ffmpeg animated gif custom palettes

I used ffmpeg to make the animated gifs for my gallery of NeoPixel goggles patterns.

In the process I discovered that as well as the expected fairly straightforward conversion mode, ffmpeg -i mymovie.mov mymovie.gif, there is another mode which can generate a custom palette. Because apparently by default, a default palette is used.

That is a slightly more awkward two step process, described for example here, but at least for my goggles GIFs looks way better (to me) although turning out to be four times the size:

Galois LFSR PRNG in NeoPixel goggles

I've been playing with software to run on my Adafruit Neopixel Goggles. The software gets to make pretty patterns on 32 x 24-bit RGB LEDs arranged in two rings.

One of the modes picks random colours to change each ring to. The Arduino stack supplies a random() function to do this, but it seemed to take up about 600 bytes of the limited 5kb flash program memory on the in-goggles microcontroller.

I wondered if I could make a smaller-but-less-random generator. After all, the colour pattern does not need to be cryptographically secure. I'm just trying to avoid getting the same sequence of colours every time.

Someone pointed me at Linear Feedback Shift Register PRNGs and I implemented one based around a description in Wikipedia. I chose the Galois one because pistols at dawn.

That seemed to work well for the basic generation of random numbers, but the Arduino always started up with the same seed, and so gave the same sequence of colours each time. Luckily, there are 512 bytes of EEPROM on this microcontroller and so I used two other those to generate and store a seed to be used next time.

Initially, I generated two random bytes at power-on, and stored those into the EEPROM. However, this rapidly proved to have a really short period: there were only two different patterns being displayed by the goggles in this mode!

So, next, which seems to work better, the code now initialises the PRNG from EEPROM, takes a single bit from it (and discards it) and then writes out the PRNG state into the EEPROM. That means that the start state advances by one bit every boot.

Code for the goggles is here on github: https://github.com/benclifford/goggles.

An Idris implementation of a reddit bot.

I wrote and host lsc-todaybot, a bot that moves the [TODAY] flair around on reddit.com/r/LondonSocialClub, a subreddit dedicated to social activities in London. I wrote it in Haskell, and over the last year or so have used it as a simple application for experimenting with different Haskell libraries and techniques. For example, I used it to learn about extensible effects (and gave a talk about it at the London Haskell meetup).

I've been interested in the dependently typed programming language Idris and someone claimed it was pacman-complete - that is, you can write Pacman in it. So rather than playing round with proofs and all that dependent-type gubbins, I set out to rewrite todaybot in Idris.

It's here: https://github.com/benclifford/idris-todaybot

It's not beautiful. But it works well enough to be running alongside the Haskell implementation.

I've littered the source with QUESTION/DISCUSSION blocks, but basically I had to patch up an existing JSON library, interface an HTTP library (I chose libcurl), and struggle with a whole new style of error message (possibly the hardest bit as I'm used to error messages really leading you to the right answer).

There's almost no use of dependent types, but I did find a straightforward place to make use of them, in the parameters to this libcurl function:

CURLcode curl_easy_setopt(CURL *handle, CURLoption option, parameter);

where the type of the supplied parameter depends on the particular option chosen: for example a pointer to a callback function, or a boolean verbosity level.

Fixing up my MSF radio clock

One of the first hardware things I did with a Raspberry Pi (sometime around March 2013) was interface it to a cheap 60kHz radio clock board, and write some driver software to interface it to ntpd. That was in one house, then in another, and then got dismantled, put in a box, and smashed up a bit. Based on the

I spent a few hours yesterday fixing it up: resoldering the antenna onto the circuit board, getting rid of the dodgy soldering and putting it on a prototype board, putting a capacitor across the power supply because I heard rumour that might make it receive better (it doesn't seem to), and implementing parity checking in the decoder.

It's still terribly awkward to position: I have it velcroed up on top of a curtain rail to try to get it high and away from all my other electronics and it is still very sensitive to antenna positioning; and it is still estimated by ntpd to be less accurate than getting time off the internet; and even with parity checking it is still fairly common for it to decode a time that is wrong.

But it has a nice flashing red LED.

Software: https://github.com/benclifford/msf