Realization of Automatic Keystone Correction for Smart mini Projector Projection Screen

Transcription

1 Applied Mechanics and Materials Online: ISSN: , Vols , pp doi: / Trans Tech Publications, Switzerland Realization of Automatic Keystone Correction for Smart mini Projector Projection Screen Baolong Zhang 1,a, Weiqi Ding 2,b, Shaojing Zhang 3,c and Huishuang Shi 4,d 1 College of Information and Automation, Tianjin University of Science and Technology, Tianjin, , China a eezbl@tust.edu.cn, b dwqqwdpl3000@163.com, c zhangshaojing111@sina.com, d shihuishuang@sina.com Keywords: Android; mini projector; keystone correction; acceleration sensor Abstract. With the rapid development of mobile internet, the projector becomes the terminal of mobile internet because of the network module embedded in smart projectors, which can provide a more convenient viewing experience. For traditional mini projector just as simple display equipment, the integration of Android operating system makes mini projector smarter, it is equivalent to a simple PC with projection function. This paper discusses the projection screen keystone distortion phenomenon of mini projector equipped with Android operating system under limiting projection environment because of different placement angle and how to eliminate the above-mentioned distortion, after elaborating the software and hardware architecture of smart mini projector, it focuses upon an analysis on how to achieve automatic correction of the projection screen using the digital acceleration sensor in hardware driver of the Android operating system. Introduction During the normal use of the projector, the axis is perpendicular to the projected plane in the projection light, and therefore, the rectangular projection screen can be displayed on the projection plane. If there is an angle θ between projector and horizontal plane, as shown in Fig.1, then there will be an angle α between projection optical axis of the projector and the projected screen in the vertical direction, at this time, the projection screen will produce vertical keystone distortion, similarly, if there is an angle between optical axis of the projector and the projected screen in the horizontal direction, then the projection screen will produce horizontal keystone distortion. During normal use of the projector, the protection optical axis usually cannot be perpendicular to the projected plane due to external environmental constraints, therefore, the projection screen keystone correction is emerged at the right moment, vertical keystone correction is more practical and common, the paper focuses on how to achieve the automatic keystone correction in vertical direction, the schematic diagram for keystone correction effect is as shown in Fig.2. The projection screen keystone correction of projector mainly includes optical keystone correction and electronic keystone correction, optical keystone correction means to correct keystone picture by adjusting the optical lenses, the electronic keystone correction is divided into hardware keystone correction and software keystone correction, respectively achieving keystone correction by electronic signal offset, software interpolation and other algorithms, the hardware keystone correction uses dedicated hardware circuits, the control operation of keystone correction is very simple, but the complexity of the hardware is increased; software keystone correction is achieved by interpolation and other software algorithms, without additional hardware circuit, but the display effect is not as good as dedicated hardware circuit, and it also takes up valuable CPU resources. This paper uses dedicated display chip integrated with keystone correction hardware module to achieve keystone correction. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-12/05/16,00:50:33)

2 Applied Mechanics and Materials Vols Fig. 1. Schematic diagram for keystone distortion Vertical keystone correction (before) Vertical keystone correction (after) Fig. 2. Schematic diagram for keystone correction effect System architecture of smart mini projector Laptop has solved the drawbacks of inconvenience to carry for desktop, but it is still very heavy and inconvenient, but smart phone, tablet, netbook and other equipment cross the gulf of "portability", however, its display screen size limitations can not meet people's visual needs. The mini projector becomes popular among people for its small size, easy to carry, large-screen display and other advantages. Traditional one will eventually be phased out only as a mini projector to display output device, and at present, the smart mini projector is combined with operating system, equipped with network capabilities and also becomes the terminal of mobile internet. Fig. 3. Framework diagram for smart mini projector hardware Fig.3 is a framework diagram for smart mini projector hardware, which is mainly divided into four parts - input module, output module, control core and network module. Input module includes the

