武汉理工大学数字图像处理英文课件 Chapter3（7）.pptVIP

Graphics Overlay Convex Hull Appearance Spatial Coordinates * im = imread(card_spade.tif); L = bwlabel(im); hull = regionprops(L,ConvexHull); [m,n]=size(im); figure(Units,pixels,Position,[100 100 n m]); h=image(255*~im); colormap(gray); set(gca,Position,[0 0 1 1]); hold; plot(hull.ConvexHull(:,1),hull.ConvexHull(:,2),LineWidth,3); axis off; hold off; f = getframe; im2=frame2im(f); figure(2); imshow(im2); Image with convex hull superimposed. Read image and find convex hull Display image, with proper scaling. Draw convex hull Copy figure to frame, then frame to image. Superimposed on image Plotted separately In the pixel coordinate system, a pixel is treated as a discrete unit, uniquely identified by a single coordinate pair, such as (5,2). From this perspective, a location such as (5.3,2.2) is not meaningful. At times, however, it is useful to think of a pixel as a square patch. From this perspective, a location such as (5.3,2.2) is meaningful, and is distinct from (5,2). In this spatial coordinate system, locations in an image are positions on a plane, and they are described in terms of x and y (not r and c as in the pixel coordinate system). The figure illustrates the spatial coordinate system used for images. Notice that y increases downward. This spatial coordinate system corresponds closely to the pixel coordinate system in many ways. For example, the spatial coordinates of the center point of any pixel are identical to the pixel coordinates for that pixel. There are some important differences, however. In pixel coordinates, the upper-left corner of an image is (1,1), while in spatial coordinates, this location by default is (0.5,0.5). This difference is due to the pixel coordinate system being discrete, while the spatial coordinate system is continuous. Also, the upper-left corner is always (1,1) in pixel coordinates. Another potentially confusing difference is largely a matter of convention: the order of the horizontal and vertical components is reversed