Shallow Thoughts : tags : gtk

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Sun, 24 Dec 2017

Saving a transparent PNG image from Cairo, in Python

Dave and I will be giving a planetarium talk in February on the analemma and related matters.

Our planetarium, which runs a fiddly and rather limited program called Nightshade, has no way of showing the analemma. Or at least, after trying for nearly a week once, I couldn't find a way. But it can show images, and since I once wrote a Python program to plot the analemma, I figured I could use my program to generate the analemmas I wanted to show and then project them as images onto the planetarium dome.

[analemma simulation] But naturally, I wanted to project just the analemma and associated labels; I didn't want the blue background to cover up the stars the planetarium shows. So I couldn't just use a simple screenshot; I needed a way to get my GTK app to create a transparent image such as a PNG.

That turns out to be hard. GTK can't do it (either GTK2 or GTK3), and people wanting to do anything with transparency are nudged toward the Cairo library. As a first step, I updated my analemma program to use Cairo and GTK3 via gi.repository. Then I dove into Cairo.

I found one C solution for converting an existing Cairo surface to a PNG, but I didn't have much luck with it. But I did find a Python program that draws to a PNG without bothering to create a GUI. I could use that.

The important part of that program is where it creates a new Cairo "surface", and then creates a "context" for that surface:

surface = cairo.ImageSurface(cairo.FORMAT_ARGB32, *imagesize)

cr = cairo.Context(surface)

A Cairo surface is like a canvas to draw on, and it knows how to save itself to a PNG image. A context is the equivalent of a GC in X11 programming: it knows about the current color, font and so forth. So the trick is to create a new surface, create a context, then draw everything all over again with the new context and surface.

A Cairo widget will already have a function to draw everything (in my case, the analemma and all its labels), with this signature:

    def draw(self, widget, ctx):

It already allows passing the context in, so passing in a different context is no problem. I added an argument specifying the background color and transparency, so I could use a blue background in the user interface but a transparent background for the PNG image:

    def draw(self, widget, ctx, background=None):

I also had a minor hitch: in draw(), I was saving the context as self.ctx rather than passing it around to every draw routine. That means calling it with the saved image's context would overwrite the one used for the GUI window. So I save it first.

Here's the final image saving code:

   def save_image(self, outfile):
        dst_surface = cairo.ImageSurface(cairo.FORMAT_ARGB32,
                                         self.width, self.height)

        dst_ctx = cairo.Context(dst_surface)

        # draw() will overwrite self.ctx, so save it first:
        save_ctx = self.ctx

        # Draw everything again to the new context,
        # with a transparent instead of an opaque background:
        self.draw(None, dst_ctx, (0, 0, 1, 0))  # transparent blue

        # Restore the GUI context:
        self.ctx = save_ctx

        print("Saved to", outfile)

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[ 19:39 Dec 24, 2017    More programming | permalink to this entry | comments ]

Sat, 06 Aug 2016

Adding a Back button in Python Webkit-GTK

I have a little browser script in Python, called quickbrowse, based on Python-Webkit-GTK. I use it for things like quickly calling up an anonymous window with full javascript and cookies, for when I hit a page that doesn't work with Firefox and privacy blocking; and as a quick solution for calling up HTML conversions of doc and pdf email attachments.

Python-webkit comes with a simple browser as an example -- on Debian it's installed in /usr/share/doc/python-webkit/examples/ But it's very minimal, and lacks important basic features like command-line arguments. One of those basic features I've been meaning to add is Back and Forward buttons.

Should be easy, right? Of course webkit has a go_back() method, so I just have to add a button and call that, right? Ha. It turned out to be a lot more difficult than I expected, and although I found a fair number of pages asking about it, I didn't find many working examples. So here's how to do it.

