In virtue of technology rapid development, PC and its peripheral apparatus are popularized for a variety of applications. In which, the image pickup device, such as a web camera (webcam) or a Digital Camera (digicam) are generally utilized with desktop PC or laptop PC to facilitate people's daily life. For instance, for further contacting the customers or the business partners after a certain business conference, it is better to file their business cards to effectively keep the communication information. To execute data filing and recognition, the image pickup device disposed within the laptop PC is always used to capture the images of the business cards to digitize the information thereon. In the same way, the image pickup device also can be utilized to capture the other images information on, such as picture, photo, credit card, debit card, identification (ID), envelop, post card, ticket, notepaper, Post-in note, label, etc. . . . for further applications.
At present, almost every prevailing webcam in the market equips with an auto focusing lens having an auto focusing function, enables it to capture a clear and sharpness image.
However, in the prior art, the auto focusing control adopts an edge detection mode to distinguish whether the edge of the document image is inside a focus area, and analyzes the resolution of the document image (such as by employing modulation transfer function to get a value) to determine if the image is in focus. That is, the prior art utilizes a varying focus, which is appeared during a process of focusing the lens on the document, to calculate the resolution of the document image inside the focus area, and try to focus on a certain location in document to get the highest resolution.
However, it is generally hard to shoot the document to capture a clear image using the prior art image pickup device, because the document is not easy to be disposed well for shooing. In practical, when the user captures the content on the held document by shooting, the image pickup device will automatically locate and focus on a location having the highest resolution in the document. While, the drawback in this operation is that the user is unable to know if the held document and the lens of the image pickup device are parallel to each other. It implies that the captured image is probably part clear, however, part blurred due to defocus as the document is not parallel with the lens of the device.