you may be wondering to yourself: “Are we dealing with GObjects here?” Luckily for us, there is a simple test for this:


I’ll spare you the effort of running this test: the assertion fails. Too bad really, but it is what it is. Libgphoto2 just uses function names similar to GObject.


gint dmp_cm_camera_capture(gchar * location) { CameraFile * file; CameraFilePath camera_file_path; gint fd; CameraEventType event_type; void * event_data; if (gp_camera_capture(camera, GP_CAPTURE_IMAGE, &camera_file_path, context) != GP_OK) { //error handling } if ((fd = g_open(location, O_CREAT | O_WRONLY, 0644)) == -1) { //error handling } do { gp_camera_wait_for_event(camera, 1000, &event_type, &event_data, context); if (event_type == GP_EVENT_CAPTURE_COMPLETE) break; } while(event_type != GP_EVENT_TIMEOUT); if (gp_file_new_from_fd(&file, fd) != GP_OK) { //error handling } do { gp_camera_wait_for_event(camera, 1000, &event_type, &event_data, context); } while(event_type != GP_EVENT_TIMEOUT); if (gp_camera_file_get(camera, camera_file_path.folder,, GP_FILE_TYPE_NORMAL, file, context) != GP_OK) { //error handling } if (gp_camera_file_delete(camera, camera_file_path.folder,, context) != GP_OK) { //error handling } gp_file_free(file); do { gp_camera_wait_for_event(camera, 1000, &event_type, &event_data, context); } while(event_type != GP_EVENT_TIMEOUT); return DMP_PB_SUCCESS; }

This function is where the meat of the process is. First we need to do some housekeeping. We create a CameraFile pointer to represent the actual image file, and a CameraFilePath struct to represent the path to the file. We also create an int for use as a file descriptor, a CameraEventType and void pointer for our calls to gp_cmaera_wait_for_event

Next we call gp_camera_capture which triggers the camera to take a picture. After that is done, we’ll open a file descriptor to save the image. You’ll notice that the call to g_open is enclosed in parentheses. THIS STEP IS 100% MANDATORY Don’t omit it, you’ll be sorry. More on this in a bit.

Next, we wait for the camera to finish working. The camera uses an event system; it will emit events when things happen. After releasing the shutter, the camera has other work to do before it is “done taking the picture”. If you try to do the next step before the camera is ready libgphoto2 will spew garbage to your STDOUT and you’ll have to ctrl+c to fix it. To avoid this, we call gp_camera_wait_for_event while event_type != GP_EVENT_TIMEOUT || GP_EVENT_CAPTURE_COMPLETE Capture complete is obviously the event we care about, but it may have happened while we weren’t listening for it. In that case, we’ll settle for a timeout.

Next up is instantiating our CameraFile. We use our File descriptor that we just opened to call gp_file_new_from_fd. Unfortunately there is no gp_file_new_from_file_pointer which means that this call is POSIX only, and there’s no portable substitute.

After creating our CameraFile we download the image we just took by calling gp_camera_file_get and then delete the file from the camera using gp_camera_file_delete

Finally we make sure no events are pending, then return.

Why Are You Yelling At Me?

Good question. The block in question of course is

if ((fd = g_open(location, O_CREAT | O_WRONLY, 0644)) == -1) { //error handling }

Inside of that if block, I’m assigning a value and testing the result inside of the if statement. This operation is about a 2 out of 10 on the cleverness scale. Normally, you could omit the parentheses around (fd = g_open(location, O_CREAT | O_WRONLY, 0644). However, if we do it here, things go off the rails. Not right away, of course, but a few function calls later we get to:

if (gp_camera_file_get(camera, camera_file_path.folder,, GP_FILE_TYPE_NORMAL, file, context) != GP_OK) { //error handling }

As soon as gp_camera_file_get(...) is evaluated, this is spewed to the console:


…and you have no choice but to kill the process.

Why does this happen? I have no idea. Why does enclosing the call to g_open in parenthesis fix it? Again, no idea. And it only happens here too. I just tried to modify the examples that come with libgphoto2 to reproduce the error and get that screenshot for this post, but it works fine there. Knowing my luck, if you download and build the program, it’ll work fine for you.