Add a toolbar button

In the WebToolbar class (derived from gtk.Toolbar): In __init__(), after initializing the parent class and before creating the location text entry (assuming you want your buttons left of the location bar), create the two buttons:

        backButton = gtk.ToolButton(gtk.STOCK_GO_BACK)
        backButton.connect("clicked", self.back_cb)
        self.insert(backButton, -1)

        forwardButton = gtk.ToolButton(gtk.STOCK_GO_FORWARD)
        forwardButton.connect("clicked", self.forward_cb)
        self.insert(forwardButton, -1)

Now create those callbacks you just referenced:

   def back_cb(self, w):

    def forward_cb(self, w):

That's right, you can't just call go_back on the web view, because GtkToolbar doesn't know anything about the window containing it. All it can do is pass signals up the chain.

But wait -- it can't even pass signals unless you define them. There's a __gsignals__ object defined at the beginning of the class that needs all its signals spelled out. In this case, what you need is

       "go-back-requested": (gobject.SIGNAL_RUN_FIRST,
                              gobject.TYPE_NONE, ()),
       "go-forward-requested": (gobject.SIGNAL_RUN_FIRST,
                              gobject.TYPE_NONE, ()),
Now these signals will bubble up to the window containing the toolbar.

Handle the signals in the containing window

So now you have to handle those signals in the window. In WebBrowserWindow (derived from gtk.Window), in __init__ after creating the toolbar:

        toolbar.connect("go-back-requested", self.go_back_requested_cb,
        toolbar.connect("go-forward-requested", self.go_forward_requested_cb,

And then of course you have to define those callbacks:

def go_back_requested_cb (self, widget, content_pane):
    # Oops! What goes here?
def go_forward_requested_cb (self, widget, content_pane):
    # Oops! What goes here?

But whoops! What do we put there? It turns out that WebBrowserWindow has no better idea than WebToolbar did of where its content is or how to tell it to go back or forward. What it does have is a ContentPane (derived from gtk.Notebook), which is basically just a container with no exposed methods that have anything to do with web browsing.

Get the BrowserView for the current tab

Fortunately we can fix that. In ContentPane, you can get the current page (meaning the current browser tab, in this case); and each page has a child, which turns out to be a BrowserView. So you can add this function to ContentPane to help other classes get the current BrowserView:

    def current_view(self):
        return self.get_nth_page(self.get_current_page()).get_child()

And now, using that, we can define those callbacks in WebBrowserWindow:

def go_back_requested_cb (self, widget, content_pane):
def go_forward_requested_cb (self, widget, content_pane):

Whew! That's a lot of steps for something I thought was going to be just adding two buttons and two callbacks.

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[ 16:45 Aug 06, 2016    More programming | permalink to this entry | comments ]

Sat, 07 May 2016

Setting "Emacs" key theme in gtk3 (and Firefox 46)

I recently let Firefox upgrade itself to 46.0.1, and suddenly I couldn't type anything any more. The emacs/readline editing bindings, which I use probably thousands of times a day, no longer worked. So every time I typed a Ctrl-H to delete the previous character, or Ctrl-B to move back one character, a sidebar popped up. When I typed Ctrl-W to delete the last word, it closed the tab. Ctrl-U, to erase the contents of the urlbar, opened a new View Source tab, while Ctrl-N, to go to the next line, opened a new window. Argh!

(I know that people who don't use these bindings are rolling their eyes and wondering "What's the big deal?" But if you're a touch typist, once you've gotten used to being able to edit text without moving your hands from the home position, it's hard to imagine why everyone else seems content with key bindings that require you to move your hands and eyes way over to keys like Backspace or Home/End that aren't even in the same position on every keyboard. I map CapsLock to Ctrl for the same reason, since my hands are too small to hit the PC-positioned Ctrl key without moving my whole hand. Ctrl was to the left of the "A" key on nearly all computer keyboards until IBM's 1986 "101 Enhanced Keyboard", and it made a lot more sense than IBM's redesign since few people use Caps Lock very often.)

I found a bug filed on the broken bindings, and lots of people commenting online, but it wasn't until I found out that Firefox 46 had switched to GTK3 that I understood had actually happened. And adding gtk3 to my web searches finally put me on the track to finding the solution, after trying several other supposed fixes that weren't.

