Older blog entries for jtauber (starting at number 202)

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

Song Project: LH Piano Riff

Having introduced the right hand of the main piano riff, let's introduce the left hand.

Firstly, here's what it sounds like by itself:

download if embed doesn't work

And together with the right hand:

download if embed doesn't work

And here's what it looks like in the score:


Now let's analyze it a little. Note that this is post hoc analysis, I didn't go through this thinking when I wrote it (at least not consciously)—it was improvised at the piano—but I find it interesting to go back and see why things worked or generated the particular effect they have.

One thing that immediately stands out is that the 3+3+2 rhythmic grouping found in the right hand is also found here (with one exception we'll come to in a minute). Also, if you look at what note is playing at the start of each of the 3+3+2 grouping: A A C♯ | D D D | B♭ B♭ - | F C E it is always the root of the current chord on the first two beats and either the root, the third or nothing on the final beat.

In the first bar, there is an additional passing note, B (notice it's natural despite the key, because the chord is A) and the C♯ is the leading note into the D chord in the next bar. The slur emphasizes this role.

In the second bar, the E is just a neighbour note between two repetitions of the root D. The final, non-accented A is in the triad of the chord but it's also the leading note of the following B♭ chord and again leading note is slurred.

In the fourth bar, the first E and D are just passing notes taking us from the F root to the C root. The final E is the third of the chord, the 5th of the A chord we return to if we repeat but it's also the leading note of the tonic F (which we'll take advantage of later). It's a great example of a note playing multiple functions both in terms of the current chord and what may follow.

The third bar (which I deliberately left until last to discuss) is a little unusual. If you imagine the second B♭ an octave higher, it's a little easier to see what's going on. In that case, the A and G are just passing notes down to the F in the next bar. But what is a little unusual is firstly that the A and G are not following the 3+3+2 pattern. It is almost as if the pattern has switched to 3+2+3 this bar. Secondly the A is quite dissonant, especially given it's only a semitone away from a note being played in the right hand. This rhythmic change coupled with the dissonance builds a nice tension that is then resolved with what I called a "triumphant" chord in a comment on the previous post.

You may notice a bit of chorusing in the piano sound of the recordings so far. Actually, there's some compression, chorusing and EQ on them, all preempting the mixing that is to follow once we add more instruments. I'll talk about each of these once we've added a few more tracks.

All material for this project is made available under a Creative Commons BY-NC-SA license so you are free to redistribute and remix with attribution but under the same license and not commercially.

Syndicated 2008-11-08 13:46:37 (Updated 2008-11-08 14:12:49) from James Tauber's Blog

Voronoi Diagrams

Back in Creating Gradients Programmatically in Python I presented some code for writing out PNGs according to some rgb-function of x and y.

The relevant write_png(filename, width, height, rgb_func) is here:


This enables one to quickly generate PNGs based on all sorts of functions of position.

For example, here's a Voronoi diagram:

Take any metric space and pick a discrete number of points in that space we'll designate "control points" (the black dots in the above example). Now colour each other point in the space based on which control point is closest. In other words, two points are the same colour if and only if they have the same "closest control point".

Here's how to generate such a diagram using write_png...

First of all, here's a function that given an (x, y) coordinate and a list of control points, returns a pair of:

  • which control point is the closest to (x, y)
  • what the distance was

def closest(x, y, control_points):
    closest = None
    distance = None
    for i, pt in enumerate(control_points):
        px, py = pt
        d = ((px - x) ** 2 + (py - y) ** 2)
        if d == 0:
            return i, 0
        if d < distance or not distance:
            closest = i
            distance = d
    return closest, distance

Now we can use this and write_png to generate a PNG Voronoi diagram for a given set of control points:

def voronoi(filename, size, control_points):
    def f(x, y):
        c, d = closest(x, y, control_points)
        # draw points in black
        if d < 5:
            return 0, 0, 0
        px, py = control_points[c]
        # just one way to generate a colour
        m = px * 255 / size
        n = py * 255 / size
        return m, 255 - ((m + n) / 2), n
    write_png(filename, size, size, f)

Of course, this is just a brute-force way of establishing the Voronoi diagram, but for just generating examples a few hundred points by a few hundred points, it's Good Enough.

Note the choice of colouring, based on the coordinates of the control point is just one of many possibilities. You could also just colour based on control_point number (i.e. c) The current approach has one disadvantage that two control points very close to one another can be given almost indistinguishable colours.

The example diagram was just randomly generated with the following code:

import random
from voronoi import voronoi
space_size = 200
num_control = 8
control_points = []
for i in range(num_control):
    x = random.randrange(space_size)
    y = random.randrange(space_size)
    control_points.append((x, y))
voronoi("voronoi.png", space_size, control_points)

You can read more about Voronoi diagrams on Wikipedia.

Syndicated 2008-11-07 20:33:58 (Updated 2008-11-07 20:57:34) from James Tauber's Blog

Song Project: RH Piano Riff

Most of my pop song ideas begin with either a chord progression voiced a particular way on piano or some bass line. The song we'll be talking about here falls in to the first category.

