OpenGL Programming Guide

Chapter 12Selection and Feedback

Selection

Hints for Writing a Program That Uses Selection

Most programs that allow a user to interactively edit some geometry provide a mechanism for the user to pick items or groups of items for editing. For two-dimensional drawing programs (for example, text editors, page-layout programs, circuit-design programs), it might be easier to do your own picking calculations instead of using the OpenGL picking mechanism. Often, it's easy to find bounding boxes for two-dimensional objects and to organize them in some hierarchical data structure to speed up searches. For example, OpenGL-style picking in a VLSI layout program that has millions of rectangles can be relatively slow. However, using simple bounding-box information when rectangles are typically aligned with the screen could make picking in such a program extremely fast. The code is probably simpler to write, too.

As another example, since only geometric objects cause hits, you might want to create your own method for picking text. Setting the current raster position is a geometric operation, but it effectively creates only a single pickable point at the current raster position, which is typically at the lower left-hand corner of the text. If your editor needs to manipulate individual characters within a text string, some other picking mechanism must be used. You could draw little rectangles around each character during picking mode, but it's almost certainly easier to handle text as a special case.

If you decide to use OpenGL picking, organize your program and its data structures so that it's easy to draw appropriate lists of objects in either selection or normal drawing mode. This way, when the user picks something, you can use the same data structures for the pick operation that you use to display the items on the screen. Also, consider whether you want to allow the user to select multiple objects. One way to do this is to store a bit for each item indicating whether it's selected, but with this method, you have to traverse your entire list of items to find the selected items. You might find it useful to maintain a list of pointers to selected items to speed up this search. It's probably a good idea to keep the selection bit for each item as well, since when you're drawing the entire picture, you might want to draw selected items differently (for example, in a different color or with a selection box around them). Finally, consider the selection user interface. You might want to allow the user to do the following:

• Select an item
  
  

• Sweep-select a group of items (see the next paragraphs for a description of this behavior)
  
  

• Add an item to the selection
  
  

• Add a sweep selection to the current selections
  
  

• Delete an item from a selection
  
  

• Choose a single item from a group of overlapping items

A typical solution for a two-dimensional drawing program might work as follows.

1. All selection is done by pointing with the mouse cursor and using the left mouse button. In what follows, cursor means the cursor tied to the mouse, and button means the left mouse button.
   
   

2. Clicking on an item selects it and deselects all other currently selected items. If the cursor is on top of multiple items, the smallest is selected. (In three dimensions, many other strategies work to disambiguate a selection.)
   
   

3. Clicking down where there is no item, holding the button down while dragging the cursor, and then releasing the button selects all the items in a screen-aligned rectangle whose corners are determined by the cursor positions when the button went down and where it came up. This is called a sweep selection. All items not in the swept-out region are deselected. (You must decide whether an item is selected only if it's completely within the sweep region, or if any part of it falls within the region. The completely within strategy usually works best.)
   
   

4. If the Shift key is held down and the user clicks on an item that isn't currently selected, that item is added to the selected list. If the clicked-upon item is selected, it's deleted from the selection list.
   
   

5. If a sweep selection is performed with the Shift key pressed, the items swept out are added to the current selection.
   
   

6. In an extremely cluttered region, it's often hard to do a sweep selection. When the button goes down, the cursor might lie on top of some item, and normally, that item would be selected. You can make any operation a sweep selection, but a typical user interface interprets a button-down on an item plus a mouse motion as a select-plus-drag operation. To solve this problem, you can have an enforced sweep selection by holding down, say, the Alt key. With this, the following set of operations constitutes a sweep selection: Alt-button down, sweep, button up. Items under the cursor when the button goes down are ignored.
   
   

7. If the Shift key is held during this sweep selection, the items enclosed in the sweep region are added to the current selection.
   
   

8. Finally, if the user clicks on multiple items, select just one of them. If the cursor isn't moved (or maybe not moved more than a pixel), and the user clicks again in the same place, deselect the item originally selected, and select a different item under the cursor. Use repeated clicks at the same point to cycle through all the possibilities.

Different rules can apply in particular situations. In a text editor, you probably don't have to worry about characters on top of each other, and selections of multiple characters are always contiguous characters in the document. Thus, you need to mark only the first and last selected characters to identify the complete selection. With text, often the best way to handle selection is to identify the positions between characters rather than the characters themselves. This allows you to have an empty selection when the beginning and end of the selection are between the same pair of characters, and to put the cursor before the first character in the document or after the final one with no special-case code.

In three-dimensional editors, you might provide ways to rotate and zoom between selections, so sophisticated schemes for cycling through the possible selections might be unnecessary. On the other hand, selection in three dimensions is difficult because the cursor's position on the screen usually gives no indication of its depth.

OpenGL Programming Guide