3 506 Computer and Information Technology interactionequipment of user and mini projector and signal input interface, such as HDMI interface, touch tablet, infrared receiver, acceleration sensor, etc. The output module is divided into two parts, respectively including the audio output module and video output module. The audio output module consists of the audio decoding chip, power amplifier and loudspeaker. The video output module is based on LCoS (Liquid Crystal on Silicon) micro-projection technology and consists of the video encoding and decoding control chip, high-power LED drive circuit, LCoS control chip and optical engine lenses. Control core includes a memory or storage equipment, a CPU with ARM cortex-a8 architecture and an 8-bit microcontroller. The memory includes Samsung's flash and DDR3, used to store the operating system and data, SD card is used to make up for lack of storage space. Core processor shall be processing chip based on ARM cortex-a8, the dominant frequency is up to 1GHz, video processor subsystem is embedded within it and with good performance on processing video decoding, etc. Network function is a major feature of the smart projector, network communication consists of wireless radio frequency module, due to the integration of network functions, the smart projector becomes the terminal of mobile internet.the software operating system of smart projector shall be Android 2.3 operating system, originally, Android operating system is specially designed for smart phones based on the Linux open-source operating system in Google in November 2007, which has experienced generations of development, by virtue of its free open source characteristics, gradually developed and improved system performance and strong application market, making the integration of smart handheld terminal equipment become imperative. Fig. 4. Framework diagram for Android operating system Android operating system framework diagram is as shown in Fig.4; the Android operating system is established on basis of Linux kernel[2] and divided into application layer from top to bottom, application framework layer, system runtime library, Android operating environment, virtual machine layer and Linux kernel layer. Since Google has excellently optimized the software in the middle layer of Android operating system, therefore, its integration part with mini projector is mainly the combination of Linux kernel layer and hardware, namely, the programming of hardware driver on the bottom layer of Linux related to projector, and thus the screen automatic keystone correction focuses on the programming related to keystone correction driver in Linux kernel.acceleration sensor is used in the design to carry out real-time monitoring on the angle between mini projector and the horizontal plane, quantify the acceleration values related to the angle and then carry out further processing on quantitative data in acceleration sensor driver of Linux kernel. Because in the design, the image keystone correction is handled by image keystone correction module in video encoding and decoding

4 Applied Mechanics and Materials Vols display chip, it only needs to simply configure register control, therefore, after corresponding conversion of the data in acceleration sensor drive, configuring video encoding and decoding display chip register simply can achieve real-time keystone correction. Realization of projection screen automatic keystone correction using acceleration sensor Fig. 5. Schematic diagram for MMA8452Q Fig. 6. Linux I2C driver architecture MMA8452Q researched and developed by Freescale Company is a 3-axis 8/12-bit digital acceleration sensor[5], also known as accelerometer, shown in Fig.5, it can measure the relationship between the deflection angles of X, Y, Z-axis and vertical axis and can accordingly measure the acceleration in each direction. Mounting the accelerometer at a suitable position of the projector closely so that the X or Y axis of accelerometer can be parallel to the projection axis, the capacitance measuring circuit in accelerometer can measure the acceleration values related to the deflection angle of projector and horizontal plane and store quantitative data. MMA8452Q uses I2C (Inter-Integrated Circuit) protocol and microprocessor to transfer data or instructions, after the MMA8452Q senses the angle between the projector projection optical axis and the horizontal plane and quantifies the information into 12 or 8-bit data, the microprocessor can read the register of MMA8452Q to obtain quantitative data related to the angle. The Linux version in design is , MMA8452Q driver is established with I2C driver architecture as the skeleton, Linux kernel I2C driver architecture is as shown in Fig.6. I2C driver can be divided into two parts including I2C equipment driver and I2C bus driver[3][4]. Operation of I2C module is very simple, but its implementation in linux is more complex, the driver becomes complex due to highly abstract software module and software layering[1], but the layering idea of software improves service efficiency of software and makes maintenance and management convenient. I2C bus driver achieves the control on internal I2C controller of microprocessor and builds communication time sequence and this part is developed and optimized by the linux kernel maintainers, and without energy from application developers. The I2C equipment driver involves in specific hardware equipment, including the use of function reference in I2C-Core to complete the configuration of hardware registers, the achievement of communication with the upper layer and establishment of the application program interface, this part must be completed by the equipment vendors or application developers. MMA8452Q driver flowchart is as shown in Fig.7. MMA8452Q driver is character-driven, this driver is mainly divided into four software modules, the first is I2C module for MMA8452Q hardware initialization, which is used to configure the acceleration sensor, such as the resolution of the sensor, operation mode, the interrupt control and reading MMA8452Q sensing angle information, etc.; the second is the input subsystem in universal input equipment architecture, used for reporting data obtained by the sensor to the upper software of Android; the third module is programming interface left for the application, which can achieve interaction of the application on upper layer with MMA8452Q through this interface to a certain extent; the fourth module is used to create the node information in the sys file system, manage the equipment and also achieve mutual communication with other processes through it.

