MAE152 Computer Graphics for Scientists and Engineers Fall 03 Display Lists

1. MAE152 Computer Graphics for Scientists and Engineers Fall 03 Display Lists

2. Objective • Understand how display lists can be used along with commands in immediate mode to organize your data and improve performance. • Maximize performance by knowing how and when to use display lists.

3. What is Display List • Display list is a group of OpenGL commands that have been stored for later execution. • When a display list is invoked, the commands in it are executed in the order in which they were issued. • Immediate mode • commands are executed immediately • You can freely mix immediate mode programming and display lists.

4. Immediate Mode Per Vertex Operations & Primitive Assembly Polynomial Evaluator DisplayList Per Fragment Operations Frame Buffer CPU Rasterization Display Listed Texture Memory Pixel Operations Immediate Mode versus Display Lists

5. Immediate Mode vs Display Listed Rendering • Immediate Mode Graphics • Primitives are sent to pipeline and displayed right away • No memory of graphical entities • Display Listed Graphics • Primitives placed in display lists • Display lists kept on graphics server • Can be redisplayed with different state • Can be shared among OpenGL graphics contexts

6. Why Use Display Lists? • It can reduce OpenGL command transmission • When running OpenGL programs remotely to another machine on the network. Display lists are stored in server computer. You can reduce the cost of repeatedly transmitting commands over the network. • Performance Gain in Local Running • Some graphics hardware may store display lists in dedicated memory or may store the data in an optimized form that is more compatible with the graphics hardware or software.

7. A Simple Example void init(void) { GLuint theTorus = glGenLists(1); glNewList(theTorus, GL_COMPILE); torus(8, 25) glEndList(); } void display(void) { glCallList(theTorus); glFlush(); }

8. A Simple Example (cont) void torus(int numc, int numt) { for(i=0; i<numc; i++) { glBegin(GL_QUAD_STRIP); for(j=0; j<=numt; j++) { for(k=1; k>=0; k--) { x = (1+.1*cos(s(twopi/umc))*cos(t*twopi/numt); y = (1+.1*cos(s(twopi/umc))*sin(t*twopi/numt); z = .1 * sin(s*twopi/numc); } } glEnd(); } }

9. Demo • Display List for Triangles

10. Display-List Design Philosophy • Display list can not be changed. • this is to MAXIMIZE the performance gain. • Display list stores only the final values. • does NOT store the intermediate results • Ex) glRotate*() directly stores the matrix • Ex) when the material properties are changed for each item • Ex) when dealing with textures

11. Naming a Display List • Each display list is identified by an integer index. • To allocate one or more unused index, use glGenLists() command. • listIndex = glGenLists(n); allocates n consecutive previously unallocated display-list indices, and returns the first index.

12. Creating a Display List • Use glNewList() and glEndList() glNewList(listIndex, GL_COMPILE); ... glEndList(); • Options for glNewList() • GL_COMPILE_AND_EXECUTE • GL_COMPILE

13. What’s Stored in a Display List • Almost all OpenGL commands can be stored in a display list. • Only the values for expressions are stored in the list. GLfloat v[3] = {0, 0, 0}; glNewList(1, GL_COMPILE); glColor3fv(v); glEndList(); v[0] = 1.0; // no effect

14. Executing a Display List • Use glCallList() command to execute commands in a display list. void display(void { glCallList(theTorus); glFlush(); }

15. Display Lists and Hierarchy • Consider model of a car • Create display list for chassis • Create display list for wheel glNewList( CAR, GL_COMPILE ); glCallList( CHASSIS ); glTranslatef( … ); glCallList( WHEEL ); glTranslatef( … ); glCallList( WHEEL ); … glEndList();

16. Hierarchical Display List • Hierarchical display list is a display list that includes other display lists in it. • Limit on the nesting = 64 glNewList(listIndex, GL_COMPILE); glCallList(frame); glTranslatef(1.0, 1.0, 0.0); glCallList(wheel); glEndList();

17. Managing Display List Indices • Use glIsList() command to determine whether a specific index is in use. GLboolean glIsList(GLuint list) • Use glDeleteLists() command to delete a contiguous range of display lists. glDeleteLists(list, n) deletes n display lists, starting at the index specified by list.

18. Executing Multiple Display Lists • Use glListBase() command to set base index of glCallLists() glListBase(base) • Use glCallLists() command to execute multiple display lists. glCallLists(n, type, lists) executes n display lists. See the next example.

19. An Example void display() { glListBase(10); GL_INT lists[] = { 1, 1, 3, 5 }; glCallLists(4, GL_INT, lists); } // Executed display lists are of // index 11, 12, 15, 20

20. Managing State Variables How to include state changes in display lists? • You can not use glGet*() in a display list. • glGetFloatv(GL_CURRENT_COLOR, &v[0]) • Instead, use glPushAttrib() command to save a group of state variables. • Use glPopAttrib() to restore the values when you’re ready for them.

21. Managing State Variables glNewList(listIndex, GL_COMPILE); // Example code. glPushMatrix(); // Save current matrix glPushAttrib(); // Save current attribute glColor3f(1.0, 0.0, 0.0); // Attribute change glBegin(GL_POLYGON); glVertex2f(0.0, 0.0); glVertex2f(1.0, 0.0); glVertex2f(0.0, 1.0); glEnd(); glTranslatef(1.5, 0.0, 0.0); // Matrix change glPopAttrib(); // Restore attribute glPopMatrix(); // Restore matrix glEndList();

22. End of Display List