9. Display Lists. Display lists optimize performance by caching and saving end results, thus avoiding many intermediate steps for

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1 9. Display Lists OpenGL commands are normally executed in immediate mode A display list is a group of OpenGL commands that have been stored for later execution Display lists are used to - Cache commands to redraw the same geometry multiple times - Manage state variables - Encapsulate mode changes - Store any series of OpenGL commands which will be repeatedly executed Display lists optimize performance by caching and saving end results, thus avoiding many intermediate steps for - Matrix Operations - Raster bitmaps and images - Lights, material properties and lighting models - Polygon stipple patterns Display lists can not be modified (must reconstruct) E.R. Bachmann & P.L. McDowell MV 4202 Page 1 of 9

2 Naming a Display List Each display list must be identified by a unique integer identifier A series of unused identifiers can be found using GLuint glgenlists( GLsizei range ); - range is the number of consecutive identifiers desired - The return value is the first of the consecutive identifiers - Each of the lists associated with the identifiers is marked as empty and used - A return value of zero indicates the requested number of identifiers are not available Example 9.1 Allocating three display lists GLuint listone, listtwo, listthree; listone = glgenlists( 3 ); listtwo = listone + 1; listthree = listtwo + 1; E.R. Bachmann & P.L. McDowell MV 4202 Page 2 of 9

3 Creating a Display List Display lists are created between glnewlist / glendlist pairs - Memory and the values of any necessary variables are allocated for OpenGL commands which appear in a list Construction is begun with void glnewlist( GLuint list, GLenum mode ); - list specifies the identifier of the list being created - mode specifies how the commands are to be executed: GL_COMPILE_AND_EXECUTEexecute the commands immediately and save them in the display list for later execution GL_COMPILE save the commands for later execution, do not execute them immediately The display list is ended with void glendlist( void ); E.R. Bachmann & P.L. McDowell MV 4202 Page 3 of 9

4 Some OpenGL commands cannot be stored in a display list; for example: glcolorpointer() gldeletelists() gldisableclientstate() glflush() glgenlists() glget*() glnormalpointer() glpixelstore() glreadpixels() - Commands that set client state and commands that retrieve state values are executed immediately - Calling glnewlist within a glnewlist / glendlist pair will generate an error E.R. Bachmann & P.L. McDowell MV 4202 Page 4 of 9

5 Example 9.2 Creating display lists BOOL COpenGLView::PreRenderScene( void ) { floorlist = glgenlists( 3 ); cubelist = floorlist + 1; balllist = cubelist + 1; glnewlist( floorlist, GL_COMPILE ); drawfloor( 0.0, -3.0, 0.0, 8.0 ); glnewlist( cubelist, GL_COMPILE ); drawcube( 0.0, 0.0, 0.0, 1.0 ); glnewlist( balllist, GL_COMPILE ); drawredball( 0.0, 0.0, 0.0, 1.0 ); return TRUE; } // end PreRenderScene Executing a Display List Once a display list has been created it is executed using void glcalllist( GLuint list ); - list specifies the list to be executed E.R. Bachmann & P.L. McDowell MV 4202 Page 5 of 9

6 Example 9.3 Executing a display list glcalllist( floorlist ); glpushmatrix(); glrotatef( 5.0f, 1.0f, 0.0f, 0.0f ); glcalllist( cubelist ); glpopmatrix(); glpushmatrix(); gltranslatef( 3.0f, 0.0f, 0.0f ); glrotatef( 5.0f, 0.0f, 1.0f, 0.0f ); glcalllist( cubelist ); glpopmatrix(); glpushmatrix(); gltranslatef( -3.0f, 0.0f, 0.0f ); glrotatef( 5.0f, 0.0f, 0.0f, 1.0f ); glcalllist( balllist ); glpopmatrix(); glflush(); // Flush all drawing commands SwapBuffers(); // swap buffers Managing Display Lists A specific display list or a contiguous range of lists can be deleted using void gldeletelist( GLuint list, GLsizei range ); - list specifies the identifier of the first list to be deleted E.R. Bachmann & P.L. McDowell MV 4202 Page 6 of 9

7 - range specifies the number of contiguous lists to be deleted Executing Multiple Display Lists Often there is a requirement to efficiently execute several display lists in succession - Commonly used when working with fonts in which the display list for each character corresponds to the ASCII value of that character n lists contained in an array can be executed using void glcalllists( GLsizei n, GLenum type, const GLvoid *lists ); - n is the number of lists to be executed - type specifies the data type of the values in lists and is normally GL_BYTE or GL_UNSIGNED_INT - list specifies the relative indices of the lists to be executed May be a GLuint array May be a character array in which the ASCII values of the characters are to be interpreted as indices When using glcalllists, the indices of the lists to be executed are computed by adding the current displaylist base to the signed integer values in list E.R. Bachmann & P.L. McDowell MV 4202 Page 7 of 9

8 The current display list base is set using void gllistbase( GLuint base ); - base specifies the offset that is to be added to the display list indices given by glcalllists() - The default display list base is 0 Example 9.4 Creating multiple lists GLuint base; base = glgenlists( 128 ); glnewlist( base+'a', GL_COMPILE ); drawletter( Adata ); glnewlist( base+'e', GL_COMPILE ); drawletter( Edata ); glnewlist( base+'p', GL_COMPILE ); drawletter( Pdata ); glnewlist( base+'r', GL_COMPILE ); drawletter( Rdata ); glnewlist( base+'s', GL_COMPILE ); drawletter( Sdata ); E.R. Bachmann & P.L. McDowell MV 4202 Page 8 of 9

9 Example 9.5 Executing multiple display lists char word1[] = PEPPERS ; GLsizei length = (GLsizei)strlen( word1 ); gllistbase( base ); glcalllists( length, GL_BYTE, (GLbyte *)word1 ); Hierarchical Display Lists Display lists may be called from within a display list by calling glcalllist between a glnewlist / glendlist pair - Useful in creating objects made from components - The called display list must already be created Sharing Display Lists Between Threads Normally each RC has a separate address space for display lists and texture objects wglsharelists allows different threads of execution to share display lists E.R. Bachmann & P.L. McDowell MV 4202 Page 9 of 9