Contents / Previous / Next

Resident Textures

Some OpenGL implementations support a working set of high-performance textures, which are said to be resident. Typically, these implementations have specialized hardware to perform texture operations and a limited hardware cache to store texture images. In this case, using texture objects is recommended, because you are able to load many textures into the working set and then control them.

If all the textures required by the application exceed the size of the cache, some textures cannot be resident. If you want to find out if a single texture is currently resident, bind its object, and then use glGetTexParameter*v() to find out the value associated with the GL_TEXTURE_RESIDENT state. If you want to know about the texture residence status of many textures, use glAreTexturesResident(). 

GLboolean glAreTexturesResident(GLsizei n, const
       GLuint*textureNames, GLboolean*residences);
Queries the texture residence status of the n texture objects, named in the array textureNames. residences is an array in which texture residence status is returned for the corresponding texture objects in the array textureNames. If all the named textures in textureNames are resident, the glAreTexturesResident() function returns GL_TRUE, and the contents of the array residences are undisturbed. If any texture in textureNames is not resident, then glAreTexturesResident() returns GL_FALSE and the elements in residences, which correspond to nonresident texture objects in textureNames, are also set to GL_FALSE.

Note that glAreTexturesResident() returns the current residence status. Texture resources are very dynamic, and texture residence status may change at any time. Some implementations cache textures when they are first used. It may be necessary to draw with the texture before checking residency.

If your OpenGL implementation does not establish a working set of high-performance textures, then the texture objects are always considered resident. In that case, glAreTexturesResident() always returns GL_TRUE and basically provides no information.

Texture Residence Strategies

If you can create a working set of textures and want to get the best texture performance possible, you really have to know the specifics of your implementation and application. For example, with a visual simulation or video game, you have to maintain performance in all situations. In that case, you should never access a nonresident texture. For these applications, you want to load up all your textures upon initialization and make them all resident. If you don't have enough texture memory available, you may need to reduce the size, resolution, and levels of mipmaps for your texture images, or you may use glTexSubImage*() to repeatedly reuse the same texture memory.

For applications that create textures "on the fly," nonresident textures may be unavoidable. If some textures are used more frequently than others, you may assign a higher priority to those texture objects to increase their likelihood of being resident. Deleting texture objects also frees up space. Short of that, assigning a lower priority to a texture object may make it first in line for being moved out of the working set, as resources dwindle. glPrioritizeTextures() is used to assign priorities to texture objects. 

void glPrioritizeTextures(GLsizei n, const GLuint *textureNames,
     const GLclampf *priorities);
Assigns the n texture objects, named in the array textureNames, the texture residence priorities in the corresponding elements of the array priorities. The priority values in the array priorities are clamped to the range [0.0, 1.0] before being assigned. Zero indicates the lowest priority; these textures are least likely to be resident. One indicates the highest priority.

glPrioritizeTextures() does not require that any of the textures in textureNames be bound. However, the priority might not have any effect on a texture object until it is initially bound.

glTexParameter*() also may be used to set a single texture's priority, but only if the texture is currently bound. In fact, use of glTexParameter*() is the only way to set the priority of a default texture.

If texture objects have equal priority, typical implementations of OpenGL apply a least recently used (LRU) strategy to decide which texture objects to move out of the working set. If you know that your OpenGL implementation has this behavior, then having equal priorities for all texture objects creates a reasonable LRU system for reallocating texture resources.