求助画图程序

ylhit

%要求在当前坐标轴下，用鼠标拖动划线。按鼠标左键出现+，之后拖动鼠标，松开点即为结束点，可多次划线。问题是划线的起点是原点，不能多次划线。源程序：<BR>function ex_draw(arg)<BR>global DRAW_HL<BR>if nargin==0<BR>   arg='ex_line'<BR>end<BR>if isstr(arg)<BR>   clear all<BR>   Hf=gcf;<BR>   set(Hf,'pointer','corssh','backingstore','off',...<BR>   'windowbuttondownfcn','ex_draw(1)');<BR>   figure(Hf);<BR>elseif arg==1<BR>       fp=get(gca,'currentpoint');<BR>       set(gca,'userdata',fp(1,1:2));<BR>       set(gcf,'windowbuttonmotionfcn','ex_draw(2)',...<BR>           'windowbuttonupfcn','ex_draw(3)')<BR>elseif arg==2<BR>       cp=get(gca,'currentpoint');cp=cp(1,1:2);<BR>       fp=get(gca,'userdata');<BR>       H1=line('xdata',[fp(1);cp(1)],'ydata',...<BR>       [fp(2);cp(2)],'erasemode','xor','color',...<BR>       'r','linestyle','-','clipping','off');<BR>       if ~isempty(DRAW_HL)<BR>           delete(DRAW_HL)<BR>       end<BR>       DRAW_H1=H1;<BR>elseif arg==3<BR>       set(gcf,'pointer','arrow',...<BR>           'backingstore','on',...<BR>       'windowbuttondownfcn','',...<BR>       'windowbuttonmotionfcn','',...<BR>       'windowbuttonupfcn','')<BR>set(gca,'userdata',[]);<BR>set(DRAW_HL,'erasemode','normal')<BR>key=waitforbuttonpress;<BR>if key<BR>   gtext('finish');<BR>   return;<BR>esle<BR>   ex_draw;<BR>end<BR>end