Shallow Thoughts : : Oct

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

Sat, 22 Oct 2016

Los Alamos Artists Studio Tour

[JunkDNA Art at the LA Studio Tour] The Los Alamos Artists Studio Tour was last weekend. It was a fun and somewhat successful day.

I was borrowing space in the studio of the fabulous scratchboard artist Heather Ward, because we didn't have enough White Rock artists signed up for the tour.

Traffic was sporadic: we'd have long periods when nobody came by (I was glad I'd brought my laptop, and managed to get some useful development done on track management in pytopo), punctuated by bursts where three or four groups would show up all at once.

It was fun talking to the people who came by. They all had questions about both my metalwork and Heather's scratchboard, and we had a lot of good conversations. Not many of them were actually buying -- I heard the same thing afterward from most of the other artists on the tour, so it wasn't just us. But I still sold enough that I more than made back the cost of the tour. (I hadn't realized, prior to this, that artists have to pay to be in shows and tours like this, so there's a lot of incentive to sell enough at least to break even.) Of course, I'm nowhere near covering the cost of materials and equipment. Maybe some day ...

[JunkDNA Art at the LA Studio Tour]

I figured snacks are always appreciated, so I set out my pelican snack bowl -- one of my first art pieces -- with brownies and cookies in it, next to the business cards.

It was funny how wrong I was in predicting what people would like. I thought everyone would want the roadrunners and dragonflies; in practice, scorpions were much more popular, along with a sea serpent that had been sitting on my garage shelf for a month while I tried to figure out how to finish it. (I do like how it eventually came out, though.)

And then after selling both my scorpions on Saturday, I rushed to make two more on Saturday night and Sunday morning, and of course no one on Sunday had the slightest interest in scorpions. Dave, who used to have a foot in the art world, tells me this is typical, and that artists should never make what they think the market will like; just go on making what you like yourself, and hope it works out.

Which, fortunately, is mostly what I do at this stage, since I'm mostly puttering around for fun and learning.

Anyway, it was a good learning experience, though I was a little stressed getting ready for it and I'm glad it's over. Next up: a big spider for the front yard, before Halloween.

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[ 20:17 Oct 22, 2016    More art | permalink to this entry | comments ]

Tue, 11 Oct 2016

New Mexico LWV Voter Guides are here!

[Vote button] I'm happy to say that our state League of Women Voters Voter Guides are out for the 2016 election.

My grandmother was active in the League of Women Voters most of her life (at least after I was old enough to be aware of such things). I didn't appreciate it at the time -- and I also didn't appreciate that she had been born in a time when women couldn't legally vote, and the 19th amendment, giving women the vote, was ratified just a year before she reached voting age. No wonder she considered the League so important!

The LWV continues to work to extend voting to people of all genders, races, and economic groups -- especially important in these days when the Voting Rights Act is under attack and so many groups are being disenfranchised. But the League is important for another reason: local LWV chapters across the country produce detailed, non-partisan voter guides for each major election, which are distributed free of charge to voters. In many areas -- including here in New Mexico -- there's no equivalent of the "Legislative Analyst" who writes the lengthy analyses that appear on California ballots weighing the pros, cons and financial impact of each measure. In the election two years ago, not that long after Dave and I moved here, finding information on the candidates and ballot measures wasn't easy, and the LWV Voter Guide was by far the best source I saw. It's the main reason I joined the League, though I also appreciate the public candidate forums and other programs they put on.

LWV chapters are scrupulous about collecting information from candidates in a fair, non-partisan way. Candidates' statements are presented exactly as they're received, and all candidates are given the same specifications and deadlines. A few candidates ignored us this year and didn't send statements despite repeated emails and phone calls, but we did what we could.

New Mexico's state-wide voter guide -- the one I was primarily involved in preparing -- is at New Mexico Voter Guide 2016. It has links to guides from three of the four local LWV chapters: Los Alamos, Santa Fe, and Central New Mexico (Albuquerque and surrounding areas). The fourth chapter, Las Cruces, is still working on their guide and they expect it soon.

I was surprised to see that our candidate information doesn't include links to websites or social media. Apparently that's not part of the question sheet they send out, and I got blank looks when I suggested we should make sure to include that next time. The LWV does a lot of important work but they're a little backward in some respects. That's definitely on my checklist for next time, but for now, if you want a candidate's website, there's always Google.

I also helped a little on Los Alamos's voter guide, making suggestions on how to present it on the website (I maintain the state League website but not the Los Alamos site), and participated in the committee that wrote the analysis and pro and con arguments for our contentious charter amendment proposal to eliminate the elective office sheriff. We learned a lot about the history of the sheriff's office here in Los Alamos, and about state laws and insurance rules regarding sheriffs, and I hope the important parts of what we learned are reflected in both sides of the argument.

