Older blog entries for jtauber (starting at number 211)

Category Feeds Available

I finally got around to doing something I've wanted to do for ages, only to discover it could be done in all of fifteen minutes. I'm talking about per-category feeds on jtauber.com

For over a year, my django-atompub has supported the parameterized feeds that django's built-in syndication feeds do but I never made use of them myself.

Fifteen minutes and roughly 20 lines of code later, the site now has a separate feed for each category. If you go to a page that is also a category, such as python or django or music theory or filmmaking or poincare project there will both be an extra feed advertised in the html head and a link from the "feed icon" after the list of "Pages in this category". So if you are only interested in a particular topic you can just subscribe to that (although I hope you don't—I did this more for topical aggregators)

One thing I always struggled with when thinking about category feeds before was how to handle subsumption relationships. If I put something in django should it automatically go in python? If in python then in some broader software category? or computing category? I certainly want to avoid the assumption of hierarchy (see some previous posts on that topic).

So for now I've left categories with no additional structure.

Syndicated 2008-11-15 03:06:02 (Updated 2008-11-15 03:06:03) from James Tauber's Blog

Getting Xcode to Work on Mac OS X 10.5.5

This morning I tried to start up Xcode but it crashed on startup.

The console revealed:

layout bitmap too short: ASKScriptView 

After a bit of searching I found this blog post. Although the main post wasn't quite relevant, one of the commenters described my problem:

Xcode 3.1 has been crashing for me since the 10.5.5 update with an error saying "layout bitmap too short: ASKScriptView". I couldn't find reference to this online and was completely stumped.

Another commenter said:

just get the '/usr/lib/libobjc.A.dylib' from a 10.5.4 system (Look for it in a 10.5.4 combo updater file) and swap it with the newer version. Then, it works again.

So I searched on Apple's site for the Mac OS X 10.5.4 Combo Update.

But then I was stuck with a 561MB .pkg I just needed extract one file from and I had no idea.

More searching revealed the shareware Pacifist from CharlesSoft which I promptly downloaded. It worked like a charm and let me extract just the libobjc.A.dylib I needed.

Now Xcode starts up fine! Sending my $20 in to Charles...

Syndicated 2008-11-14 11:25:57 (Updated 2008-11-14 11:31:32) from James Tauber's Blog

RED Changes the Game Again

Back in 2005, I talked about the vague announcement of an upcoming a 2540p camera based on a full frame 4K CMOS from a company founded by the founder of Oakley. The camera, RED ONE, was launched in 2007 and was viewed by many as a game changer in digital cinematography.

Now RED is changing the game again. They've just announced a new modular system that separates out the sensor, lens mount, I/O, recording module, battery, viewfinder, remote, etc all as separate components you can mix and match. They also announced a line of sensor modules (what they call 'brains') that I'll talk about in a moment.

I'm blown away just like I was in 2005 and there is some nice tech pr0n at http://www.red.com/epic_scarlet/ for you to look at to see what I'm talking about.

In addition to the modular approach, which I find very compelling, the other thing that blew me away is a couple of the 'brains' they offer and their sensors.

I've talked about sensor sizes before. Most consumer video cameras have 1/3" sensors. Professional video cameras are generally 2/3". Even Star Wars Episode II was shot on 2/3" cameras capable of 1080-line resolution. Remember the 1920x1080 is 2 megapixels. Video was a fair way behind digital still cameras, which was why the announcement from RED back 2005 was so tantalizing. A typical DSLR has an APS-C sized sensor (or similar).

But look what RED has done now. They have sensors designated S35, FF35, 645 (medium format) and 617 (large format panoramic). I've shown all the sensors sized I've mentioned below for comparison (done in the same scale and style as my previous post on sensor sizes). Yes, that huge rectangle is (roughly, depending on your screen resolution) the sensor dimensions in actual size.

The full-frame FF35 is 24MP. So it's already as good as any professional DSLR and it can shoot video up to 100fps!!

