Thursday, 13 June 2013

Grue detection t-shirts

When adventuring it is very important to stay away from grues. I have developed a grue-detecting t-shirt which will warn your party about potential grue danger. When in a good light source or using your lamp, the t-shirt will look as it does in this picture.
In low light conditions the t-shirt looks like this. Find a light source quickly.

I had this prototype made at; in case it's not obvious, it's a combination of reflective and glow-in-the-dark vinyl print. Unfortunately, spreadshirt can't guarantee the alignment of these two print materials, otherwise I'd be offering a link to buy these essential pieces of safety equipment.

Thursday, 31 January 2013

Reimplementing TinyHack in PyGame

This is a small exercise I did today. It's a reimplementation of Rob Beschizza's TinyHack in PyGame. Of course, I don't expect anyone to download and play a game which you can play a better version of in your browser. This is just for fun and possibly as a starting point for other Roguelikes.

It's a reimplementation rather than a port; Rob did provide the source, but it's in Flash format which I've no idea how to read. The 'artwork', levels and sounds are remade from scratch, using SFXR again for the latter. It's stripped down somewhat - the world is smaller, only having three 'dungeons', there's no magic, and no village, although the pub still exists.

Behind the scenes, there are two main global stores of information in the game. There's a large map, a 2D array of integers, which changes occasionally (squares which represent gold or items are replaced with empty squares when the player moves over them). The second part is a 1D array of 'Entity' objects which contains the mobile things; the player and any enemies. Entities have an x and y position and HP (health) as well as a few other bits. To make things simple, the player is always at index 0 in the entity array. To draw the screen, it's only necessary to draw the map around the player and then the entities. Timing is basic; the game waits for a keystroke to do a player move, and then does an enemy move if the player did actually move anywhere. Enemies have very basic AI, and will just move towards the player if they can. When trying to move in any direction, the game needs to check both the map and the entity list for space. If there's an entity blocking the target square, the entity may attack the target. Enemies are marked as one team which is different to the player, so they won't attack each other. Finally, the player has the extra option of moving into 'teleporters' which are the red squares on the map.

Items and gold aren't part of the movement routine, but are checked for after moving. Gold just disappears and a counter is incremented. Items need to do different things, so there's a static Python dictionary which maps coordinates of items to types. (39,14) is the shield, for example. A similar dictionary maps teleporters to their destinations. There are also some special teleporters, such as the pub and the jetty, which are marked as special by having a destination x coordinate of 0. The y coordinate in these is an action index which is taken instead of moving the player. 1 is the pub which restores the player's health and 2 is the jetty which ends the game.

The map and teleporter/object data could be coded directly in Python, however it's a lot more convenient to do it in Tiled. As well as making the base map, it's also possible to use Tiled to specify the teleporter relationships and other bits using an object layer. The collectible items, for example, are marked by small objects on the map with the 'item' property set to either 'hp', 'sword' or 'boat'. The code to read this data out of Tiled's XML and into Python structures is in the separate file ''.

TinyHack removed a lot of common Roguelike features to make it work in a simple graphical environment: Lighting, line of sight, decent enemy AI, inventory, statistics, skills, fine-grained turn timing and random level creation are all missing. It's still a fun game though. As I said, MicroHack is a starting point, and I'd like to add to it. The question is, which of the missing features are actually important?

As usual, the code is in github at, in the 'microhack' directory.

Friday, 2 November 2012

Error correction of the NPL time signal

There's a project on the back burner at London Hackspace to make a better alarm clock. One of the first things we need for this is a way to figure out the current time, instead of bothering the user to set it.

There are a number of ways to obtain this automatically. Both GPS and GSM could be used, and in most houses, WiFi could be used to connect to a time server. DAB is also a possibility in Europe. None of these are particularly cheap though. It would be difficult to get a receiver working for under 20 pounds with any of these methods.

The National Physical Laboratory transmits a time signal by low frequency radio from Cumbria. This is also known as MSF from its old radio call sign. Similar services exist in Germany and the USA, at least. A receiver can be had for 9 pounds from PV Electronics, however, using it hasn't been particularly easy. The signal coming from the module is TTL level, but in my experience, very noisy in the time domain. The signal I got on the first try looked more like this:

Eurgh. Some of this is due to the power supply; the module needs a very clean power supply and it took me a long time to get any sensible data while running from a mains supply. Giving the Symtrik module its own linear regulator, some big capacitors and a choke seem to have improved it. The orientation and position of the antenna is also important; I've noticed that it will produce garbage if it's too close even to an AVR running at 1MHz. LCD monitors in particular seem to drive it crazy.

We could use analogue means to try and get an average value over the expected length of a pulse, or sample it several times and take the most common value. Those are approaches I might go back to, but for now, I'm sampling it just once in the middle of the pulse.