The Voter Guides also have a link to a Youtube recording of the first Los Alamos LWV candidate forum, featuring NM House candidates, DA, Probate judge and, most important, the debate over the sheriff proposition. The second candidate forum, featuring US House of Representatives, County Council and County Clerk candidates, will be this Thursday, October 13 at 7 (refreshments at 6:30). It will also be recorded thanks to a contribution from the AAUW.

So -- busy, busy with election-related projects. But I think the work is mostly done (except for the one remaining forum), the guides are out, and now it's time to go through and read the guides. And then the most important part of all: vote!

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[ 16:08 Oct 11, 2016    More politics | permalink to this entry | comments ]

Wed, 05 Oct 2016

Play notes, chords and arbitrary waveforms from Python

Reading Stephen Wolfram's latest discussion of teaching computational thinking (which, though I mostly agree with it, is more an extended ad for Wolfram Programming Lab than a discussion of what computational thinking is and why we should teach it) I found myself musing over ideas for future computer classes for Los Alamos Makers. Students, and especially kids, like to see something other than words on a screen. Graphics and games good, or robotics when possible ... but another fun project a novice programmer can appreciate is music.

I found myself curious what you could do with Python, since I hadn't played much with Python sound generation libraries. I did discover a while ago that Python is rather bad at playing audio files, though I did eventually manage to write a music player script that works quite well. What about generating tones and chords?

A web search revealed that this is another thing Python is bad at. I found lots of people asking about chord generation, and a handful of half-baked ideas that relied on long obsolete packages or external program. But none of it actually worked, at least without requiring Windows or relying on larger packages like fluidsynth (which looked worth exploring some day when I have more time).

Play an arbitrary waveform with Pygame and NumPy

But I did find one example based on a long-obsolete Python package called Numeric which, when rewritten to use NumPy, actually played a sound. You can take a NumPy array and play it using a pygame.sndarray object this way:

import pygame, pygame.sndarray

def play_for(sample_wave, ms):
    """Play the given NumPy array, as a sound, for ms milliseconds."""
    sound = pygame.sndarray.make_sound(sample_wave)
    sound.play(-1)
    pygame.time.delay(ms)
    sound.stop()

Then you just need to calculate the waveform you want to play. NumPy can generate sine waves on its own, while scipy.signal can generate square and sawtooth waves. Like this:

import numpy
import scipy.signal

sample_rate = 44100

def sine_wave(hz, peak, n_samples=sample_rate):
    """Compute N samples of a sine wave with given frequency and peak amplitude.
       Defaults to one second.
    """
    length = sample_rate / float(hz)
    omega = numpy.pi * 2 / length
    xvalues = numpy.arange(int(length)) * omega
    onecycle = peak * numpy.sin(xvalues)
    return numpy.resize(onecycle, (n_samples,)).astype(numpy.int16)

def square_wave(hz, peak, duty_cycle=.5, n_samples=sample_rate):
    """Compute N samples of a sine wave with given frequency and peak amplitude.
       Defaults to one second.
    """
    t = numpy.linspace(0, 1, 500 * 440/hz, endpoint=False)
    wave = scipy.signal.square(2 * numpy.pi * 5 * t, duty=duty_cycle)
    wave = numpy.resize(wave, (n_samples,))
    return (peak / 2 * wave.astype(numpy.int16))

# Play A (440Hz) for 1 second as a sine wave:
play_for(sine_wave(440, 4096), 1000)

# Play A-440 for 1 second as a square wave:
play_for(square_wave(440, 4096), 1000)

Playing chords

That's all very well, but it's still a single tone, not a chord.

To generate a chord of two notes, you can add the waveforms for the two notes. For instance, 440Hz is concert A, and the A one octave above it is double the frequence, or 880 Hz. If you wanted to play a chord consisting of those two As, you could do it like this:

play_for(sum([sine_wave(440, 4096), sine_wave(880, 4096)]), 1000)

Simple octaves aren't very interesting to listen to. What you want is chords like major and minor triads and so forth. If you google for chord ratios Google helpfully gives you a few of them right off, then links to a page with a table of ratios for some common chords.

For instance, the major triad ratios are listed as 4:5:6. What does that mean? It means that for a C-E-G triad (the first C chord you learn in piano), the E's frequency is 5/4 of the C's frequency, and the G is 6/4 of the C.