The 645 is 56mm x 42mm and 65MP. The 617 is...wait for it...186mm x 56mm with 261MP. And both these shoot video at 50fps and 25fps respectively.

Finally, the dynamic range on the FF35, 645 and 617 are supposedly 13+ stops. That is incredible (although admittedly the larger pixels sizes of such large sensors make that possible)

Simple mind blowing!

Syndicated 2008-11-13 18:59:48 (Updated 2008-11-13 18:59:46) from James Tauber's Blog

Book Meme

Too slow a tempo is unsuitable to the ornamental melodic motions characteristic of the third species.

from Counterpoint in Composition by Felix Salzer and Carl Schachter

Meme from Greg Newman, Justin Lilly and Brian Rosner:

  • Grab the nearest book.
  • Open it to page 56.
  • Find the fifth sentence.
  • Post the text of the sentence in your journal along with these instructions.
  • Don't dig for your favorite book, the cool book, or the intellectual one: pick the CLOSEST.

Syndicated 2008-11-12 21:22:58 (Updated 2008-11-12 21:22:59) from James Tauber's Blog

12 Nov 2008 (updated 12 Nov 2008 at 12:09 UTC) »

X will cost Y jobs

One type of clause that I have long had an issue with is that of the form

X will create Y jobs

which I most recently read in descriptions of the newly passed proposition for a high-speed rail project in California (which will, supposedly, create 450,000 jobs)

The problem is the assumption that job creation is a benefit. The benefit of the high-speed rail project is presumably supposed to be energy-efficient transportation that will reduce greenhouse gases. I'm all for that. But if you can achieve that with 350,000 jobs that's actually better than taking 450,000 jobs to do it.

Say two start ups are founded and one can build the product with 10 people and another will take 20 people to do the same thing. It seems crazy to say "well the second company is creating 10 extra jobs". No, the second company is wasting 10 jobs. Those people could be doing something more productive (either within the company or somewhere else).

And that gets to the heart of the matter. Creating jobs means taking people away from doing something else.

"What about helping unemployment?" you ask.

What are the chances that the 450,000 people that will work on the California rail project, will all otherwise be unemployed? Pretty slim. Now, to the extent that otherwise unproductive people are made productive (and are paid accordingly) then that is definitely a benefit.

Who knows, maybe all 450,000 people will be doing something more productive than they otherwise would have been doing. But the articles don't say "X will employ Y people more productively", they just say "X will create Y jobs" which, taken as is, is a cost, not a benefit.

I've suggested before that many precepts in economics come down to the concept of opportunity cost. This one is no different.

And, of course, this is all just economics 101. Go read Hazlitt's classic Economics in One Lesson. There's a nice treatment of the "make-work bias" in Bryan Caplan's excellent The Myth of the Rational Voter.

Let me finish with a classic story:

An economist visits China under Mao Ze dong. He sees hundreds of workers building a dam with shovels. He asks: 'Why don't they use a mechanical digger?''That would put people out of work,' replies the foreman. 'Oh,' says the economist, 'I thought you were making a dam. If it's jobs you want, take away their shovels and give them spoons.'

UPDATE: I actually preferred how I explain somethings in a comment below in response to a question so I thought I'd bump up my response into the main body:

whether diverting those 10 people to that startup is a good thing ultimately depends if the start up produces something worth that diversion. It is not prima facie a good thing just because it's using up 10 people. Similarly, the diversion of 450K people to build a high-speed train system may be a good thing (and hopefully will be) but would not be because it diverted 450K people, it would be that despite the fact it diverted 450K away from other things, it was still worth it for the benefit of having the train system.

Syndicated 2008-11-12 05:53:34 (Updated 2008-11-12 06:55:01) from James Tauber's Blog

Storing HTTP_X_FORWARDED_FOR in Django

I occasionally get a

ProgrammingError: value too long for type character(15)

when people post to my blog. There aren't any fields in my model declared to have a max_length of 15 so I was always a little confused and in almost all cases, it was a spammer anyway so never took the time to investigate further.