Here's what actually worked: edit ~/.config/gtk-3.0/settings.ini and add, inside the [Settings] section, this line:

gtk-key-theme-name = Emacs

I think that's all that was needed. But in case that doesn't do it, here's something I had already tried, unsuccessfully, and it's possible that you actually need it in addition to the settings.ini change (I don't know how to undo magic Gnome settings so I can't test it):

gsettings set org.gnome.desktop.interface gtk-key-theme "Emacs"

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[ 18:11 May 07, 2016    More linux | permalink to this entry | comments ]

Sun, 26 Sep 2010

GIMP Wallpaper script improvements

Dave was using some old vacation photos to test filesystem performance, and that made me realize that I had beautiful photos from the same trip that I hadn't yet turned into desktop backgrounds.

Sometimes I think that my GIMP Wallpaper script is the most useful of the GIMP plug-ins I've written. It's such a simple thing ... but I bet I use it more than any of my other plug-ins, and since I normally make backgrounds for at least two resolutions (my 1680x1050 desktop and my 1366x768 laptop), it certainly saves me a lot of time and hassle.

But an hour into my background-making, I started to have nagging doubts. I wasn't renaming these images, just keeping the original filenames from the camera, like pict0828.jpg. What if if some of these were overwriting images of the same name? The one thing my script doesn't do is check for that, and gimp_file_save doesn't pop up any warnings. I've always meant to add a check for it.

Of course, once the doubts started, I had to stop generating backgrounds and start generating code. And I'm happy with the result: warns and won't let you save over an old background image, but keeps all the logic in one dialog rather than popping up extra warnings.

[ overwrite warning dialog]

Now I can generate backgrounds without worrying that I'm stomping on earlier ones.

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[ 22:25 Sep 26, 2010    More gimp | permalink to this entry | comments ]

Tue, 15 Sep 2009

GTK dialogs in GIMP (and updated wallpaper script)

[Grosvenor Arch] I've been getting tired of my various desktop backgrounds, and realized that I had a lot of trip photos, from fabulous places like Grosvenor Arch (at right), that I'd never added to my background collection.

There's nothing like lots of repetitions of the same task to bring out the shortcomings of a script, and the wallpaper script I threw together earlier this year was no exception. I found myself frequently irritated by not having enough information about what the script was doing or being able to change the filename. Then I could have backgrounds named grosvenor.jpg rather than img2691.jpg.

Alas, I can't use the normal GIMP Save-as dialog, since GIMP doesn't make that dialog available to plug-ins. (That's a deliberate choice, though I've never been clear on the reason behind it.) If I wanted to give that control to the user, I'd have to make my own dialogs.

It's no problem to make a GTK dialog from Python. Just create a gtk.Dialog, add a gtk.Entry to it, call, then check the return value and get the entry's text to see if it changed. No problem, right?

Ha! If you think that, you don't work with computers. The dialog popped up fine, it read the text entry fine ... but it wouldn't go away afterward. So after the user clicked OK, the plug-in tried to save and GIMP popped up the JPEG save dialog (the one that has a quality slider and other controls, but no indication of filename) under my text entry dialog, which remained there.

All attempts at calling dialog.hide() and dialog.destroy() and similar mathods were of no avail. A helpful person on #pygtk worked with me but ended up as baffled as I was. What was up?

The code seemed so simple -- something like this:

    response =
    if response == gtk.RESPONSE_OK :
        pathname = pathentry.get_text()
        pdb.gimp_file_save(newimg, newimg.active_layer, pathname, pathname,

In the end, GIMP guru Sven pointed me to the answer. The problem was that my dialog wasn't part of the GTK main loop. In retrospect, this makes sense: the plug-in is an entirely different process, so I shouldn't be surprised that it would have its own main loop. So when I hide() and destroy(), those events don't happen right away because there's no loop in the plug-in process that would see them.

