During image processing, if the pixels in some region of the image are operated, those pixels to be processed must be extracted. The area including pixels to be processed may be regular, and may also be irregular. As for regular area(for example, polygon, rectangle, circle and ellipse), it can be picked up by classical graphics algorithm; if the area is irregular, it can be converted into regular area by appropriate method. This paper discusses the following problems: the structure of 24-bit true color BMP file; the relationship between pixels in bmp data array and pixels to be showing on screen; extracting pixels, which belong to circle area of image, by Bresenham drawcircle; the visibility of those pixels be extracted.