This is a poor man's latency measurement tool to quantify the "glass-to-glass" latency of my Erlang IP camera setup. Glass-to-glass latency refers to the time it takes light that hits the glass of a camera's lens to be displayed on a screen (the glass of a CRT). This program measures that time indirectly by using the screen as input to the camera and measuring how long it takes to receive the result (but not display the result) over the network.
To use the program, point the camera at the shell console where you're going to run this program. When run, it will repeatedly print the current time and capture frames from the camera. Each frame is logged with the current time. The total latency is then the difference between the logged time and the time in the image.
For example, here's an example picture:
The time that the frame was received over HTTP was at 121347.131938. The time in the picture is 121347.026054, so the latency is about 106 ms. Yes, you have to manually go through the frames one by one.
It's also possible to use a Sparkfun 7-segment Arduino shield to remove frame delays in the display path. LEDs are nice since they change almost instantly. Connect a 5 V FTDI cable to the shield and attach it to a USB port. Set the 7-segment display to run at 115200 baud (it defaults to 9600). Point the camera at the shield. Here's my setup:
It might take some lighting adjustments to make sure that the LEDs don't wash out. Setting the LED brightness to the lowest level helped me get a decent picture with office lighting.
I measure latencies at least 30 ms shorter with the 7-segment display. Apparently, my laptop and LCD monitor take two frame delays to get text output to the shell window to my eyes. I had hoped for only one frame delay.