OpenGL.GL.APPLE.client_storage

OpenGL extension APPLE.client_storage

This module customises the behaviour of the OpenGL.raw.GL.APPLE.client_storage to provide a more Python-friendly API

Overview (from the spec)

This extension provides a simple mechanism to optimize texture data handling by clients. GL implementations normally maintain a copy of texture image data supplied clients when any of the various texturing commands, such as TexImage2D, are invoked. This extension eliminates GL's internal copy of the texture image data and allows a client to maintain this data locally for textures when the UNPACK_CLIENT_STORAGE_APPLE pixel storage parameter is TRUE at the time of texture specification. Local texture data storage is especially useful in cases where clients maintain internal copies of textures used in any case. This results in what could be considered an extra copy of the texture image data. Assuming all operations are error free, the use of client storage has no affect on the result of texturing operations and will not affect rendering results. APPLE_client_storage allows clients to optimize memory requirements and copy operations it also requires adherence to specific rules in maintaining texture image data.

Clients using this extension are agreeing to preserve a texture's image data for the life of the texture. The life of the texture is defined, in this case, as the time from first issuing the TexImage3D, TexImage2D or TexImage1D command, for the specific texture object with the UNPACK_CLIENT_STORAGE_APPLE pixel storage parameter set to TRUE, until the DeleteTextures command or another TexImage command for that same object. Only after DeleteTextures has completed, or new texture is specified, can the local texture memory be released, as it will no longer be utilized by OpenGL. Changing the UNPACK_CLIENT_STORAGE_APPLE pixel storage parameter will have no additional effect once the texturing command has been issued and specifically will not alleviate the client from maintaining the texture data.

Client storage is implemented as a pixel storage parameter which affects texture image storage at the time the texturing command is issued. As with other pixel storage parameters this state may differ from the time the texturing command in executed if the command is placed in a display list. The PixelStore command is used to set the parameter UNPACK_CLIENT_STORAGE_APPLE. Values can either be TRUE or FALSE, with TRUE representing the use of client local storage and FALSE indicating the OpenGL engine and not the client will be responsible for maintaining texture storage for future texturing commands issued per the OpenGL specification. The default state for the UNPACK_CLIENT_STORAGE_APPLE parameter is FALSE

Client storage is only available for texture objects and not the default texture (of any target type). This means that a texture object has to generated and bound to be used with client storage. Setting UNPACK_CLIENT_STORAGE_APPLE to TRUE and texturing with the default texture will result in normally texturing with GL maintaining a copy of the texture image data.

Normally, client storage will be used in conjunction with normal texturing techniques. An application would use GenTextures to generate texture objects as needed. BindTexture to the texture object name of interest. Enable client storage via the PixelStore command setting the UNPACK_CLIENT_STORAGE_APPLE parameter to TRUE. Then use TexImage3D, TexImage2D or TexImage1D to specify the texture image. If no further use of client storage is desired, it is recommended to again use the PixelStore command, in this case setting the UNPACK_CLIENT_STORAGE_APPLE parameter to FALSE to disable client storage, since this pixel state is maintained unless explicitly set by the PixelStore command.

If an application needs to modify the texture, using TexSubImage for example, it should be noted that the pointer passed to TexSubImage1D, TexSubImage2D or TexSubImage3D does not have to the same, or within the original texture memory. It if is not, there is the likelihood of GL copying the new data to the original texture memory owned by the client, thus actually modifying this texture image data. This does not affect requirement to maintain the original texture memory but also does not add the requirement to maintain the sub image data, due to the copy.

Once a client has completed use of the texture stored in client memory, it should issue a DeleteTextures command to delete the texture object or issue a texture command, with the same target type, for the object, with either a different data pointer, or UNPACK_CLIENT_STORAGE_APPLE set to false, in any case, breaking the tie between GL and the texture buffer. An implicit Flush command is issued in these cases, ensuring all access to the texture by OpenGL is complete. Only at this point can the texture buffer be safely released. Releasing the texture buffer prior has undefined results and will very possibly display texel anomalies at run time. System level memory management and paging schemes should not affect the use of client storage. Consider in any case, that GL has an alias of the base pointer for this block of texture memory which is maintained until GL is finished rendering with the texture and it has been deleted or reassigned to another set of texture data. As long as this alias exists, applications must not de-allocate, move or purge this memory.

The official definition of this extension is available here: http://www.opengl.org/registry/specs/APPLE/client_storage.txt