5 508 Computer and Information Technology I2C match N Get device ID Y Config control circuit interrupt Register input device Register misc deviece Creat node in sysfs Config accelerator End Fig. 7. MMA8452Q driver flowchart Fig. 8. Realization of keystone correction data stream in Android operating system The data is converted after the driver obtains the data related to the angle between the projector and the horizontal direction, then the software processing shall be conducted to obtain a stable angle value. And the value shall be reported to the android upper layer and used for adjusting the screen. The angle values after filtering software shall be transmitted to the driver of display chip and complete keystone correction through configuring the corresponding register of display chip. Processing of related keystone correction data in Android operating system The schematic diagram for processing of related keystone correction data in Android operating system is as shown in Fig.8, when the capacitance measuring circuit of accelerometer MMA8452Q detects there is deflection angle between projector projection axis and the horizontal plane, the acceleration values of three axis for accelerometer are changed, thereby generating data related to the offset angle, the accelerometer will quantify this data and send the interruption to the microprocessor. After the MMA8452Q driver in Linux kernel obtains the data after quantization through I2C, on the one hand, it will convert the data format, process them in package and report data to the Android system runtime library layer through Input subsystem, in hardware abstraction layer, it contains the function set of operating accelerometer, data can be further reported to the Android application framework layer through JNI (java native interface)[2]. In application framework layer, there is sensor manager to be responsible for the communication between application layer and the system runtime library layer so that the accelerometer data can be captured by the sensor event listener in application finally. Similarly, the upper software can call the set of functions from the system runtime library through sensor manager and JNI and obtain the configuration parameters through interface functions created by the misc equipment such as mma8452_open, mma8452_release, mma8452_ioctl and other function interfaces or operating mode for configuring MMA8452Q. On the other hand, the corresponding deflection angle value can be obtained after extracting the data of X -axis or Y-axis parallel to the projection axis from the obtained quantitative data and converting the data format, Due to mechanical shaking, measurement error and other reasons, a stable

6 Applied Mechanics and Materials Vols deflection angle value is required to obtain, therefore, it needs to conduct software processing on the values, simply taking average and taking mid-value in sequence can not meet the requirements in case of a occasional larger mechanical shaking value, this paper uses a kind of simple software state processing method, the data enters into the software processing module as entering into the queue, as shown in Fig.9, if there are m stable values in n continuous input data, the output shall be this stable value and the comparison parameter in software modules shall be updated to the stable value, if the new input data is equal to the comparison parameter, the data shall be output directly, adjusting m value is to regulate the error-tolerant rate, this method can obtain a more stable output value. After obtaining a stable output angle, the keystone correction module in function configuration image processing engine use exported from Optical Engine Driver through EXPORT_SYMBOL[4] can be used to obtain ideal keystone correction effect, when the deflection angle of the projector changes, real-time keystone correction can be carried out. Conclusion Fig. 9. Processing of software output stable value This paper analyzes the real-time keystone correction using digital acceleration sensor under the Android operating system platform, keystone correction display effect is better in the test. In Linux drive, it achieves the drive of optical engine driver and MMA8452Q digital accelerometer related to video display encoding and decoding chip, MMA8452Q driver conducts real-time monitoring on declination value of smart mini projector and the horizontal plane, using I2C communication to obtains data and using deflection angle value to configure the relevant register in optical engine driver can obtain keystone correction effect. This paper makes a general description on the realization of keystone correction data processing in Android operating system, briefly describes how to manipulate the data in the different Android software layers and also analyzes the stable output process of the data in software. References [1] Yu Zhang, Linux Kernel Module Programming Guide, third ed., Tsinghua University Press, Beijing, 2004 (in Chinese). [2] Zhuan Wu, Inside the Android Framework, Posts and Telecom Press, Beijing, 2012(in Chinese). [3] Miao Liu, Embedded System Interface Design and Linux Driver Development, Beihang University Press, Beijing, 2006 (in Chinese). [4] Guojin Fung, Embedded Linux Driver Design, Tsinghua University Press, Beijing, 2008 (in Chinese). [5] Information on

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