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Friday, 18 March 2011

Customizable Virtual Keyboard

It is becoming increasingly difficult for users to interact with the slew of portable gadgets they carry, especially in the area of text entry. Although miniature displays and keyboards make some portable devices, such as cell phones and PDAs, amazingly small, users’ hands do not shrink accordingly.

To solve this problem, we proposed a Virtual Keyboard. This device will replace a physical keypad with a customizable keyboard printed on a standard A3 size paper whose “keystrokes” are read and translated to real input. This virtual keyboard can be placed on any flat surface, such as desktops, airplane tray tables, kitchen counters, etc. and can theoretically be interfaced with any computing device that requires text entry. This would eliminate the need to carry anything around and also prevent any chance of mechanical damage to the keypad in harsh environments if a simple lamination is used to protect the paper. In addition, buttons on this device can be reconfigured on-the-fly to give a new keyboard layout using a GUI we built in Java and then transferring that data to the device using a computer’s serial port.

At first we initialize PORTA on the Mega32 to take UV input from the camera and PORTC to communicate with the camera over the I2C interface. The baud rate is set to 19,200bps for serial communication. We then run the calibrate function on the camera, which looks at a black keyboard to determine a distinguishable value for red color threshold. Then we call a function called "init_cam" which performs a soft reset on the camera before writing the required values to corresponding camera registers. These registers change the frame size to 176x144, turn on auto white balance, set the frame rate to 6 fps, and set the output format to 16-bit on the Y/UV mode with Y=G G G G and UV = B R B R. The code then enters an infinite loop which checks for the status of the PS2 transmitting queue and tries to process the next captured frame if the queue is empty. If not, the queue is updated and the PS2 transmission is allowed to continue.

Image Processing

The getRedIndex function captures rows of data from the camera and processes each of them. We first wait until a neg edge on the VSYNC, which indicates the arrival of new frame data on the UV and Y lines. We then wait for one HREF to go by since the first row of data is invalid. At this point, we can clock in 176 pixels of data for a given vertical line in the Bayer format.

Bayer format

Figure 4: Bayer color pattern

In the mode where the UV line receives BR data, the output is given by: B11 R22 B13 R24 and so on. Since we only needed red data, we stored an array of 88 values in which we captured the data on the UV line every 2 PCLKS. The OV6630 also repeats the same set of pixels for consecutive rows and thus 2 vertical lines processed would have data about the same pixels. We considered optimizing this by completely dropping data about the even rows, but this was not going to save us anything since all our processing could be done between one neg edge and a pos edge (when data becomes valid again) of HREF.

Since we don’t have enough memory to store entire frames of data to process, we do the processing after each vertical line. After each vertical line of valid data, HREF stays negative for about 0.8ms and the camera data becomes invalid; this gives us ample time to process one line worth of data. After each vertical line was captured, we looped through each pixel to check if it exceeded the red threshold found during calibration. For every pixel that met this threshold, we then checked if the pixel was part of a contiguous line of red pixels, which would indicate the presence of a key press. If such a pixel was found, we then mapped this pixel to a scan code by binary searching through an array of x, y values. If this scan code was found to be valid, we debounced the key by checking for 4 continuous presses, and then added the detected key to the queue of keys to send to the PC.

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