The plug-in passes control back to GIMP to do the gimp_file_save(). GIMP's main loop doesn't have access to the hide and destroy signals I just sent. So the gimp_file_save runs, popping up its own dialog (under mine, because the JPEG save dialog is transient to the original image window while my python dialog isn't). That finishes, returns control to the plug-in, the plug-in exits and at that point GTK cleans up and finally destroys the dialog.

The solution is to loop over GTK events in the plug-in before calling gimp_file_save, like this:

    response =
    if response == gtk.RESPONSE_OK :
        pathname = pathentry.get_text()
        while gtk.events_pending() :
        pdb.gimp_file_save(newimg, newimg.active_layer, pathname, pathname,

That loop gives the Python process a chance to clean up the dialog before passing control to GIMP and its main loop. GTK in the subprocess is happy, the user is happy, and I'm happy because now I have a much more efficient way of making lots of desktop backgrounds for lots of different machines.

The updated script, along with a lot more information on how to use it and how to set up tool presets for it.

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[ 23:21 Sep 15, 2009    More gimp | permalink to this entry | comments ]

Sat, 20 Jun 2009

Pytopo 0.8 released

On my last Mojave trip, I spent a lot of the evenings hacking on PyTopo.

I was going to try to stick to OpenStreetMap and other existing mapping applications like TangoGPS, a neat little smartphone app for downloading OpenStreetMap tiles that also runs on the desktop -- but really, there still isn't any mapping app that works well enough for exploring maps when you have no net connection.

In particular, uploading my GPS track logs after a day of mapping, I discovered that Tango really wasn't a good way of exploring them, and I already know Merkaartor, nice as it is for entering new OSM data, isn't very good at working offline. There I was, with PyTopo and a boring hotel room; I couldn't stop myself from tweaking a bit.

Adding tracklogs was gratifyingly easy. But other aspects of the code bother me, and when I started looking at what I might need to do to display those Tango/OSM tiles ... well, I've known for a while that some day I'd need to refactor PyTopo's code, and now was the time.

Surprisingly, I completed most of the refactoring on the trip. But even after the refactoring, displaying those OSM tiles turned out to be a lot harder than I'd hoped, because I couldn't find any reliable way of mapping a tile name to the coordinates of that tile. I haven't found any documentation on that anywhere, and Tango and several other programs all do it differently and get slightly different coordinates. That one problem was to occupy my spare time for weeks after I got home, and I still don't have it solved.

But meanwhile, the rest of the refactoring was done, nice features like track logs were working, and I've had to move on to other projects. I am going to finish the OSM tile MapCollection class, but why hold up a release with a lot of useful changes just for that?

So here's PyTopo 0.8, and the couple of known problems with the new features will have to wait for 0.9.

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[ 20:49 Jun 20, 2009    More programming | permalink to this entry | comments ]

Sat, 14 Mar 2009

The new gtk file selector fstab viewer

[New gtk 2.14.4 file selector] When I upgraded to Ubuntu Intrepid recently, I pulled in a newer GTK+, version 2.14.4. And when I went to open a file in GIMP, I got a surprise: my "bookmarks" were no longer visible without scrolling down.

In the place where the bookmarks used to be, instead was a list of ... what are those things? Oh, I see ... they're all the filesystems listed with "noauto" in my /etc/fstab --the filesystems that aren't mounted unless somebody asks for them, typically by plugging in some piece of hardware.

