Week 02 - Laser Detection I

0114

The week began with a question: How do I differentiate the laser dot from its background? The bright green dot could easily be a bullet on a presentation slide, or the desktop background, or the mouse pointer, or ....

I would like to think that I am able to differentiate a laser point from the background via:

1. Color separation
2. Motion tracking
3. Canny edge detection

I decided to use OpenCV libraries to implement these methods, with the aim of finding the capabilities and limitations of what I can do for each of the three. Hopefully, one method would be able to make up for the shortcomings of the others. Then, a combination of methods will give me a better approximate for the location of the laser point.

I began working on the HSV colour model. I know that I will be looking for a green hue of high saturation and brightness. The green hue would be in a extremely small range of approximately 180 degrees. I tried to use OpenCV to convert the image from a RGB to HSV colour model, before extracting the individual H,S and V channels from the resulting image. Getting the full channels is no problem at all, but I am only interested in a certain hue. Unfortunately for me, OpenCV uses a very different scale for HSV.

Usually, H spans from 0 to 360 degrees, and S, V would take on values from either 0 to 1, or 0 to 100. OpenCV did not state explicitly what range of values HSV would take on, but a quick look at the sample code (camshiftdemo) reveals that in OpenCV, H spans from 0 to 180 degrees, and values of S,V spans from 0 to 255. Confusion! I would have to try and error to understand the scale used in OpenCV.

Took a little time off after work to help debug Peng's program and caught two nasty bugs.



0115

By Tuesday, I was still unable to extract the laser hue from the webcam feed. I found out that my webcam sees the red and green laser dots as white instead of their respective colours, mainly because of their intensity. I am able to get the webcam to pick up their colours only on two occassions; when the laser dot is in motion, and when the camera is ridiculously out of focus. A comparison between my webcam and the iSight shows that the Apple iSight is able to pick up the colour of the laser dots better than my Logitech Quickcam IM

As progress was slow with HSV, I had to put a hold on this until I can grab hold of a better webcam or think of a solution. I spent the rest of the day working on motion tracking, mainly with the use of the template included in OpenCV.



0116

Motion tracking is far more successful at detecting the laser dot. By using a relaxed version of the problem, I am able to "draw" on a black background by making the motion persistent. The setup involves a smooth non-glossy black wall approximately 2m away from the quickcam.

Motion tracking is also able to give a rough figure of how intense the dot is. The red laser, being less intense than its green counterpart, gives a much smaller dot. Also, gaps appear when trying to draw a line quickly using red laser. This did not occur with the green laser. Noise can be observed around the trail left by the green laser but the noise is not present the red one.



0117

Added removeNoise, which works by downsampling the image followed by an upsampling, using Gaussian pyramid decomposition. The noise caused by green laser is gone for good!

I proceeded to get the setup to work with a projector screen after having success working with a relaxed environment. I noticed that "Whiteouts" always occur at the beginning (and when I clear the image buffer by pressing 'c'), because the image buffer is initially empty, and the next and also first incoming frame is not, so the resulting difference between the two is picked up as motion.

Note: clock() behaves differently on light and my laptop.



0118

Had our weekly update today. Next week, mornings will be spent on design pattern as the project will be shifting to OOP design. I will have to get the draft UML diagrams ready. We plan to buy parts to make the IR pen, and a 4.5m long firewire cable. Parts of IR pen include: 40-70 Ohm resistor for usb powered, usb power adapter, momentary switch, 1.6v infrared LED. I might want to check out light blocking filters too. (I am looking for a 532nm pass-band filter for the green YAG-medium laser, or 650 longpass filter for the common red Krypton-medium laser.
Ref: http://www.seattlerobotics.org/encoder/200110/vision.htm)

The team tried with various setups of the iSight. Having the camera at the behind the screen is not feasible as there is more interference there. [Edit: see 0211, camera @ back is better] So we ended up placing the iSight in front of projector screen to test how well the iSight is able to see the laser dot. We cheated a little by overlaying terminal transparency on the desktop to improve detection of dot with iSight.