明慧--The proposed CAPTCHA mechanism

2.The proposed CAPTCHA mechanism
The proposed visual CAPTCHA mechanism is relatively simple. First, a collection of images should be prepared. To generate a test, select an image from the database at random. In this image, choose two non-overlapping blocks of the same size and exchange the content. We will name this procedure the EXCHANGE-2 algorithm. Fig.2 depicts an image before and after applying the EXCHANGE-2 routine. To pass the test, the user needs to use the pointing device to click on the switched regions. In the above example, it should not require too much effort to identify these areas due to the size of the exchanged block.

在這裡提出的視覺CAPTCHA方法相當簡單，首先將影像準備好，然後隨機選擇一張影像作測試用。在這張影像中，選擇兩個沒有重疊的區塊作交換，此方法在此命名為EXCHANGE-2演算法。如Fig.2所示，一張為改變前的影像，一張為套用EXCHANGE-2演算法之後的影像。若要通過此測試，使用者必須在交換的區塊上做點擊的動作，而在上例中，他應該不需要花費很多心力去辨識所交換的區塊。


The original methodology can be generalized as follows: randomly select K non-overlapping regions in the image and perform content reassignment. A user will be granted access if he/she can correctly identify J exchanged blocks where K>=J>=2. Intuitively, the lager K and J are, the more difficult it is for both human and computer to pass the test. On the other hand, incrementing K also increases the probability of overlapping, hindering the test generation process. Therefore, proper choices of K and J must be made to allow smooth operation of this EXCHANGE-K-J algorithm.

原始的方法論可能被推斷如下：在一張影像中隨機選擇K的區塊，然後將其重新配置，如果能正確辨認出J塊已交換區塊，而這J塊區塊介於2塊到K塊之間，則使用者將認同此方法是可行的。在直覺上，隨機選擇的區域K及辨認出來的區域J越多，就越難通過人機辨識系統。就另一方面而言，增加K塊區域且增加重疊的可能性，也阻擋了測試生成過程。所以，適當的選擇K和J，可讓EXCHANGE-K-J演算法更流暢的操作。


The proposed visual CAPTCHA is designed to enable quick inspection and identification of the switched regions by a human user. At the same time, these regions should not be easily detected by a computer algorithm. To this end, we must look into two important issues. The first one has to do with the size of the exchanging blocks. The lager the size is, the easier it is for a user, and perhaps the computer, to locate the exchanged block is too small, even users with acute eyesight will have difficulty finding these regions, as shown in Fig.3, where the block size is 0.05x0.05 of the original image. The second issue is regarding the image collection. What type of images should be stored in the database? Does the choice of a specific type of images affect the performance we attempt to answer in this paper.

在此提出的視覺CAPTCHA是設計成讓使用者能夠快速檢查及辨識交換的區塊，同時，這些區塊也不能讓電腦程式容易破解。最後，我們必須調查兩個重要的問題。第一個問題為交換區塊的大小，size越大越容易讓使用者辨識，也或許讓電腦更容易破解，若交換的區塊太小，使用者較難找出這些區塊，像Fig.3，區塊大小僅0.05x0.05。第二個問題則為影像匯集。什麼樣的影像該被存放在資料庫中？在本文中我們嘗試著回答具體類型的影像所影響的效能。