Selection of the subsection point is the key to enhance low contrast image under light background by subsection linear transformation. Present two methods to choose the subsection point:multiscale app roach (MSA) and constant enhance ratio (CER). The former is based on the princip le that target and background would express a different dynamic behavior in scale-space. It gets the subsection point in transition area between target and background by repetitious app roach. The latter defines a fixed enhance ratio, and find out the gray level that satisfied the ratio as the subsection point from high to low in the histogram. These two methods are simp le and fast. Experimental results prove that those methods can find the subsection point accurately, and enhance the contrast ratio of image effectively.