As long as it works, I guess…

DMP Photo Booth: First Commit

Well, it’s been a long time coming, but this weekend 45 days of work has culminated in my first milestone: DMP Photo Booth has received its first commit. If you go to this page, you’ll find the newly created Github repository for it. Feel free to go judge all the choices I made, I’ll wait…

…done? Good, now I’d like to use this post to talk about it.

The Story So Far


So far, most of the work is done in the core. There are module stubs for each module, and they are loadable and “function”, but they don’t do much of anything. However, the UI for the core is, for the most part, done. If you build the core (Either open the projects in NetBeans or cd into its directory and enter make build), and run it (as of this writing, it must be run from the project root directory, since the supporting files are located here and referenced by relative paths) it will launch. At this point, you can launch the about window, the config window, or exit the program. From the config window you can load your modules (built stub modules are included for your convenience), and the status indicators on the main window will detect it.

All this is fine and good, but the real story here is the plumbing that has been laid.

Said Plumbing

Not much works at the moment, but the groundwork has been laid for things to come. Going down the list:

Global Defines

In this header, you’ll find all the magic integer constant return values used throughout DMP Photo Booth and friends. I touched on this topic briefly in a previous post, but the cliff’s notes version is that I feel that using a bare integer instead of an enum reduces the burden on module developers, and that the benefits of an enum aren’t great enough to justify this burden.

As this header has no accompanying .c file, it can be #included by module developers so they can make use of it if they like. I do this in my modules.


One of the first things I worked on was module handling. This header holds all the functions required for the loading and unloading of modules, and the calling of module functions. At first I was implementing all of this with POSIX calls, but it occurred to me that I was defeating the purpose of my modular architecture by using non-portable calls. For this reason, I switched to GModule for module handling. I wrote a post about this if you want to hear more.

Module Callbacks

This header contains functions that will be added to modules as callbacks. Currently, this is the function that will be called when the user presses “the button”, and the function that will be called to push a message to the console queue.

Error Handling

My general approach to function design is that functions should not return void. All functions that would, instead return gint. This way, I have a mechanism to detect error conditions. In a language without Exception Handling, this is a nice thing to have. This header contains functions to extend this. Since sometimes you can’t return gint, I’ve created a facility to log error codes, similar to how the POSIX errno function works.

Console Queue

This header contains a thread safe queue to store messages to print to the console. Currently, this is implemented with a GQueue and mutex locking, but I made that choice before I knew about GAsyncQueue. I may refactor this at some point to use GAsyncQueue, but for now it works fine, and I see no immediate issue with my implementation.

The idea behind the console queue is that there should be a generic way to store messages. Currently the console is a GtkTextBuffer, but if I build around this, it limits future functionality. Suppose I wanted to support a command line only option? It’d be silly to require GTK for that, wouldn’t it?

That said, the default implementation uses a function placed in the GtkMainLoop to pop all messages out of the queue and place them into the GtkTextBuffer.

Module Status Watchdog

This header file is in charge of keeping those fancy module status indicators up to date. There are two main moving parts here: a thread that polls the module status, and a function run by the GtkMainLoop.

The Thread polls the modules every 3 seconds and pushes any status changes to a GAsyncQueue. Meanwhile the GtkMainLoop checks the GAsyncQueue for updates, and sets the indicator status accordingly. Why doesn’t the GtkMainLoop just check the module status you ask? I wanted to make sure the GtkMainLoop does as little work as possible. Currently the calls to check module status are pretty trivial, but there is no guarantee that they will stay that way. Since nobody likes an unresponsive interface, I kept them separate.

User Interface

This rather hefty header contains two things: a function to build and launch the UI, and all the callbacks called by said UI. All the callbacks are named according to their glade widget name to maintain some semblance of readability.

The Next Step

Obviously, looking through the User Interface source, there are signals to be hooked up. But aside from that, the core is coming along. The one big portion of the core remaining to be done is the photo booth strip logic. The camera module will take X pictures and download them. At this point it is up to the core to assemble them into on image file to be printed.

For now, I will continue to hook up signals and refine the existing portions of the core. While I do this, I will research image handling libraries. I’d like to keep things within the GLib family, but failing this, I will try to keep it limited to libraries shipped with Ubuntu (The default platform). Windows compatibility will come, but version 1.0 will work on Ubuntu.