There are a lot of these. Of course there's one for the CDROM drive (I never use floppies so at some point I dropped that entry). I have another entry for Windows-formatted partitions that show up on USB, like when I plug in a digital camera or a thumb drive. I also have one of those front panel flash card readers with 4 slots, for reading SD cards, memory sticks, compact flash, smart media etc. Each of those shows up as a different device, so I treat them separately and mount SD cards as /sdcard, memory sticks as /stick and so on. In addition, there are entries corresponding to other operating systems installed on this multi-boot machine, and to several different partitions on my external USB backup drive. These are all listed in /etc/fstab with entries like this:

/dev/hdd   /cdrom  udf,iso9660  user,noauto               0  0
/dev/sde1  /pix    vfat         rw,user,fmask=133,noauto  0  0

[Places in the gtk 2.14.4 file selector]

The GTK developers, in their wisdom, have realized that what the file selector really needs to be. I mean, I was just thinking while opening a file in GIMP the other day,

"Browsing image files on filesystems that are actually mounted is so tedious. I wish I could do something else instead, like view my /etc/fstab file to see a list of unmounted filesystems for which I might decide to plug in an external device."

Clicking on one of the unmounted filesystems (even right-clicking!) gives an error:

Could not mount sdcard
mount: special device /dev/sdb1 does not exist
So I guess the intent is that I'll plug in my external drive or camera, then use the gtk file selector from a program like GIMP as the means to mount it. Um ... don't most people already have some way of mounting new filesystems, whether it's an automatic mount from HAL or typing mount in a terminal?

(And before you ask, yes, for the time being I have dbus and hal and fam and gamin and all that crap running.)

The best part

But I haven't even told you the best part yet. Here it is:

If you mount a filesystem manually, e.g. mount /dev/sdb1 /mnt ... it doesn't show up in the list!

So this enormous list of filesystems that's keeping me from seeing my file selector bookmarks ... doesn't even include filesystems that are really there!

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[ 12:59 Mar 14, 2009    More linux | permalink to this entry | comments ]

Sun, 14 Dec 2008

Sometimes a horrible hack is the best solution (and building gtk on Tiger)

Dave has been fighting for I don't know how many weeks trying to get a buildable set of gtk libraries installed on his Mac.

He doesn't need them to build GIMP -- the GIMP on OS X project (split off from Wilber Loves Apple) provides binaries complete with all the libraries needed. Alas, it's just a binary package with no development headers, so if you want to build any other gtk packages, like pho, or maybe some GIMP plug-ins, you're in for a much longer adventure.

Mac Ports used to make that easy, but the Ports version of gtk2 doesn't build on OS X "Tiger". It's a long story and I don't (want to) know all the hairy details, but this weekend he finally gave up on it and began downloading all the gtk2 packages and dependencies (cairo, pango, bonobo, atk etc.) from their various project sites.

Oddly enough, building them went much more smoothly than Ports had, and after a little twiddling of --disable flags in configure and a lot of waiting, he had most of the libraries built. Even gtk2 itself! Except ... gtk2's make install failed.

Seems that although gtk is configured to disable building docs by default (configure --help shows a --enable-gtk-doc option), nevertheless make install calls something called gtkdoc-rebase from a lot of the subdirectories. And gtkdoc-rebase doesn't exist, since it wasn't ever built. So the whole make install process fails at that point -- after installing the libraries but before telling pkg-config that gtk-2.0 is indeed present.

After twiddling configure dependencies all day, Dave was getting frustrated. "How do I configure it to really disable docs, so it won't try to run this gtkdoc-rebase thing I don't have?"

I was in the middle of a timed quiz for a class I'm taking. "I have no idea. You'd think they'd check for that. Um ... all you need is for gtkdoc-rebase to return success, right? What if you make a script somewhere in your path that contains nothing but a shebang line, #! /bin/sh? It's a horrible hack, but ..."

"Horrible hacks R us!" he exclaimed, and created the script. 10 minutes later, he had gtk-2.0 installed, pkg-config notified and pho built.

Sometimes horrible hacks are the best.

The gtk2 package list

Incidentally, for anyone trying to accomplish the same thing, the packages he needed to download were:

pgk-config gettext glib pango atk jpeg jasper libpng tiff pixman freetype libxml fontconfig cairo gtk2

and he had to configure gtk2 with --disable-cups (because it introduced other errors, not because of CUPS itself). The trickiest dependency was atk, because it wasn't in the place that points to and it wasn't on its own project site either; he eventually found it by poking around on the gnome ftp site.