The MSF Specification is pleasantly simple. The most important thing to recognise is the minute marker, since all the remaining data frames' meaning is dependent on their sequence after it. If you treat all the frames as 4-bit frames, sampled at 150, 250, 350 and 450ms from the first rising edge, then the minute frame is all 1s, while the other frames always look like XX00. By using four samples it's unlikely to detect a false minute marker, and it's better to miss a minute marker than detect one when there wasn't one. Note that the Symtrik module outputs an inverted signal, which is why the diagram above is upside down compared with NPL's diagrams.

Unfortunately the signal only has four parity bits for error detection, and only one of those covers the hour and minute information that's really interesting to us. So the best resort we have is to collect data over several minutes, and compare them until we get a sequence of minutes which give us enough confidence to change our current idea of the time.

This means we now maintain three notions of time:

Display MinutesExpected MinutesRadio Minutes
Display HoursExpected HoursRadio Hours

Display time is what we currently display and consider the current time to be; this isn't something we want to change lightly and of course increments by itself. With a crystal oscillator we can expect an AVR to keep good time for months.

Expected time is what we'd expect the next radio frame to say; this also needs to be incremented automatically when we don't get a sensible radio time. Radio time is the time read from the radio this minute. At the end of the minute it's compared with the (pre-incremented) expected time and if it matches, we increment a sequence counter. If it doesn't match, that counter is reset and the radio time is incremented and becomes the expected time for the next frame.

Once we get a high enough sequence (at the moment, 3 agreeing minutes) we can use that to set the display time. If we don't have a current display time (just after power-on, for example) then the standards can be lower.

I'll provide some C code for the AVR when I have it working well. Debugging is an annoyingly slow process, being locked to the minute cycles of the NPL signal.

Sunday, 12 August 2012

Simple, headphone-controlled phone robot

TL;DR: The headphone socket on some Android phones is a good enough signal generator to control servo motors almost directly.

I had an idea recently that servo motors could be driven directly from the headphone socket on an Android mobile phone. I was probably subconsciously remembering this old this old Hackaday link; anyway, it turns out that it's very easy to do.

Servo motors expect a square pulse of 5V of between 1 and 2 milliseconds, usually once every 20 milliseconds. 1 millisecond is canonically fully left, 2 is fully right, and 1.5ms is in the centre, although there is quite a lot of variation. So, I created a suitable square wave by hand in Audacity, saved it as a .WAV file and played it back on my phone's default media player app, while looking at the headphone levels on an oscilloscope. There are lots of reasons to not expect a square wave to come out of an audio amplifier, but the result was actually a very faithful reproduction of my square wave. The maximum voltage I could get out of it was just under one volt, not enough to drive a servo directly, but enough to turn a transistor on. On both the phones I've tried - a HTC Hero (G2) and Desire C - the voltage out of the headphone port is negative with respect to the value in the wave file, so the wave file needs to be between 0 (off) and -32768 (pulses) for a 16-bit sample. As well as playing back standard audio files, it's pretty easy to generate audio on the fly, so you can control servos direct from your application.

Here's my circuit diagram (I've only done shown one channel - it's identical for both). I'm not suggesting you try this - it might break your phone in some way I haven't thought of. This is just to show what I've done.

I've used two bog standard NPN transistors in my circuit; the second is acting as a second inverter as the first one inverts the signal once. I think it should be possible to do this with one transistor, and an inverted wave file (using 0 as the pulses and -32768 the remainder of the time) but it hasn't worked when I've tried it and I don't have ready access to an oscilloscope to figure out why.

Nonetheless, this is a very cheap way of controlling servos from a mobile phone and I would like to find out whether it works on more Android devices. The transistors cost about £5 for a hundred and there's just another 3 resistors per channel to make it work. It is limited to two servos, so if you want more connectivity, you'll probably be after more powerful devices like the IOIO.

This idea gets more useful when you use continuous rotation servos. Here's a video of my old phone running two such servos with wheels attached. Even old Android phones have cameras, accelerometers and wifi, which makes them great brains for simple robots. Running two servos from the headphone socket gives them very cheap mobility.

PS. I've since become aware of which is a very similar device. They don't have any transistors; but use a capacitor on each channel to provide AC coupling. I couldn't get this to work when I tried it with my Android phone - maybe with more experimentation. Still, nice work.

Two more demos: Meteor Miner and Platformatic

Here's another couple of PyGame demos. Meteor Miner is a fairly straight clone of an old BBC Micro game, Thrust. Thrust was probably cloned from another platform, but it's not a style of game that has been used much recently. You control a spaceship by turning left or right and accelerating forwards, much like Asteroids, but the goal is to fly inside the structure of a meteor and recover ore. You can also shoot forwards to destroy the guns which will attack you inside the meteor.

Fuel is limited and ore makes you heavier. You can escape the meteor with just one tonne of ore, but the goal is to recover all of it.

This is the first game I've done with sound, although on all the platforms I've tried (Ubuntu, Windows 7 and Android) the sound suffers from an annoying latency. The game works well with the PyGame subset for Android but as PyGame doesn't handle multi-touch devices yet, I can't emulate the buttons on a touchscreen, so it will only be playable with a keyboard.