But then someone just emailed me and told me they were getting a 500 when posting a comment to my blog. So I decided to investigate and that's where it started getting interesting...


./manage.py sql leonardo

revealed no sign of a field of length 15 either. So I went into the DB (in my case PostgreSQL)'s shell.

A quick

\d leonardo_comment


author_ipaddress | character(15) 

Like many blogs, I capture the IP address of the commenter so I can block spam.

In my model I have:

author_ipaddress = models.IPAddressField(null=True)

Which Django's ORM translates to:

"author_ipaddress" inet NULL

which PostgreSQL is obviously storing as a character(15).

Why would an IP Address be more than 15 characters, though?

Well, I went back to the error log and noticed this:


(note: I changed the second address to protect the original poster)

You see, because the Apache instance running django is behind another webserver (on the same machine), I can't rely on REMOTE_ADDR because it's always So I log HTTP_X_FORWARDED_FOR.

What I didn't realise until now is that HTTP_X_FORWARDED_FOR can be a list.

I guess the best solution is to just change the field to a CharField.

Other Djangonauts who are logging HTTP_X_FORWARDED_FOR might want to heed this warning: don't use IPAddressField.

Syndicated 2008-11-11 04:36:52 (Updated 2008-11-11 04:46:36) from James Tauber's Blog

Chrome Overtakes IE

I was just looking at the analytics for http://pinaxproject.com/ and noticed the following browser stats for the last month:

Firefox                       68.79%
Safari                        13.84%
Chrome                        5.88%
Internet Explorer             4.31%
Opera                         3.26%

What stood out to me is that more people have accessed the website using Chrome than IE!

Syndicated 2008-11-10 18:53:54 (Updated 2008-11-10 18:55:13) from James Tauber's Blog

You Need More Than Equipment to be a Cinematographer

With its full frame 1080p video, I've seen a bunch of articles like this one that claim "The Canon 5D Mk II Will Turn Us All Into Professional Cinematographers".

Yes, the 5D Mk II, with a full-frame sensor and EOS lens system is potentially a game changer, but to suggest that we'll all be able to produce results like professional cinematographers is stretching it.

While a good understanding of photography helps with cinematography, it's necessary but not sufficient knowledge.

  • composition is important but you also need to know how to move the camera during a shot
  • you also need to understand concepts like "crossing the line"
  • with the narrow depth of field full-frame affords, you need to be able to pull focus while shooting—that's a whole new skill to learn (and you probably need to use a follow focus)
  • you can't rely on shutter speed or aperture to control exposure the same way you do with still photography—you have to learn to rely more on ND filters and lighting
  • colour correction becomes even more important as mismatches are more jarring

I hope that the Canon 5D Mk II will encourage people to learn more about the art and craft of cinematography. But the equipment is not the end of the story, it's just the beginning.

Syndicated 2008-11-10 14:11:50 (Updated 2008-11-10 14:13:50) from James Tauber's Blog

10 Nov 2008 (updated 10 Nov 2008 at 07:09 UTC) »

Groups, Tribes and Projects in Pinax

From fairly early on, Pinax had tribes and projects. The intention of the distinction was that tribes are a loose grouping of people with a common interest (see Seth Godin's great little book Tribes: We Need You to Lead Us) whereas projects were more focused around managing a group of people working on common tasks.

This distinction was reflected in the differences in implementation: projects are invite only, tribes are open to anyone. Projects have tasks, tribes don't. But there are a lot of similarities: both have threaded discussion, wikis and photo pools. There's also a lot of duplicated code.

It wasn't long before I realised that really projects and tribes we just two subclasses of the same class (or instances of the same metaclass—more on that in a moment). So one of the things we're working on for the next release of Pinax is to merge the two into a single model but then allow a site developer to create as many differently configured subclasses/instances of this model as they like.