I remember when I first started composing in high school, I did a lot of songs that were just permutations of I, IV, V and vi chords (so in C, that would be C, F, G and Am).

I remember one instrumental I wrote in Year 10 (called "Mystical Movements in Green")—that my drama class choreographed a dance to—used the chord progression vi IV I V and in particular was voiced with the vi and I in the second inversion. I always liked the way it sounded.

A couple of weeks ago, I was improvising on my digital piano and took a liking to the following variation:

    III . vi . IV . I V

with the vi and I again in the second inversion. I was playing in F at the time with a driving 3+3+2 rhythm in the right hand, so the resultant riff was:


which sounds something like this:

download if embed doesn't work

This will form the basis for the song.

All material for this project is made available under a Creative Commons BY-NC-SA license so you are free to redistribute and remix with attribution but under the same license and not commercially.

Syndicated 2008-11-06 08:30:24 (Updated 2008-11-06 08:59:24) from James Tauber

Song Project

One series I thought would be fun to kick off this month is to talk about music composition and record producing and engineering by working through a song. I've chosen a song I just started working on last month and the idea is I'll go through the process from initial idea to final produced song over a series of blog entries.

All material for this project is made available under a Creative Commons BY-NC-SA license so you are free to redistribute and remix with attribution but under the same license and not commercially.

I'll get started with the music in a separate post right away.

Syndicated 2008-11-06 08:04:55 (Updated 2008-11-06 08:19:49) from James Tauber

Atom, Google Reader and Duplicates on Planets

For a while I've wondered why posts syndicated across multiple planets don't get picked up by Google Reader as duplicates (and automatically marked read when I've read it the first time around).

I wasn't sure whether the problem was:

  • the source feeds
  • the planet feeds
  • Google Reader itself

so I decided to investigate further with my own feed as the source and the three planets my site is syndicated to (that I know of).

Let's take my post Cleese and Disk Images.

My feed gives both an id and a link for both the feed itself and each individual entry. That makes it possible, at least, for planets and readers to do the Right Thing. So I don't think the problem is my feed.

On the Official Planet Python:

  • there is an RSS 1.0 feed
  • the rdf:about is not the same as the id in my feed but is the link URL (subtlety different but the planet is doing the wrong thing, IMHO)
  • there is no authorship or source information

On both the Unofficial Planet Python and on Sam Ruby's Planet Intertwingly:

  • there is an Atom feed
  • the entry Cleese and Disk Images has the same id as in my feed and a distinct link.
  • the source element gives appropriate information about my blog and the original feed
  • the author (which I only give on the source feed not each entry) is not inherited on to the entry in the planet feed, only included in the source

Note that the handling of the author by the latter two feeds is correct per the Atom RFC, although I have noticed that Safari's feed reader gets this wrong and, despite the author in the source element, uses the inherited author from the planet feed itself.

But, in short, the Atom-feed-based Planets do the Right Thing, although IMHO the RSS-1.0-based Official Planet Python does not. That may not be the Planet's fault. The RSS 1.0 Spec (or any RSS for that matter) may not make the distinction between id and link.

So given that my feed and two of the planet feeds do the right thing, I guess that places the blame with Google Reader.

Why does Google Reader not honour the entry id and automatically mark duplicates as already read when you've read it the first time. That's my pony request for Google Reader.

And by the way, the same thing applies to feeds themselves, not just entries. Feedburner, for example, does the right thing and passes through the id of a source Atom feed into its own Atom feed version. However, if you subscribe to both the source and Feedburner version of of a feed, Google Reader doesn't not identify them as the same feed. Of course, if either are RSS, I'd assume all bets are off.

So, in summary, Atom supports doing the Right Thing. The Atom-based Planets do the Right Thing. Google Reader doesn't take advantage of this.

Syndicated 2008-11-06 06:04:34 (Updated 2008-11-06 06:04:35) from James Tauber

Dear America

I'm glad you have elected someone you think brings hope and change. I hope he turns out to be one of the truly great presidents.

However, after the last eight years, you need a strong dose of fiscal conservatism. I hope your choice turns out to be the right one for that. I am not yet convinced.

Syndicated 2008-11-05 19:55:43 (Updated 2008-11-05 19:55:44) from James Tauber

Cleese and Disk Images

Previously I talked about setting up a toolchain to compile i386-elf binaries for hobby OS writing on Mac OS X.

The next step in getting Cleese development working on Mac OS X was working how to build disk images for VMware Fusion for a "hello world" kernel. I got about half way over the weekend, but Brian Rosner worked out the rest Monday night.

VMware can mount a normal disk image as a floppy but can't do the same for hard drives. Turns out, though, you can create floppy images larger than 1.44MB (although I don't know if there's an upper limit).

Here's the make target Brian came up with:

cleese.img: KERNEL.BIN
        hdiutil create -size 5M -fs "MS-DOS" -layout NONE cleese
        mv cleese.dmg cleese.img
        mkdir -p mnt
        mount_msdos -o nosync `hdid -nomount cleese.img` ./mnt
        cp -r boot KERNEL.BIN ./mnt
        umount -f ./mnt
        rm -r ./mnt

This creates a 5MB disk image, mounts it and copies the "boot" directory from GRUB and our kernel KERNEL.BIN on to the image.