Platformatic is a puzzle game which looks like a platformer; rather than asking the player to time jumps exactly, it asks him or her to place instruction symbols onto the screen, which the character will follow when it treads on them. This partly came out of my frustration with platform games which require you to make a long sequence of carefully timed actions, often repeating the first ones hundreds of times until you finally get the last one correct.

I'm not particularly pleased with either of these games, but I've forced myself to bring them to a point at which I can publish them, as I otherwise tend to abandon old projects while half-finished and start on new ones.

Both games are available from my 2dgames repository on github and are MIT licenced: As before you'll need PyGame to play them; as I get better at creating games, I'll consider packaging some of them up or porting them so they can be played more easily.

Wednesday, 18 July 2012

Investigating Raspberry Pi performance problems

I bought a Raspberry Pi some time ago and have been trying to run some of my PyGame games on it. I'm going to describe one performance issue I've seen with it. This isn't a criticism of the board in general, which I still think is excellent for the price.

I wanted to show something off at the Raspberry Jam last weekend in Cambridge, so shunted one of my games onto the SD card and started it up. The good news is that the Pi's standard distributions come with Python 2.7 and PyGame already installed, which saves a lot of time and effort. The bad news is that my game, previously running at 120+ frames per second on my desktop was down to 5 FPS on the Pi, which makes it unplayable.

Of course you wouldn't expect a 700MHz ARM1176 to be as quick as my Core i5, but this isn't a particularly taxing game, and I expected it to be well within the Raspberry Pi's capabilities. It's a 2D game, using lots of blits to draw tiled scenery, some bitmap rotates and vector arithmetic for collision detection.

My first suspect was floating point, as my game uses a fair amount to do polygon overlap calculations and I'm using PyGame's libraries to rotate bitmaps. The old Debian linux distribution for the Pi didn't use hardware floating point. Yesterday, the new new Raspbian image was released with hardware floating point support, so I gave that a try. No luck - it's still at about 5 FPS.

Python has a built-in profiler which can be invoked with
python -m cProfile

It's best to sort samples by time and save this to a file, so I use:
python -m cProfile -s time > profileoutput

(You can also use "-o file" to redirect the profile output to a file, but it's saved in a binary format if you do that, so it's easier to redirect stdout, although this does mix the profiler output with the output from your program)

Here's the profile from the Rasbperry Pi, with some information stripped out for brevity:

   225868   65.576 {method 'blit' of 'pygame.Surface' objects}
        1   47.159
      605    3.082 {method 'fill' of 'pygame.Surface' objects}
      605    2.802 {pygame.display.flip}
    64433    1.967 {range}
    76268    1.741 {shipIntersectsBlock)
    10030    0.977 {}

It's often useful to look at the profile on the well-performing case, so this is the profile from my desktop machine:

   748686   11.493 method 'blit' of 'pygame.Surface' objects}
        1    2.859
     2321    2.523 {pygame.display.flip}
     2321    0.405 {method 'fill' of 'pygame.Surface' objects}
   499709    0.246 (shipIntersectsBlock)
   247663    0.081 {range}

Time at the file level of has gone up proportionally on the Pi, but this is probably because the desktop version ran for longer and initialization code wasn't such a significant part. This wasn't a very carefully controlled test. The main thing to note is that 'blit' is what we're spending most of our time on, in most cases. 

To test this, I removed one blit from the code which blits the static background onto the screen every frame. As might be expected, the performance went up from 5.2 FPS to 8.8 FPS. That's small in absolute terms but a very significant chance proportionally.

Now, there's a number of ways to reduce that - the scenery doesn't change often, so it may be more efficient to construct a single scenery image and keep blitting this to the screen rather than blitting each tile individually, each frame. There's also a lot of settings to play with regarding pixel depth.

I've got a suspicion that one of the biggest problems is the resolution I'm running - 1824x1104, which is a lot of work for the graphics hardware. My game only runs at 640x480, but I haven't figured out how to get the Raspberry Pi's framebuffer to use a lower res - using a smaller monitor would be the easiest way, if I had one.

Update 19th July 2012: Tried this on an Android tablet (Asus TF300) and got 25FPS; and on a laptop with an Intel U4100 @1.3GHz; Radeon 4550 - 50 FPS. Both of these devices are quite capable of doing lots of 2D blits at 50FPS without breaking a sweat, so the culprit is likely to be a software problem.

Tuesday, 3 July 2012

2D Platform games with PyGame

I've been writing computer games for quite a long time but rarely finish anything and never publish them. Just to show that I can actually finish things, here's Object Collector 2D, a tiny game very similar to Manic Miner and a hundred other old platform games. It took one day to program and comes in at 388 lines of Python while still, hopefully, being fairly readable in case any newcomers to Python or PyGame want to use it as an example. Don't expect a competitor to Braid.

I have a tendency to spend a lot of time working on the mechanics of games, finely tuning collision resolution and other rules. There's a great talk on YouTube by Martin Jonasson & Petri Purho called "Juice it or lose it" which shows how much more you need than good mechanics to make an enjoyable game, and it's these things I need to work on.

The code is in my github repository "2dgames", under the directory "collector": You'll need PyGame to run it.