Also looming on the horizon are the modules. I have a general idea of how I will deal with these, but nothing is set in stone. The first module that I will likely handle is the trigger module. I plan to implement the trigger module using Arduino. This is a topic that interest me, but I’ve never had a good project idea. Whenever I get tired of dealing with GTK, it should be a nice change of pace.

DMP Photo Booth: Going All In With GLib Primitives

While working on DMP Photo Booth, I’ve been trying to use GLib library calls as often as possible as opposed to standard library calls. Why? Portability, of course. I’ve been doing my best to ensure my application is as portable as possible. While doing this, I find myself having to work with lots of glib types. After a few weeks of this, I find myself wondering if I shouldn’t be using these in my own code…

GLib Primitives

GLib defines g[type] versions of all standard C primitives, plus a few extras. The GLib reference manual describes these as falling into 4 different categories:

  • New types which are not part of standard C (but are defined in various C standard library header files) – gboolean, gsize, gssize, goffset, gintptr, guintptr.
  • Integer types which are guaranteed to be the same size across all platforms – gint8, guint8, gint16, guint16, gint32, guint32, gint64, guint64.
  • Types which are easier to use than their standard C counterparts – gpointer, gconstpointer, guchar, guint, gushort, gulong.
  • Types which correspond exactly to standard C types, but are included for completeness – gchar, gint, gshort, glong, gfloat, gdouble.

Let’s talk about these.

New Types

The new types represent boolean values, and size types. Both of these may have been new when GLib defined them, but they’ve since found their way into the C standard library since C99. That said, these are types that we often neglect to use. Who can honestly say they’ve never used a bare integer when a size_t would have been appropriate?

I feel that these are all good types to use, when needed. Maybe they no longer solve a real problem since C99, but GLib still uses them. I will use these for consistency’s sake, but the standard library types should suffice.

Integer Types

As we should all know, integers are not guaranteed to be the same size across different platforms. On platform A, an int may be 2 bytes, and on platform B, it may be 4 bytes. Meanwhile, these types provide this guarantee. Like the new types, C99 brings these types also, but in my opinion gint8 is easier to type than int_8.

Aside from ease, GLib uses its types, and for consistency I will too.

Types That Are “Easier To Use”

In my opinion, this one is debatable. The people over at Gnome may think that gpointer “looks better” and is “easier to use”, but I find it incredibly confusing. I’ve even mentioned this in past posts. Gpointer is a typedef of void *. In other words, where you may have done this before:

void * foo = malloc(sizeof(foo));

…you would now do this:

gpointer foo = g_malloc(sizeof(foo))

…and now you’re left with this gpointer foo;. Aside from the fact that it has the word “pointer” in the type name, this doesn’t look like a pointer. Even worse is when you see gpointer * bar;. At first glance, this looks like it should be a void * bar;, but it’s actually void * * bar;. I know I’ve spent more time than I care to admit trying to figure out why bar->the_bar was throwing compiler errors.

Maybe it’s just amateur hour in the DMP Undersea Laboratory, but I like to see the *. That way it’s right in my face that I’m dealing with a pointer. I can also count the number of *s to see how many array indices I’m dealing with. If I see void *** super_foo; I know I’m dealing with a 3 dimensional array just by counting the stars. I don’t have to think about it. On the other hand if I see gpointer ** super_foo; I have to know what gpointer is a typedef of; I can’t just trust my instincts.

For this reason, I will not be using gpointer or gconstpointer. The rest of the “easier to use” types can stay though.

Types That Correspond Exactly

There is not much to say about these. These types exist just for consistency’s sake. If you’re using GLib primitives, you should use these as well for consistency’s sake.

Seems Kind Of Pointless…

At first glance, yes it does. I must admit that while I was typeing this up, I wavered. “If all the GLib types are in the standard library in C99, why bother?” I thought. None of these types are objectivly better than the standard library versions, it’s just another name to remember. So why should we care?

