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By using a "retained mode" approach, client calls do not directly cause actual rendering, but instead update an internal model (typically a list of objects) which is maintained within the library's data space. This allows the library to optimize when actual rendering takes place along with the processing of related objects.
Some techniques to optimize rendering include:
- managing double buffering
- performing occlusion culling
- only transferring data that has changed from one frame to the next from the application to the library
Immediate mode is an alternative approach; the two styles can coexist in the same library and are not necessarily exclusionary in practice. For example, OpenGL has immediate mode functions that can use previously defined server side objects (textures, vertex and index buffers, shaders, etc.) without resending unchanged data.