Efficient drawing with OpenGL

High Performance Computing Techniques in 3d Graphics
Instructor: Pourya Shirazian

How to choose best method when drawing
geometries in OpenGL?
Available methods
Costs and Complexity
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Immediate Mode
Display Lists
Vertex Arrays (Client Memory)
Vertex Buffer Objects (VBO) (Server Memory)
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Primitives being drawn as they are defined:
glBegin(GL_TRIANGLES);
glVertex3fv(v0);
glVertex3fv(v1);
glVertex3fv(v2);
glEnd();
V0
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V1
V2

V4
V7
Shared vertices should be repeated
V3
V0
V5
V2
V1
Each Vertex is shared between 3 quads/
glBegin(GL_QUADS);
glVertex3fv(v0); glVertex3fv(v1);
glVertex3fv(v2); glVertex3fv(v3);
glVertex3fv(v0); glVertex3fv(v3);
glVertex3fv(v7); glVertex3fv(v4);
glVertex3fv(v0); glVertex3fv(v4);
glVertex3fv(v5); glVertex3fv(v1);
// draw other 3 faces
…
glEnd();
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A List of commands stored for later execution
Lists reside on server (static object)
Client Server
index = glGenLists(1);
glNewList(index, GL_COMPILE);
glBegin(GL_TRIANGLES);
glVertex3fv(v0);
glVertex3fv(v1);
glVertex3fv(v2);
glEnd();
glEndList();
List 1
List 2
List n
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Not all OpenGL commands can be used:
No Client Commands:
No result returnable funcs:
glFlush
glFinish
glRenderMode
glEnableClientState
glVertexPointer
glIsEnabled
glGet
glReadPixels
glFeedbackBuffer
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Build Once, Draw Many Times
List 1
List 2
List 3
glCallList(1);
glListBase(index);
glCallLists(3, GL_INT, {1,2,3});
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V0
V1
V2

Benefits:
▪ Lists are on the server side no data transfer overhead
Disadvantages:
▪ Once a list is compiled its data cannot be modified
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glEnableClientState, glDisableClientState
Are used to enable and disable 6 type of arrays:
1. Vertices glVertexPointer
2. Normals glNormalPointer
3. Colors glColorPointer
4. Indices glIndexPointer
5. Textures glTexCoordPointer
6. EdgeFlags glEdgeFlagPointer
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All objects are on the client application
memory and the server will be given access.
EdgeFlags: used to show if a vertex is on the
boundary edge or not.
Only edges with set edgeflag will be visible
with glPolygonMode(GL_LINES);
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Draw Mesh:
1. glColorPointer(3, GL_FLOAT, 0, mesh.vColor);
2. glEnableClientState(GL_COLOR_ARRAY);
3. glNormalPointer(GL_FLOAT, 0, mesh.vNorm);
4. glEnableClientState(GL_NORMAL_ARRAY);
5. glVertexPointer(3, GL_FLOAT, 0, mesh.vPos);
6. glEnableClientState(GL_VERTEX_ARRAY);
7. glDrawElements(GL_TRIANGLES, (GLsizei)mesh.ctTriangles * 3,
GL_UNSIGNED_SHORT, mesh.triangles);
8. glDisableClientState(GL_COLOR_ARRAY);
9. glDisableClientState(GL_NORMAL_ARRAY);
10. glDisableClientState(GL_VERTEX_ARRAY);
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Benefits:
▪ Reduces number of functions calls for drawing
primitives
▪ Reduces redundant usage of shared vertices
Disadvantages
▪ Functions are called on the client side
▪ Memory should be resent to server upon each call
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Store VERTEX VBO, Store PIXELPBO
Hybrid of Vertex Arrays and DisplayLists
Buffer Objects for vertex attribute in high
performance memory
Buffer Objects are server side
3 Kinds of memory usages: System, AGP,
Video Memory
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Buffer Objects can be mapped to client
memory space for modification
Buffer Objects can be accessed by multiple
clients using identifiers.
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glGenBuffer: Generates a new buffer
▪ void glGenBuffersARB(GLsizei n, GLuint* ids)
▪ Param 1: Number of buffers to generate
▪ Param 2: Array to store buffer ids
glBindBuffer
▪ void glBindBufferARB(GLenum target, GLuint id)
▪ Param 1: target is vertex or index
▪ GL_ARRAY_BUFFER_ARB, GL_ELEMENT_ARRAY_BUFFER_ARB
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glBufferData
▪ void glBufferDataARB(GLenum target, GLsizei size,
const void* data, GLenum usage)
▪ target: GL_ARRAY_BUFFER_ARB ,
GL_ELEMENT_ARRAY_BUFFER_ARB
▪ size: Number of bytes to transfer
▪ data: pointer to buffer data
▪ usage: static, dynamic, stream and read, copy or draw
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GL_STATIC_DRAW_ARB
GL_STATIC_READ_ARB
GL_STATIC_COPY_ARB
GL_DYNAMIC_DRAW_ARB
GL_DYNAMIC_READ_ARB
GL_DYNAMIC_COPY_ARB
GL_STREAM_DRAW_ARB
GL_STREAM_READ_ARB
GL_STREAM_COPY_ARB
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Static/Dynamic Draw/Read/Copy
Static: Does not change Draw: Application GL
VBO
Dynamic: Changes Read: GL Application
PBO
Copy: GL GL
FBO
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Map memory to client address space:
▪ void* glMapBufferARB(GLenum target, GLenum access)
▪ target: GL_ARRAY_BUFFER, GL_ELEMENT_ARRAY_BUFFER
▪ access: READ, WRITE, READ/WRITE
▪ Returns NULL on failure.
Unmap memory after making changes
▪ GLboolean glUnmapBufferARB(GLenum target)
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Benefits:
▪ Unlike display lists memory can be read and mapped on
client address space for modifications.
▪ Memory objects can be shared among many clients
using identifiers.
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