The answer is because not all compilers/platforms support C99. The most nobale offender here is Microsoft Visual Studio. To quote Herb Sutter: “Our primary goal is to support “most of C99/C11 that is a subset of ISO C++98/C++11.”” This allegedly includes “C99 preprocessor and library extensions”, but this requires compiling with Visual Studio’s C++ compiler and accepting all the baggage that C++ brings. If you want to keep your code pure C, then you are stuck using C90 only.

So, long story short: if you want easy portability to Windows, consider using GLib primitives.

Moving Forward

It is time to get with the program. Development of DMP Photo Booth is still early-enough along that a refactor to use GLib primitives won’t be too difficult. I will not change the API for the modules because, while I will most likely use GLib with the reference modules, I don’t want to impose a requirement to use GLib. Int and char are exactly equivilent to gint and gchar, so this should not cause any issues.

However, moving forward, I’ll be using GLib primitives.

Bringing The Portability With GModule

As I’ve been writing DMP Photo Booth, I’ve been taking great pains to improve portability. I’ve got a fancy module-based architecture designed to segregate the non-portable sections of the project from the Core. The only problem? A bunch of ugly POSIX calls: dlopen(), dlsym(), and dlclose(). Kind of defeats the purpose of using modules for portability if I don’t load said modules in a portable way, doesn’t it? I thought so too…

Enter GModule

GModule is part of the GLib family of libraries. GModule provides a portable way to handle working with shared libraries. It works on any POSIX compliant platform, as well as Windows, and HP-UX via its shl_load() mechanism. You can read more about GModule in the GLib Reference Manual. While I’m sure there is some edge case that GLib doesn’t cover, this is far more portable than I’d initially envisioned DMP Photo Booth being. (Yay, HP-UX support!)

Another consideration in all of this is the adding of dependencies. However, since I’m already using GTK3 for my GUI, I already have a GLib dependency, so there is no added burden to using GModule.

The How

GModule is actually quite similar to using POSIX dlfcn.h functions. Some semantics are different, but GModule has functions that are roughly equivalent to the POSIX functions.

GModule * g_module_open(const gchar *file_name, GModuleFlags flags);

G_module_open() is the replacement for dlopen() in POSIX. The GModule pointer that it returns is the replacement for the void pointer returned by dlopen(). GModuleFlags is an integer flag that can be boolean or’d in. Your options for this are G_MODULE_BIND_LAZY and G_MODULE_BIND_LOCAL which are equivalent to RTLD_LAZY and RTLD_LOCAL.

gboolean g_module_symbol(GModule *module, const gchar *symbol_name, gpointer *symbol);

This is your replacement for dlsym(), and functions mostly in the same way. module is the gmodule pointer to extract symbols from, symbol_name is the symbol to get, and symbol is the function pointer to populate. This function returns true if successful, and false if not. This function is commonly called like this:

if (!g_module_symbol(dmp_pb_camera_module, "dmp_cm_capture", (gpointer *) &dmp_cm_capture)) { /* error handling here */ }

This idiom can be found throughout the GLib documentation. Did you see the craziness that is argument number 3? Dmp_cm_capture is a function pointer, as you may remember, but GObject tends to make things a little tricky, and will throw thousands of warnings if you don’t cast your function pointer to a gpointer *. The definition of gpointer is:

typedef void* gpointer;

That means that a gpointer * is actually void **. Therefore, you are expected to pass in the address of your pointer using an ampersand, hence (gpointer *) &dmp_cm_capture.

gboolean g_module_close(GModule *module);

As a nice change of pace from the last function, this one is quite straight forward. You pass in your GModule pointer, and it closes it, just like dlclose(), then returns true (or false, if an error occurs). Nothing particularly noteworthy here.

gboolean g_module_supported();

While this function does not line up with a POSIX function, I felt it was important to mention this one. This function returns true if you are on a platform that GModule supports. If you’re on Linux/UNIX/OSX, or Windows, or HP-UX, this should return true. If you’re hacking your Atari-2600, it will most likely return false. Stick this in front of calls to GModule functions and save yourself a headache.

Moving Forward

My design philosophy for DMP Photo Booth is that the Core should compile on any (typical) platform with no changes. Using GLib, this seems to be within reach. With GModules, GTK3 User Interfaces, I’ve kept the faith so far. Looking through the GLib documentation, it has GThreads, so when I inevitably get mired in threaded programming, I should be golden. GLib also has support for Pipes and file IO via GIOChannel, but the documentation claims only partial support in Windows.