We haven't worked out the details yet, but basically it would allow you to define a new subclass/instance of "group", pick the membership model and what apps and object types can be attached to it.

Quite separately, I've noticed that there is another kind of structure that tacitly exists between tribes on Cloud27. I started a Python tribe but others have started Ruby and PHP tribes. There are also geographic tribes and language-based tribes. So there is another sense in which a set of tribes could be labelled as "programming language" tribes, or "geographical" tribes or "natural language" tribes.

So we have:

  • the root group type (that will unify projects and tribes)
  • types of groups (e.g. projects, tribes, other combinations of features and membership patterns defined by site developers)
  • instances of one of these types (e.g. a particular tribe, like 'Python')
  • a category of instances (e.g. 'Programming Language Tribes')

To slightly complicate things, one could imagine an "Australian Pythonistas" tribe. Or various intersections of "movies", "food", etc with geography-based tribes (e.g. Italian Food, French Films). These sorts of semantic relationships seems quite orthogonal to defining a type of group that has a wiki but not threaded discussions or is invitation-only versus free-for-all.

I'm trying to wrap my head around which to view as instances versus subclasses. As a longtime data modeler, I thought I had a grasp on this stuff but my brain is hurting :-)

Any suggestions for a metamodel?

Syndicated 2008-11-09 22:11:44 (Updated 2008-11-10 01:46:22) from James Tauber's Blog

From Focus and Directrix to Bézier Curve Parameters

For reasons that will become clear in a couple of posts, I wanted to be able to calculate quadratic Bézier curve parameters from a focus and horizontal directrix.

A focus and directrix are enough to define a parabola (in fact a parabola is the locus of points equidistance from a point, the focus, and a line, the directrix).

A quadratic Bézier curve is a section of a parabola and is defined by three points, according to the formula:

B(t) = (1-t)²P₀ + 2t(1-t)P₁ + t²P₂, t ∈ [0, 1]

Here's how I came to my result...

Given the directrix is horizontal,

  • P₁ must have the same x-coordinate as focus
  • P₁ must have an x-coordinate midway between P₀ and P₂

and by definition:

  • the parabola's low point is midway between the focus and directrix

Now even though somewhat arbitrary and assuming the directrix is below the focus,

  • P₀ and P₂ can have a y-coordinate of 0


  • the parabola's low point's y-coordinate is midway between P₀y and P₁y
  • ⇒ the y-coordinate midway between focus and directrix is midway between P₀y and P₁y
  • ⇒ the y-coordinate midway between focus and directrix is half P₁y


  • P₀ = (Fx - α, 0)
  • P₁ = (Fx, Fy + Dy)
  • P₂ = (Fx + α, 0)

for some α where (Fx, Fy) is focus and Dy is y-coordinate of directrix.

By definition,

  • P₀ must be equidistant between focus and directrix
  • P₂ must be equidistant between focus and directrix

But, given P₀ and P₂ have a y-coordinate of 0 and directrix is horizontal,

  • distance between P₀ and focus = Dy
  • distance between P₂ and focus = Dy


  • (P₀x - Fx)² + (P₀y - Fy)² = Dy²
  • ⇒ ((Fx-α) - Fx)² + (0 - Fy)² = Dy²
  • ⇒ (-α)² + (-Fy)² = Dy²
  • ⇒ α² + Fy² = Dy²
  • ⇒ α = √(Dy²-Fy²)

And so, assuming the directrix is horizontal and below the focus, the following Bézier curve parameters can also be used to create the parabola:

  • α = √(Dy²-Fy²)
  • P₀ = (Fx - α, 0)
  • P₁ = (Fx, Fy + Dy)
  • P₂ = (Fx + α, 0)

Syndicated 2008-11-08 15:55:26 (Updated 2008-11-08 15:55:27) from James Tauber's Blog

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