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[ 21:44 Dec 14, 2008    More programming | permalink to this entry | comments ]

Fri, 12 Oct 2007

PyTopo and PyGTK pixbuf memory leakage

On a recent Mojave desert trip, we tried to follow a minor dirt road that wasn't mapped correctly on any of the maps we had, and eventually had to retrace our steps. Back at the hotel, I fired up my trusty PyTopo on the East Mojave map set and tried to trace the road. But I found that as I scrolled along the road, things got slower and slower until it just wasn't usable any more.

PyTopo was taking up all of my poor laptop's memory. Why? Python is garbage collected -- you're not supposed to have to manage memory explicitly, like freeing pixbufs. I poked around in all the sample code and man pages I had available but couldn't find any pygtk examples that seemed to be doing any explicit freeing.

When we got back to civilization (read: internet access) I did some searching and found the key. It's even in the PyGTK Image FAQ, and there's also some discussion in a mailing list thread from 2003.

Turns out that although Python is supposed to handle its own garbage collection, the Python interpreter doesn't grok the size of a pixbuf object; in particular, it doesn't see the image bits as part of the object's size. So dereferencing lots of pixbuf objects doesn't trigger any "enough memory has been freed that it's time to run the garbage collector" actions.

The solution is easy enough: call gc.collect() explicitly after drawing a map (or any other time a bunch of pixbufs have been dereferenced).

So there's a new version of PyTopo, 0.6 that should run a lot better on small memory machines, plus a new collection format (yet another format from the packaged Topo! map sets) courtesy of Tom Trebisky.

Oh ... in case you're wondering, the ancient USGS maps from Topo! didn't show the road correctly either.

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[ 22:21 Oct 12, 2007    More programming | permalink to this entry | comments ]

Fri, 25 Aug 2006

PyTopo 0.5

Belated release announcement: 0.5b2 of my little map viewer PyTopo has been working well, so I released 0.5 last week with only a few minor changes from the beta. I'm sure I'll immediately find six major bugs -- but hey, that's what point releases are for. I only did betas this time because of the changed configuration file format.

I also made a start on a documentation page for the .pytopo file (though it doesn't really have much that wasn't already written in comments inside the script).

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[ 22:10 Aug 25, 2006    More programming | permalink to this entry | comments ]

Sat, 03 Jun 2006

Cleaner, More Flexible Python Map Viewing

A few months ago, someone contacted me who was trying to use my PyTopo map display script for a different set of map data, the Topo! National Parks series. We exchanged some email about the format the maps used.

I'd been wanting to make PyTopo more general anyway, and already had some hacky code in my local version to let it use a local geologic map that I'd chopped into segments. So, faced with an Actual User (always a good incentive!), I took the opportunity to clean up the code, use some of Python's support for classes, and introduce several classes of map data.

I called it 0.5 beta 1 since it wasn't well tested. But in the last few days, I had occasion to do some map exploring, cleaned up a few remaining bugs, and implemented a feature which I hadn't gotten around to implementing in the new framework (saving maps to a file).

I think it's ready to use now. I'm going to do some more testing: after visiting the USGS Open House today and watching Jim Lienkaemper's narrated Virtual Tour of the Hayward Fault, I'm all fired up about trying again to find more online geologic map data. But meanwhile, PyTopo is feature complete and has the known bugs fixed. The latest version is on the PyTopo page.

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[ 18:25 Jun 03, 2006    More programming | permalink to this entry | comments ]

Tue, 21 Jun 2005

A Fast Volume Control App

I updated my Debian sid system yesterday, and discovered today that gnome-volume-control has changed their UI yet again. Now the window comes up with two tabs, Playback and Capture; the default tab, Playback, has only one slider in it, PCM, and all the important sliders, like Volume, are under Capture. (I'm told this is some interaction with how ALSA sees my sound chip.)

