In time-lapse photography, the camera is placed in a fixed position and photos are taken at equal intervals over a period of time, then combined to make a movie of greatly sped-up action.
This process is similar, except the camera is attached to a vehicle, and instead of taking photos at equal time intervals, the camera is connected to a computer that takes photos at equal *distance* intervals. Additionally, the computer knows how fast the car is moving at any given instant, and adjusts the shutter speed and aperture (and other camera settings like ISO) to provide a balanced exposure where the shutter is open for the same distance each exposure. For example, the shutter speed might be 1/20 at 30mph, but 1/40 at 60mph.
The net result, when the images are combined into a movie, should be a speed-normalized version of the trip. Unlike existing sped-up trip movies that can be found on the Internet, there will be no stops and starts, and the terrain should move at the same apparent speed regardless of the actual speed of the car when a specific photo was taken. It would appear as if you're floating through the landscape at a constant speed.-- ytk, Aug 28 2013 Dash cam Dash_20CamClicks a shot every X mile based on the odometer. [tatterdemalion, Aug 28 2013] Note: I have actually made progress towards implementing this idea, but since it got stalled over a year ago and I haven't worked on it since then, I figured I'd post it here.
I did some tests and discovered that a GPS receiver wasn't as accurate at determining speed as would be needed, nor was the update frequency even close to what I was hoping. However, I managed to modify my car to add a jack that taps into the speed sensor wire, and read the data via an Arduino circuit connected to a laptop. So I can get the speed information with a high degree of precision.
I also started working on writing a driver to control the camera from the laptop. I got about 50% done with that when I got distracted by something shiny, and never finished it. That's the easy part, though—I just need to find the time for it.
I also need to figure out how to mount the camera in my car so that it's stable. I fear this is going to be the hardest part. I'm not really sure what the best way to do it is. Rigid mount? Gyroscopic gimbal? No clue. I think it would need to be really stable and free from vibration for the effect to work. A frame that's a bit “buzzed” from vibration during exposure could totally ruin the effect.-- ytk, Aug 28 2013 I would imagine that this is how the Googlemobile does its thing (or, to be more exact, how I imagine it would be accomplished efficiently).-- FlyingToaster, Aug 28 2013 They're a bit similar, but not really that much. The Google van is only concerned with taking pictures at specified locations, and doesn't really care about modifying the exposure based on speed. It also has a fairly coarse spatial resolution (e.g. one photo every 50 feet or so), so a GPS receiver is perfectly suited for that purpose. For this application, the location isn't specified, and indeed, doesn't really even matter. The only things that are important are instantaneous speed and distance travelled—factors that are irrelevant to the Google van.-- ytk, Aug 28 2013 It would be nice to implement this using only mechanical means, using a series of levers connecting the dials to the camera. If your car only has a digital odometer you could build a mechanical integrator out of mechano.-- mitxela, Aug 28 2013 // So I can get the speed information with a high degree of precision.//
I doubt that.
Legislature decrees that motor vehicles have some indicator of speed for the driver, and also that vehicles keep within speed limits on designated roads. In order to avoid the obvious litigation when motorists charged with speeding blame the readout on the car's speedometer, the speedo always reads higher than true. Tolerance may be 5%, so when travelling at 50mph your speedo may read anything between 50 and 55. The spread varies with speed and the way the speedo gets its data may also give rise to further discrepancies. If it counts the number of revs your road wheels make, as a function of the drive shaft/axle and gearing ratios, it's also going to be affected by the amount of tread remaining on your tyres and the overall wheel size. The difference between new and bald tyres may be 200 revolutions per kilometre.
Even a cheap GPS sensor should be more reliable than your car's mechanical measurement.
Surveyors and road planners use a "wheel on a stick" as their preferred measurement device, with controlled circumference and accurate gearing. Trail one of those around attached to your car and use that as your trigger instead.-- Tulaine, Aug 28 2013 //I doubt that.//
I don't care what the *actual* speed is, so long as the reported speed is consistent between measurements. The way the speed wire works is it pulses as the car moves. The pulse rate increases as the car speeds up.
//Even a cheap GPS sensor should be more reliable than your car's mechanical measurement.//
In actual fact, I've done some testing and compared the car's reported speed to the GPS speed and found it to be more than accurate enough for my purposes, and it updates significantly faster and is more consistent as well. The GPS speed tends to lag behind the actual speed by as much as a few seconds, and it has random dips and spikes from time to time. That simply won't do.
By “precision” I mean ability to get a measurement that reflects the speed of the car in as close to real time as possible. GPS receivers just aren't capable of doing that; at least, none of the ones I've ever used are. But even if there is some receiver out there capable of the degree of precision I need, I don't really care that much, since I already have it from the car's sensor wire.-- ytk, Aug 28 2013 Quite so; a car's speedometer has high precision, but low accuracy. Conversely, GPS has high accuracy, but low precision. The obvious upshot of which is that you can combine the two data streams to calculate position and velocity with great precision _and_ accuracy.-- spidermother, Aug 30 2013 //update frequency// yeah, that's a problem - I have an old Magellan that updates the display every second, but the data out lines only update every 2 seconds. It's set up to give it time to cycle a full set of NMEA sentences at 4800 baud, and even if you select less output data, you still have to wait.
However, there are some GPS units I've noticed on Adafruit which update at 10 Hz, which may be much more like what you want. I don't know the output format, but I'm sure it's something reasonable.-- lurch, Aug 30 2013 random, halfbakery