Here’s hoping all goes well!

Introducing DMP Photo Booth

In June of 2014, I will be getting married. One problem: I am currently an unemployed student, and my fiancee Liz is also between jobs. Recent studies show that the average couple in this day and age spends an average of Twenty Four Bajillion Dollars on their wedding. Capital Bajillion. Italicised. Even were I still employed, this would pose a problem for me. Needless to say, we began looking for costs to cut. Liz really wants a photo booth, and I think it sounds fun. Unfortunatley photo booths run from $750-1000 to rent. The solution: DMP Photo Booth.

On Reinventing The Wheel

DMP Photo Booth wasn’t actually my first thought. That would be “well, there must be some open-source photo booth software we can use!” It turns out that there is. People like me who’ve had this issue and whipped something up for themselves. Some of these projects even have fancy websites. Unfortunately, none of these seem to have reached the level of maturity where you can just grab them and go. No, we’re talking old-school Linux status projects. If I’m going to have to put that much effort into making this work, I might as well roll my own. Besides, it will be good experience.


The photo booth must do the usual photo booth stuff; it should programatically take 3-4 pictures, arrange them in a vertical strip with a background picture, and print them on photo paper. This should all work with the user only having to press a button(not a key on the laptop, an actual button), and there should be some sort of indicator when the pictures are going to be taken.

Some reasearch turns up the existence of a Picture Transfer Protocol, or PTP, that has been “supported” by the major camera vendors since 2002 or so. PTP allows you to programatically control a digital camera, and should serve nicely. I own a printer that can print on photo paper; it seems likely that I will be able to use the operating system’s printing facilities to handle this. Finally, I will construct a button and indicator thingamabober using Arduino.


I have decided to go with a modular architecture. There will be four components: a Camera Module, a Printer Module, a Trigger Module, and the Core Application.


The Core Application will bring the GUI, tenatively planned to be implemented using GTK+3, and will interface with the component modules. The core expects the component modules to implement specific functions that will be called to handle their operation. The component modules will be dynamic libraries that can be swapped out at runtime.

Camera Module

The Camera Module will handle the operation of the camera. The Camera Module currently is expected to implement two functions:

int dmp_cm_capture(); int dmp_cm_download(char * location);

These two functions do what they say: the first takes a picture, and the second downloads the picture to the passed-in path. The reference Camera Module will use libgphoto2 to implement these functions, but this module exists to provide the capability to use any method. A module that uses some other PTP library, or some custom protocol, or a dummy module that uses a picture on the filesystem, or uses a scanner, or anything else can be swapped in at runtime and this will all be transparent to the Core.

Printer Module

Shockingly, the Printer Module will handle printing. The Printer Module is currently expected to implement one function:

int dmp_pm_print(char * to_print);

This function prints the file at the path that is passed into the function. This module prevents the Core from needing to know how to print. The Core tells the Printer Module to print, and the Printer Module can figure out how to print in Linux, or Mac OSX, or how to print to a Game Boy Printer, or whatever. The Core doesn’t care.

Trigger Module

The Trigger Module is the module that knows how to operate the custom photo booth equipment. The Trigger Module currently is expected to implement two functions:

int dmp_tm_add_trigger_handler(void (*th)()); int dmp_tm_set_countdown(int current);

The first function adds a callback function that will be called by the Trigger Module when it detects that the button has been pressed. The second function updates the countdown indicator. Since some dealybopper that I made out of Arduino is basically the definition of non-portable, I decided to create a module for it. The reference implementation will be written for my equipment, but this module can be replaced with one that uses some other input device with minimal effort.

Moving Forward

DMP Photo Booth represents my first major open source project, and it has many moving parts. With this groundwork laid, I have broken the project down into more manageable chunks. I have four main components that each represent different challenges: GUI programming in the Core, poorly documented mystery libraries in the Camera Module, Learning to Arduino to finish the Trigger Module, and learning the Linux printing system for the Printer Module.

Rome wasn’t built in a day, but it was started in one. There will surely be many challenges and roadblocks on the way. Expect to hear all about them here!

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