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Locking a Subvolume of a Volume Texture

Last Updated: 2/14/2017

DirectX 8.1 introduces a new feature that lets a driver lock just a subvolume of a volume texture. When a driver's DdLock function is called, the driver can improve system performance by locking just a subvolume instead of the whole volume texture.

To indicate support of this feature, the driver must set the D3DDEVCAPS_SUBVOLUMELOCK bit in the DevCaps member of the D3DCAPS8 structure. The driver returns a D3DCAPS8 structure in response to a GetDriverInfo2 query as described in Reporting DirectX 8.0 Style Direct3D Capabilities. Support of this query is described in Supporting GetDriverInfo2.

After support of this feature is determined, the driver can receive a DdLock call with the DDLOCK_HASVOLUMETEXTUREBOXRECT bit set in the dwFlags member of the passed DD_LOCKDATA structure. This bit informs the driver to lock down the specified subvolume texture. The driver must then obtain the front and back coordinates of the locked subvolume from the left and right members of the RECTL structure that is specified in the rArea member of DD_LOCKDATA. The driver obtains the front and back coordinates from the higher 16 bits of the left and right members respectively.

The left and right coordinates of the locked subvolume are constrained to the lower 16 bits of the left and right members. The driver uses the top and bottom members of the RECTL structure in rArea unchanged to specify the top and bottom coordinates of the locked subvolume. In this way, the rArea member effectively provides three coordinate sets to specify the locked subvolume. The RECTL structure is described in the Microsoft Windows SDK documentation.

The following code shows how to obtain the front and back coordinates:

"real" left = rArea.left && 0xFFFF;
"real" right = rArea.right && 0xFFFF;
front = rArea.left >> 16;
back = rArea.right >> 16;

This feature is available on Windows Me and Windows XP and later versions. This feature is also available on Windows 2000 and Windows 98 operating system versions that have the DirectX 8.1 runtime installed on them.

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