This image isn't bootable by VMware yet. You need to boot off another floppy that has GRUB and is bootable but this is a one off operation. You can easily create a bootable GRUB disk with just

cat boot/grub/stage1 boot/grub/stage2 > grub.img

Once you've booted to the GRUB command line, you can switch to cleese.img as your floppy and type

setup (fd0)

and that will copy GRUB onto the boot sector. From that point on, cleese.img is all you need.

To avoid having to do that step every time KERNEL.BIN updates, I wrote an additional make target that just updates KERNEL.BIN on an existing image.

        mkdir -p mnt
        mount_msdos -o nosync `hdid -nomount cleese.img` ./mnt
        cp KERNEL.BIN ./mnt
        umount -f ./mnt
        rm -r ./mnt

As a quick guide to what that's doing:

  • the -p option to mkdir just stops it complaining if mnt already exists
  • hdid -nomount cleese.img binds the disk image to a /dev and returns the device path
  • that device path is then used as an argument to mount_msdos (hence the backticks) which mounts that device as ./mnt
  • the file(s) are copied on, the image unmounted and the mount point deleted

I'm not sure why the -o nosync is needed. Maybe it isn't.

In the original target, the -layout NONE option to hdiutil ensures no partition map is created for the drive.

Syndicated 2008-11-05 09:26:14 (Updated 2008-11-05 09:49:40) from James Tauber

Daylight Saving Time

Yesterday I was asked at work what the origins of daylight savings were. People who know me know I can never just say "I don't know" to a question like that—I had to go do some research.

The short answer is "war and golf" but here is a longer version, gleaned from various articles online and a little prior knowledge on the topic.

While Benjamin Franklin is sometimes credited with the idea of setting clocks differently in the summer, his idea was well before its time as there wasn't a notion of standard time in his day. The notion that clocks would be set according to the "real time" (i.e. based on the Sun) of some other location has its origin with the railroad system. In November 1840, the Great Western Railway in England adopted London Time for all their schedules. The US and Canada followed suit with their own Standard Time in November 1883.

While Standard Time was initially for the railroads, it began to be adopted across the board, eventually being enacted into law in the US by the Standard Time Act of 1918.

An Englishman, William Willet made the observation, a century after Ben Franklin had done the same, that people were wasting the early hours of the day in summer by sleeping in. He was also an avid golfer who was frustrated at dusk cutting short his game. So he started campaigning for clocks to be advanced during the summer months. The idea was ridiculed and he died in 1915 without seeing his idea adopted.

In April 1916, however, Germany started advancing the clock an hour to reduce electricity usage and hence fuel consumption during the war. Many European countries immediately followed suit and Britain started in May 1916. When the US joined the war, they too adopted this daylight saving measure.

US Congress repealed the law in 1919, but Woodrow Wilson (incidentally also an avid golfer) vetoed the repeal. Congress overrode the veto and so daylight saving stopped, although was adopted locally in some places.

In World War II, it was reintroduced, this time all year around. The US had daylight saving from February 1942 to September 1945. After the war, it went back to being a local issue.

It was a controversial issue through the early 1960s but the confusion caused by so many local differences resulted in the US passing the Universal Time Act in 1966 which reintroduced it across the country unless overridden by state law.

My own home state of Western Australia is currently in a three-year trial of daylight saving and will hold a vote next year as to whether to keep it.

Syndicated 2008-11-04 08:21:40 (Updated 2008-11-04 18:46:31) from James Tauber

Python's re.DEBUG Flag

Eric Holscher points out a Python gem I never knew about. If you pass in the number 128 (or, as I have a preference for flags in hex, 0x80) as the second arg to re.compile, it prints out the parse tree of the regex:

>>> import re
>>> pattern = re.compile("a+b*\s\w?", 0x80)
max_repeat 1 65535
  literal 97
max_repeat 0 65535
  literal 98
  category category_space
max_repeat 0 1
    category category_word

While re.compile is documented as having the signature

compile(pattern[, flags])

the particular flag 0x80 is not documented as far as I can tell.

I thought I'd dig in further.

Firstly, note that re appears to cache patterns as if you repeat the same re.compile, it returns the same object and doesn't spit out the parse tree. There is a re.purge function for purging this cache but while this is mentioned in help(re) it is not in the main documentation.

Secondly, note that the flag 0x80 is actually defined as DEBUG in the re module, so a more robust form would be:

re.compile(pattern, re.DEBUG)

A source code comment for DEBUG and another undocumented flag TEMPLATE (which supposedly disables backtracking) mentions:

# sre extensions (experimental, don't rely on these)

which explains why they aren't documented.

In the Python source code, there is also a Scanner class defined with the comment "experimental stuff (see python-dev discussions for details)"

A quick search of the python-dev mailing list found nothing. Perhaps a python core development could fill us in.

Syndicated 2008-11-03 11:47:43 (Updated 2008-11-03 11:49:27) from James Tauber

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