You can pass that list, [4, 5, 5] to a function that will calculate the right ratios to produce the set of waveforms you need to add to get your chord:

def make_chord(hz, ratios):
    """Make a chord based on a list of frequency ratios."""
    sampling = 4096
    chord = waveform(hz, sampling)
    for r in ratios[1:]:
        chord = sum([chord, sine_wave(hz * r / ratios[0], sampling)])
    return chord

def major_triad(hz):
    return make_chord(hz, [4, 5, 6])

play_for(major_triad(440), length)

Even better, you can pass in the waveform you want to use when you're adding instruments together:

def make_chord(hz, ratios, waveform=None):
    """Make a chord based on a list of frequency ratios
       using a given waveform (defaults to a sine wave).
    """
    sampling = 4096
    if not waveform:
        waveform = sine_wave
    chord = waveform(hz, sampling)
    for r in ratios[1:]:
        chord = sum([chord, waveform(hz * r / ratios[0], sampling)])
    return chord

def major_triad(hz, waveform=None):
    return make_chord(hz, [4, 5, 6], waveform)

play_for(major_triad(440, square_wave), length)

There are still some problems. For instance, sawtooth_wave() works fine individually or for pairs of notes, but triads of sawtooths don't play correctly. I'm guessing something about the sampling rate is making their overtones cancel out part of the sawtooth wave. Triangle waves (in scipy.signal, that's a sawtooth wave with rising ramp width of 0.5) don't seem to work right even for single tones. I'm sure these are solvable, perhaps by fiddling with the sampling rate. I'll probably need to add graphics so I can look at the waveform for debugging purposes.

In any case, it was a fun morning hack. Most chords work pretty well, and it's nice to know how to to play any waveform I can generate.

The full script is here: play_chord.py on GitHub.

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[ 11:29 Oct 05, 2016    More programming | permalink to this entry | comments ]

Sat, 01 Oct 2016

Zsh magic: remove all raw photos that don't have a corresponding JPEG

Lately, when shooting photos with my DSLR, I've been shooting raw mode but with a JPEG copy as well. When I triage and label my photos (with pho and metapho), I use only the JPEG files, since they load faster and there's no need to index both. But that means that sometimes I delete a .jpg file while the huge .cr2 raw file is still on my disk.

I wanted some way of removing these orphaned raw files: in other words, for every .cr2 file that doesn't have a corresponding .jpg file, delete the .cr2.

That's an easy enough shell function to write: loop over *.cr2, change the .cr2 extension to .jpg, check whether that file exists, and if it doesn't, delete the .cr2.

But as I started to write the shell function, it occurred to me: this is just the sort of magic trick zsh tends to have built in.

So I hopped on over to #zsh and asked, and in just a few minutes, I had an answer:

rm *.cr2(e:'[[ ! -e ${REPLY%.cr2}.jpg ]]':)

Yikes! And it works! But how does it work? It's cheating to rely on people in IRC channels without trying to understand the answer so I can solve the next similar problem on my own.

Most of the answer is in the zshexpn man page, but it still took some reading and jumping around to put the pieces together.

First, we take all files matching the initial wildcard, *.cr2. We're going to apply to them the filename generation code expression in parentheses after the wildcard. (I think you need EXTENDED_GLOB set to use that sort of parenthetical expression.)

The variable $REPLY is set to the filename the wildcard expression matched; so it will be set to each .cr2 filename, e.g. img001.cr2.

The expression ${REPLY%.cr2} removes the .cr2 extension. Then we tack on a .jpg: ${REPLY%.cr2}.jpg. So now we have img001.jpg.

[[ ! -e ${REPLY%.cr2}.jpg ]] checks for the existence of that jpg filename, just like in a shell script.

So that explains the quoted shell expression. The final, and hardest part, is how to use that quoted expression. That's in section 14.8.7 Glob Qualifiers. (estring) executes string as shell code, and the filename will be included in the list if and only if the code returns a zero status.

The colons -- after the e and before the closing parenthesis -- are just separator characters. Whatever character immediately follows the e will be taken as the separator, and anything from there to the next instance of that separator (the second colon, in this case) is taken as the string to execute. Colons seem to be the character to use by convention, but you could use anything. This is also the part of the expression responsible for setting $REPLY to the filename being tested.

So why the quotes inside the colons? They're because some of the substitutions being done would be evaluated too early without them: "Note that expansions must be quoted in the string to prevent them from being expanded before globbing is done. string is then executed as shell code."

Whew! Complicated, but awfully handy. I know I'll have lots of other uses for that.

One additional note: section 14.8.5, Approximate Matching, in that manual page caught my eye. zsh can do fuzzy matches! I can't think offhand what I need that for ... but I'm sure an idea will come to me.

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[ 15:28 Oct 01, 2016    More linux/cmdline | permalink to this entry | comments ]