16. The apparatus of claim 15 , wherein the processor detects or allows other device to detect the at least one text candidate by changing intensity level of the pixels of the specific region, and builds or allows other device to build a tree by allocating (i) a low-level text candidate whose intensity level in a specific region is relatively high and (ii) a text candidate whose intensity level is the highest among high-level text candidates, including a region of the low-level text candidate respectively, whose intensity level in the specific region is relatively low, to a child node and a parent node, respectively.
16. The apparatus of claim 15 , wherein the processor detects or allows other device to detect the at least one text candidate by changing intensity level of the pixels of the specific region, and builds or allows other device to build a tree by allocating (i) a low-level text candidate whose intensity level in a specific region is relatively high and (ii) a text candidate whose intensity level is the highest among high-level text candidates, including a region of the low-level text candidate respectively, whose intensity level in the specific region is relatively low, to a child node and a parent node, respectively. 17. The apparatus of claim 16 , wherein, if there occurs no less than a certain percentage of an overlap between a first region where a text candidate corresponding to a particular node in the tree is included and a second region including the first region where the text candidate corresponding to the particular node is included and where a text candidate, with a relatively low intensity level of the pixel, corresponding to a parent node is included for no less than a specified number of times, the processor selects or allows other device to select a text candidate corresponding to a node whose variant of a corresponding region is small compared to its parent node; and the processor classifies or allows other device to classify (i) the text candidate satisfying the predetermined condition as the strong text or the non-strong text in comparison to the first threshold value by referring to the comparison result between the first threshold value and the first feature value of at least one pixel selected within the corresponding region where the selected text candidate is included or the value converted from the first feature value, and (ii) the text candidate classified as the non-strong text as the weak text or the non-text by referring to the comparison result between the second threshold value and the second feature value of at least one pixel selected within the corresponding region where the text candidate classified as the non-strong text is included or the value converted from the second feature value.