How to freedraw with a mouse, like in paint?

So for this schoolproject I need to remake paint without using any of the preprogrammed functions that matlab provides. Currently I'm trying to code a drawing tool with your mouse, so that a user can hold his leftmousebutton, draw, and finish the action by releasing his leftmousebutton. So tried with ButtonDownFcn and WindowButtonMotionFcn but I couldn't figure out how to work with them...At the moment my code is as follows:

      function workspace_ButtonDownFcn(hObject, eventdata, handles)
      if valPen == 1
          %msgbox('Button pressed'); This was just so I could see if it worked as
          %intended...
          set(gcf,'WindowButtonMotionFcn',@FreeDraw)
      end

      function FreeDraw(hObject, handles)
      CurrP = get(gca, 'CurrentPoint');
      handles.im(CurrP(1,1),CurrP(1,2),:)=0;
      image(handles.im);
      axis image
      guidata(hObject,handles);

Answer 1

[Rik]

Use ButtonDownFcn (of the axes object) to set a flag, use WindowButtonUpFcn (of the figure) to reset the flag. Now you can set WindowButtonMotionFcn to draw pixels, but exit immediately if a button is not pressed.

Inside FreeDraw I would explicitly set figure to normalized and check if CurrP is in the [0 1] range. Then you can convert those units to pixel locations inside im (don't forget to round and check for 0).