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OpenGL Programming GuideChapter Objectives
After reading this chapter, you'll be able to do the following:
Create applications that allow the user to select a region of the screen or pick an object drawn on the screen
Use OpenGL's feedback mode to obtain the results of rendering calculations
Some graphics applications simply draw static images of two- and three-dimensional objects. Other applications allow the user to identify objects on the screen and then to move, modify, delete, or otherwise manipulate those objects. OpenGL is designed to support exactly such interactive applications. Since objects drawn on the screen typically undergo multiple rotations, translations, and perspective transformations, it can be difficult for you to determine which object a user is selecting in a three-dimensional scene. To help you, OpenGL provides a selection mechanism that automatically tells you which objects are drawn inside a specified region of the window. You can use this mechanism together with a special utility routine to determine which object within the region the user is specifying, or picking, with the cursor.
Selection is actually a mode of operation for OpenGL; feedback is another such mode. In feedback mode, you use your graphics hardware and OpenGL to perform the usual rendering calculations. Instead of using the calculated results to draw an image on the screen, however, OpenGL returns (or feeds back) the drawing information to you. If you want to draw three-dimensional objects on a plotter rather than the screen, for example, you would draw the items in feedback mode, collect the drawing instructions, and then convert them to commands the plotter can understand.
In both selection and feedback modes, drawing information is returned to the application rather than being sent to the framebuffer, as it is in rendering mode. Thus, the screen remains frozen - no drawing occurs - while OpenGL is in selection or feedback mode. This chapter explains each of these modes in its own section:
"Selection" discusses how to use selection mode and related routines to allow a user of your application to pick an object drawn on the screen.
"Feedback" describes how to obtain information about what would be drawn on the screen and how that information is formatted.
OpenGL Programming Guide