That's just silly. I've never liked the app anyway -- it takes forever to come up, so I end up missing too much of any clip that starts out quiet. All I need is a simple, fast window with a single slider controlling master volume. But nothing like that seems to exist, except panel applets that are tied to the panels of particular window managers.

So I wrote one, in PyGTK. vol is a simple script which shows a slider, and calls aumix under the hood to get and set the volume. It's horizontal by default; vol -h gives a vertical slider.

Aside: it's somewhat amazing that Python has no direct way to read an integer out of a string containing more than just that integer: for example, to read 70 out of "70,". I had to write a function to handle that. It's such a terrific no-nonsense language most of the time, yet so bad at a few things. (And when I asked about a general solution in the python channel at [large IRC network], I got a bunch of replies like "use int(str[0:2])" and "use int(str[0:-1])". Shock and bafflement ensued when I pointed out that 5, 100, and -27 are all integers too and wouldn't be handled by those approaches.)

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[ 15:54 Jun 21, 2005    More programming | permalink to this entry | comments ]

Sat, 09 Apr 2005

Python Expose vs. Focus

A few days ago, I mentioned my woes regarding Python sending spurious expose events every time the drawing area gains or loses focus.

Since then, I've spoken with several gtk people, and investigated several workarounds, which I'm writing up here for the benefit of anyone else trying to solve this problem.

First, "it's a feature". What's happening is that the default focus in and out handlers for the drawing area (or perhaps its parent class) assume that any widget which gains keyboard focus needs to redraw its entire window (presumably because it's locate-highlighting and therefore changing color everywhere?) to indicate the focus change. Rather than let the widget decide that on its own, the focus handler forces the issue via this expose event. This may be a bad decision, and it doesn't agree with the gtk or pygtk documentation for what an expose event means, but it's been that way for long enough that I'm told it's unlikely to be changed now (people may be depending on the current behavior).

Especially if there are workarounds -- and there are.

I wrote that this happened only in pygtk and not C gtk, but I was wrong. The spurious expose events are only passed if the CAN_FOCUS flag is set. My C gtk test snippet did not need CAN_FOCUS, because the program from which it was taken, pho, already implements the simplest workaround: put the key-press handler on the window, rather than the drawing area. Window apparently does not have the focus/expose misbehavior.

I worry about this approach, though, because if there are any other UI elements in the window which need to respond to key events, they will never get the chance. I'd rather keep the events on the drawing area.

And that becomes possible by overriding the drawing area's default focus in/out handlers. Simply write a no-op handler which returns TRUE, and set it as the handler for both focus-in and focus-out. This is the solution I've taken (and I may change pho to do the same thing, though it's unlikely ever to be a problem in pho).

In C, there's a third workaround: query the default focus handlers, and disconnect() them. That is a little more efficient (you aren't calling your nop routines all the time) but it doesn't seem to be possible from pygtk: pygtk offers disconnect(), but there's no way to locate the default handlers in order to disconnect them.

But there's a fourth workaround which might work even in pygtk: derive a class from drawing area, and set the focus in and out handlers to null. I haven't actually tried this yet, but it may be the best approach for an app big enough that it needs its own UI classes.

One other thing: it was suggested that I should try using AccelGroups for my key bindings, instead of a key-press handler, and then I could even make the bindings user-configurable. Sounded great! AccelGroups turn out to be very easy to use, and a nice feature. But they also turn out to have undocumented limitations on what can and can't be an accelerator. In particular, the arrow keys can't be accelerators; which makes AccelGroup accelerators less than useful for a widget or app that needs to handle user-initiated scrolling or movement. Too bad!

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[ 21:52 Apr 09, 2005    More programming | permalink to this entry | comments ]

Wed, 06 Apr 2005

PyTopo is usable; pygtk is inefficient

While on vacation, I couldn't resist tweaking pytopo so that I could use it to explore some of the areas we were visiting.

It seems fairly usable now. You can scroll around, zoom in and out to change between the two different map series, and get the coordinates of a particular location by clicking. I celebrated by making a page for it, with a silly tux-peering-over-map icon.

