Display Lists & Text

1. Display Lists & Text Glenn G. ChappellCHAPPELLG@member.ams.org U. of Alaska Fairbanks CS 381 Lecture Notes Monday, September 22, 2003

2. Review:Client-Server Model • Many operations in a computing environment, especially in a networked environment, are provided via a client-server model. • The server sits and waits for requests. • A client can request the server to provide services. • This model works well in a CG context. • Client: Application program. • Server: Graphics hardware (or low-level software). • The model affects optimization. • Assumption: client-server communication may be slow. CS 381

3. Display Lists:Motivation • Many factors can slow down OpenGL rendering. • Scenes modeled in real time may require large computations. • *Function-call overhead. • *Slow client-server communication. • Slow rendering hardware. • In order to mitigate some of these effects (marked with *), OpenGL provides “display lists”. • In OpenGL-speak, these are often called “call lists”. CS 381

4. Display Lists:What Are They? • A display list (or call list) is a way for an OpenGL implementation to store a sequence of OpenGL commands for later playback. • Application programmers need no knowledge of how display lists are stored. You simply tell OpenGL: Store these commands as a display list. • A display list can be compiled once and executed many times. • Display lists always reduce function-call overhead. • Since display lists may be stored on the graphics hardware, they may reduce client-server communication delays. CS 381

5. Display Lists:Usage Overview • We compile a display list in four steps: • Allocate a “name” (actually an integer) for the list. (glGenLists). • Tell OpenGL that we are creating a display list (glNewList). • Make the OpenGL function calls that we want to store in the display list. • Tell OpenGL that we are finished creating the list (glEndList). • We can later execute the commands in the display list. • We use a display list by calling it (glCallList). • This tells OpenGL to execute all the commands stored in the list, just as if we made all those calls again, but generally a bit faster. CS 381

6. Display Lists:Usage Details — Naming • An OpenGL display-list “name” is just a number. • But that number must be assigned by OpenGL. • Get a display-list name with glGenLists. • 1 parameter: # of list names to generate. • Return value: 1st list name generated, or 0 if error. • Put the name into a global int. • Check for errors! int my_list; // global // The following goes in function init, maybe? my_list = glGenLists(1); // Give me 1 list name if (my_list == 0) { cerr << "Could not make display-list" << endl; exit(1); // Error } CS 381

7. Display Lists:Usage Details — Compilation • We compile a display list using glNewList. 2 parameters: • The list name. • Either GL_COMPILE or GL_COMPILE_AND_EXECUTE. • The latter is useful when making a display list in the display function. • Then give the OpenGL commands to be stored in the list. • Last, glEndList (no parameters). glNewList(my_list, GL_COMPILE); // Lots of OpenGL commands here // Indentation helps, as with glBegin/glEnd glEndList(); • You can do glBegin…glEnd inside the above, but do not do the above inside glBegin…glEnd. CS 381

8. Display Lists:Usage Details — Execution • We execute a display list using glCallList. • 1 parameter: The list name. // The following goes in the display function! glCallList(my_list); • There is also glCallLists, for executing multiple display lists. CS 381

9. EXAMPLE 1 • Modify intro2d.cpp to use a display list. • Modified the program so that the color was set and the square was drawn using a display list. • The display list was created in function init. • The display function was reduced to 3 lines: • Clear the viewport. • Execute the display list. • Flush. CS 381

10. Text:Introduction • In CG, text comes in two varieties: • Raster/bitmap fonts. • Text described as an image. • Outline/stroke fonts. • Text described as a sequence of curves. • Both are available, in a limited form, in GLUT. • We will discuss bitmap text here. CS 381

11. Text:Raster Position & Bitmaps [1/2] • When we make bitmap text, we are telling OpenGL to draw a raster image. • A raster image is drawn with the left-hand point on its baseline at the current raster position. • The OpenGL raster position is specified with glRasterPos*. • Coordinates passed via glRasterPos* pass through the same transformations as those passed via glVertex*. • However, glRasterPos* is not called between glBegin & glEnd. CS 381

12. Text:Raster Position & Bitmaps [2/2] • Bitmap images are raster images with 1 bit per pixel. • They are intended specifically for rendering bitmap text. • OpenGL draws bitmap images with the primitive glBitmap. • This command includes an amount to move the raster position by. • We will use GLUT commands, not glBitmap, but it is important to remember that glBitmap is what GLUT calls. CS 381

13. Text:Using GLUT Bitmap Fonts • We draw a character at the current raster position using glutBitmapCharacter. 2 parameters: • The GLUT font to use. • I use GLUT_BITMAP_9_BY_15. See doc’s for others. • The character to render. • The raster position is advanced by an appropriate amount. CS 381

14. EXAMPLE 2 • See the program bmptext.cpp, which draws bitmap text using GLUT. • Source code is on the web page. CS 381