A texture is a rectangular array of colored pixels. (The rectangle does not necessarily have to be square, although the system deals most efficiently with square textures.) You can use textures for texture-mapping faces, in which case their dimensions must be powers of two. If your application uses the RGB color model, you can use 8-, 24-, and 32-bit textures. If you use the monochromatic (or ramp) color model, however, you can use only 8-bit textures.
You can retrieve an interface to a Direct3D texture object by calling the IDirectDrawSurface::QueryInterface method and specifying IID_IDirect3DTexture. An IDirect3DTexture interface is actually an interface to a DirectDrawSurface object, not to a distinct Direct3D texture object. For more information about the relationship between textures in Direct3D and surfaces in DirectDraw, see Direct3D Texture Interface.
The following example demonstrates how to create an IDirect3DTexture interface and then how to load the texture by calling the IDirect3DTexture::GetHandle and IDirect3DTexture::Load methods.
lpDDS->QueryInterface(IID_IDirect3DTexture,
lpD3DTexture); // Address of a DIRECT3DTEXTURE object
lpD3DTexture->GetHandle(
lpD3DDevice, // Address of a DIRECT3DDEVICE object
lphTexture); // Address of a D3DTEXTUREHANDLE
lpD3DTexture->Load(
lpD3DTexture); // Address of a DIRECT3DTEXTURE object
Texture objects reside on the interface list and point both to the next texture in a device's list and back to their associated device or devices. (For more information about this hierarchy, see Object Connectivity.) The texture handle is used in materials and execute buffers, and as a z-buffer for a viewport. You can use the IDirect3DTexture interface to load and unload textures, retrieve handles, and track changes to palettes.
Texture Wrapping
The texture coordinates of each face define the region in the texture that is mapped onto that particular face. Your application can use a wrap to calculate texture coordinates. For more information, see Direct3DRMWrap.
Your application can use the D3DRENDERSTATE_WRAPU and D3DRENDERSTATE_WRAPV render states (from the D3DRENDERSTATETYPE enumerated type) to specify how the rasterizer should interpret texture coordinates. The rasterizer always interpolates the shortest distance between texture coordinates; that is, a line. The path taken by this line, and the valid values for the u- and v-coordinates, varies with the use of the wrapping flags. If either or both flags is set, the line can wrap around the texture edge in the u or v direction, as if the texture had a cylindrical or toroidal topology.
·In flat wrapping mode, in which neither of the wrapping flags is set, the plane specified by the u- and v-coordinates is an infinite tiling of the texture. In this case, values greater than 1.0 are valid for u and v. The shortest line between (0.1, 0.1) and (0.9, 0.9) passes through (0.5, 0.5).
·If either D3DRENDERSTATE_WRAPU or D3DRENDERSTATE_WRAPV is set, the texture is an infinite cylinder with a circumference of 1.0. Texture coordinates greater than 1.0 are valid only in the dimension that is not wrapped. The shortest distance between texture coordinates varies with the wrapping flag; if D3DRENDERSTATE_WRAPU is set, the shortest line between (0.1, 0.1) and (0.9, 0.9) passes through (0, 0.5).
·If both D3DRENDERSTATE_WRAPU and D3DRENDERSTATE_WRAPV are set, the texture is a torus. Because the system is closed, texture coordinates greater that 1.0 are invalid. The shortest line between (0.1, 0.1) and (0.9, 0.9) passes through (0, 0).
Although texture coordinates that are outside the valid range may be truncated to valid values, this behavior is not defined.
Typically, applications set a wrap flag for cylindrical wraps when the intersection of the texture edges does not match the edges of the face, and do not set a wrap flag when more than half of a texture is applied to a single face.
For more information about wrapping, see Wrapping Types in the introduction to Direct3D Retained-Mode objects.
Texture Filtering and Blending
After a texture has been mapped to a surface, the texture elements (texels) of the texture rarely correspond to individual pixels in the final image. A pixel in the final image can correspond to a large collection of texels or to a small piece of a single texel. You can use texture filtering to specify how to interpolate texel values to pixels.
You can use the D3DRENDERSTATE_TEXTUREMAG and D3DRENDERSTATE_TEXTUREMIN render states (from the D3DRENDERSTATETYPE enumerated type) to specify the type of texture filtering to use.
The D3DRENDERSTATE_TEXTUREMAPBLEND render state allows you to specify the type of texture blending. Texture blending combines the colors of the texture with the color of the surface to which the texture is being applied. This can be an effective way to achieve a translucent appearance. Texture blending can produce unexpected colors; the best way to avoid this is to ensure that the color of the material is white. The texture-blending options are specified in the D3DTEXTUREBLEND enumerated type.
You can use the D3DRENDERSTATE_SRCBLEND and D3DRENDERSTATE_DSTBLEND render states to specify how colors in the source and destination are combined. The combination options (called blend factors) are specified in the D3DBLEND enumerated type.
Mipmaps
A mipmap is a sequence of textures, each of which is a progressively lower resolution, prefiltered representation of the same image. Mipmapping is a computationally low-cost way of improving the quality of rendered textures. Each prefiltered image, or level, in the mipmap is a power of two smaller than the previous level.
You can use mipmaps when texture-filtering by specifying the appropriate filter mode in the D3DTEXTUREFILTER enumerated type. To find out what kinds of mipmapping support are provided by a device, use the flags specified in the dwTextureFilterCaps member of the D3DPRIMCAPS structure.
For more information about how to use DirectDraw to create mipmaps, see Mipmaps.
Transparency and Translucency
As already mentioned, one method for achieving the appearance of transparent or translucent textures is by using texture blending. You can also use alpha channels and the D3DRENDERSTATE_BLENDENABLE render state (from the D3DRENDERSTATETYPE enumerated type).
A more straightforward approach to achieving transparency or translucency is to use DirectDraw's support for color keys. Color keys are colors or ranges of colors that can be part of either the source or destination of a blit or overlay operation. You can specify whether these colors should always or never be overwritten.
For more information about DirectDraw's support for color keys, see Color Keying.