One annoyance: it repaints every time it gets a focus in or out, which means, for people like me who use mouse focus, that it repaints twice for each time the mouse moves over the window. This isn't visible, but it would drag the CPU down a bit on a slow machine (which matters since mapping programs are particularly useful on laptops and handhelds).

It turns out this is a pygtk problem: any pygtk drawing area window gets spurious Expose events every time the focus changes (whether or not you've asked to track focus events), and it reports that the whole window needs to be repainted, and doesn't seem to be distinguishable in any way from a real Expose event. The regular gtk libraries (called from C) don't do this, nor do Xlib C programs; only pygtk.

I filed bug 172842 on pygtk; perhaps someone will come up with a workaround, though the couple of pygtk developers I found on #pygtk couldn't think of one (and said I shouldn't worry about it since most people don't use pointer focus ... sigh).

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[ 17:26 Apr 06, 2005    More programming | permalink to this entry | comments ]

Sun, 27 Mar 2005

Python GTK Topographic Map Program

I couldn't stop myself -- I wrote up a little topo map viewer in PyGTK, so I can move around with arrow keys or by clicking near the edges. It makes it a lot easier to navigate the map directory if I don't know the exact starting coordinates.

It's called PyTopo, and it's in the same place as my earlier two topo scripts.

I think CoordsToFilename has some bugs; the data CD also has some holes, and some directories don't seem to exist in the expected place. I haven't figured that out yet.

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[ 18:53 Mar 27, 2005    More programming | permalink to this entry | comments ]

Topographic Maps for Linux

I've long wished for something like those topographic map packages I keep seeing in stores. The USGS (US Geological Survey) sells digitized versions of their maps, but there's a hefty setup fee for setting up an order, so it's only reasonable when buying large collections all at once.

There are various Linux mapping applications which do things like download squillions of small map sections from online mapping sites, but they're all highly GPS oriented and I haven't had much luck getting them to work without one. I don't (yet?) have a GPS; but even if I had one, I usually want to make maps for places I've been or might go, not for where I am right now. (I don't generally carry a laptop along on hikes!)

The Topo! map/software packages sold in camping/hiking stores (sometimes under the aegis of National Geographic are very reasonably priced. But of course, the software is written for Windows (and maybe also Mac), not much help to Linux users, and the box gives no indication of the format of the data. Googling is no help; it seems no Linux user has ever tried buying one of these packages to see what's inside. The employees at my local outdoor equipment store (Mel Cotton's) were very nice without knowing the answer, and offered the sensible suggestion of calling the phone number on the box, which turns out to be a small local company, "Wildflower Productions", located in San Francisco.

Calling Wildflower, alas, results in an all too familiar runaround: a touchtone menu tree where no path results in the possibility of contact with a human. Sometimes I wonder why companies bother to list a phone number at all, when they obviously have no intention of letting anyone call in.

Concluding that the only way to find out was to buy one, I did so. A worthwhile experiment, as it turned out! The maps inside are simple GIF files, digitized from the USGS 7.5-minute series and, wonder of wonders, also from the discontinued but still useful 15-minute series. Each directory contains GIF files covering the area of one 7.5 minute map, in small .75-minute square pieces, including pieces of the 15-minute map covering the same area.

A few minutes of hacking with python and Image Magick resulted in a script to stitch together all images in one directory to make one full USGS 7.5 minute map; after a few hours of hacking, I can stitch a map of arbitrary size given start and end longitude and latitude. My initial scripts, such as they are.

Of course, I don't yet have nicities like a key, or an interactive scrolling window, or interpretation of the USGS digital elevation data. I expect I have more work to do. But for now, just being able to generate and print maps for a specific area is a huge boon, especially with all the mapping we're doing in Field Geology class. GIMP's "measure" tool will come in handy for measuring distances and angles!

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[ 12:13 Mar 27, 2005    More programming | permalink to this entry | comments ]