1. Field of the Invention
The present invention relates to a coordinate input apparatus which detects a coordinate position input to a coordinate input surface by a pointer such as a finger in order to input or select information, and a method thereof.
2. Description of the Related Art
Various types of coordinate input apparatuses (touch panels and digitizers) have conventionally been proposed or commercialized. For example, a touch panel is used, which makes it possible to easily operate a terminal such as a PC (Personal Computer) by touching the screen with a finger without using a special tool or the like.
There are various coordinate input methods ranging from a method using a resistive film to a method using an ultrasonic wave. As a method using light, a method (light shielding method) is known, in which a retroreflecting member is arranged outside a coordinate input surface and reflects light from a light projecting unit, and a light receiving unit detects the light amount distribution (see, for example, Japanese Patent Laid-Open No. 2004-272353). This method detects the direction of a light shielding portion (region) where light is cut off with a finger or the like in the coordinate input region, and determines the coordinates of the light shielding position, that is, the coordinate input position.
Japanese Patent Laid-Open No. 2012-234413 has disclosed an invention for implementing a coordinate input apparatus for a large screen by a compact system using many sensor units. United States Patent Publication No. 2012/0065929 has disclosed a system in which a device can be set at an arbitrary position to improve user friendliness.
By integrating a coordinate input apparatus of this type with a display apparatus, the display state can be controlled by touching the display screen of the display apparatus, or the locus of a pointed position can be displayed as handwriting as in the relationship between paper and a pencil. A typical example of this is a mobile device such as a smartphone. Along with upsizing of flat panel displays, it is becoming popular to combine the flat panel display with a large-size touch panel and introduce it into, for example, the digital signage field.
In such an apparatus, several errors are generated owing to its structure. For example, a light receiving device such as a CCD line sensor or CMOS sensor is used as a light receiving unit which detects light. Since each pixel receives light in the light receiving device, a quantization error is generated at this time. When converting the number of a pixel detected by the light receiving device into an angle value, a method such as table lookup or conversion by polynomial approximation is adopted. However, a slight error is generated in either method.
For example, when the apparatus is permanently attached to a housing, the number of a pixel indicating a reference angle in the horizontal direction or the like that is used for coordinate calculation is measured and recorded in the internal memory of the apparatus at the time of, for example, assembling the apparatus. At this time, a measurement error or the like is generated. If the attaching position of the apparatus shifts owing to aged deterioration or the like, an error may be further generated. Owing to various error factors as mentioned above, an angle detected by the sensor has an error.
If an angle detected by the sensor unit has an error, calculated coordinates on the screen do not match a touched position. To reduce the difference between them, a generated error needs to be reduced by, for example, increasing the pixel resolution of the light receiving device or the mechanical precision of the apparatus. In practice, the design level is determined in consideration of the cost, the function/performance balance, and the like.
Japanese Patent Laid-Open No. 2012-234413 has disclosed a system which changes a combination of sensor units, divides the screen into a plurality of regions, and calculates a pointed position. In this system, if an error is generated in an angle detected by the sensor unit, another problem that coordinate values do not coincide with each other at the boundaries between the plurality of divided regions is generated, in addition to the problem that a touched position and calculated coordinates on the screen do not match each other. The other problem arises from the difference in the appearance of an error (direction and distance of an error) between the plurality of divided regions. For example, when drawing is performed on the screen, coordinates become discontinuous at specific positions. Such an error is very noticeable.