当Direct3D渲染一个图元时，必须将它通过坐标变换映射到二维屏幕上。如果图元有纹理，Direct3D就需要用纹理来产生图元的二维渲染图像上每个像素的颜色。对于图元在二维屏幕上图像的每个像素来说，都必须从纹理中获得一个颜色，从纹理中为每个像素获取颜色的过程称为纹理过滤（texture filtering）。

大多数情况下，屏幕显示的图形大小与纹理贴图大小不相同，换句话说，这个纹理将被映射到一个比它大或小的图元的图像上，这样纹理常常会被放大或缩小。对纹理的放大会造成许多像素被映射到同一个纹理元素，图形渲染结果会有色块的感觉。缩小一个纹理意味着一个像素被映射到许多纹理元素，图形看上去会闪烁失真或有锯齿。为了解决这些问题，可以将相关纹理元素的颜色融合到一个像素上，如何将多个纹理元素的颜色融合到一个像素上取决于纹理过滤方式。

Direct3D支持4种纹理过滤方式，分别是：最近点采样（nearest-point sampling）、线性纹理过滤（linear texture filtering）、各向异性纹理过滤（anisotropic texture filtering）和多级渐进纹理过滤（texture filtering with mipmaps）。不同的纹理过滤方式产生的图像效果差别可能很大。

4种纹理过滤方法各有优缺点。例如，线性过滤生成的图像较粗糙，但计算量小。多级渐进纹理过滤的效果通常最好，特别是和各项异性过滤联合使用时效果更好，但是它需要Direct3D提供的内存空间最多，计算量也最大。

纹理采样属性设置函数IDirect3DDevice9::SetSamplerState()可以用来设置纹理过滤方式。该函数的声明如下：

Sets the sampler state value.

HRESULT SetSamplerState(
  DWORD Sampler,
  D3DSAMPLERSTATETYPE Type,
  DWORD Value
);

Parameters

Sampler

[in] The sampler stage index.

Type

[in] This parameter can be any member of the D3DSAMPLERSTATETYPE enumerated type.

Value

[in] State value to set. The meaning of this value is determined by the Type parameter.

Return Values

If the method succeeds, the return value is D3D_OK. If the method fails, the return value can be D3DERR_INVALIDCALL.

参数Type属于D3DSAMPLERSTATETYPE类型，用来指定对哪种纹理采样属性赋值。

D3DSAMPLERSTATETYPE

Sampler states define texture sampling operations such as texture addressing and texture filtering. Some sampler states set-up vertex processing, and some set-up pixel processing. Sampler states can be saved and restored using stateblocks (see State Blocks Save and Restore State (Direct3D 9)).

typedef enum D3DSAMPLERSTATETYPE
{
    D3DSAMP_ADDRESSU = 1,
    D3DSAMP_ADDRESSV = 2,
    D3DSAMP_ADDRESSW = 3,
    D3DSAMP_BORDERCOLOR = 4,
    D3DSAMP_MAGFILTER = 5,
    D3DSAMP_MINFILTER = 6,
    D3DSAMP_MIPFILTER = 7,
    D3DSAMP_MIPMAPLODBIAS = 8,
    D3DSAMP_MAXMIPLEVEL = 9,
    D3DSAMP_MAXANISOTROPY = 10,
    D3DSAMP_SRGBTEXTURE = 11,
    D3DSAMP_ELEMENTINDEX = 12,
    D3DSAMP_DMAPOFFSET = 13,
    D3DSAMP_FORCE_DWORD = 0x7fffffff,
} D3DSAMPLERSTATETYPE, *LPD3DSAMPLERSTATETYPE;

Constants

D3DSAMP_ADDRESSU

Texture-address mode for the u coordinate. The default is D3DTADDRESS_WRAP. For more information, see D3DTEXTUREADDRESS.

D3DSAMP_ADDRESSV

Texture-address mode for the v coordinate. The default is D3DTADDRESS_WRAP. For more information, see D3DTEXTUREADDRESS.

D3DSAMP_ADDRESSW

Texture-address mode for the w coordinate. The default is D3DTADDRESS_WRAP. For more information, see D3DTEXTUREADDRESS.

D3DSAMP_BORDERCOLOR

Border color or type D3DCOLOR. The default color is 0x00000000.

D3DSAMP_MAGFILTER

Magnification filter of type D3DTEXTUREFILTERTYPE. The default value is D3DTEXF_POINT.

D3DSAMP_MINFILTER

Minification filter of type D3DTEXTUREFILTERTYPE. The default value is D3DTEXF_POINT.

D3DSAMP_MIPFILTER

Mipmap filter to use during minification. See D3DTEXTUREFILTERTYPE. The default value is D3DTEXF_NONE.

D3DSAMP_MIPMAPLODBIAS

Mipmap level-of-detail bias. The default value is zero.

D3DSAMP_MAXMIPLEVEL

level-of-detail index of largest map to use. Values range from 0 to (n - 1) where 0 is the largest. The default value is zero.

D3DSAMP_MAXANISOTROPY

DWORD maximum anisotropy. The default value is 1.

D3DSAMP_SRGBTEXTURE

Gamma correction value. The default value is 0, which means gamma is 1.0 and no correction is required. Otherwise, this value means that the sampler should assume gamma of 2.2 on the content and convert it to linear (gamma 1.0) before presenting it to the pixel shader.

D3DSAMP_ELEMENTINDEX

When a multielement texture is assigned to the sampler, this indicates which element index to use. The default value is 0.

D3DSAMP_DMAPOFFSET

Vertex offset in the presampled displacement map. This is a constant used by the tessellator, its default value is 0.

D3DSAMP_FORCE_DWORD

Forces this enumeration to compile to 32 bits in size. Without this value, some compilers would allow this enumeration to compile to a size other than 32 bits. This value is not used.

其中，D3DSAMP_MAGFILTER、D3DSAMP_MINFILTER、D3DSAMP_MIPFILTER、D3DSAMP_MIPMAPLODBIAS、D3DSAMP_MAXMIPLEVEL和D3DSAMP_MAXANISOTROPY用来控制纹理过滤方式。

D3DTEXTUREFILTERTYPE

Defines texture filtering modes for a texture stage.

typedef enum D3DTEXTUREFILTERTYPE
{
    D3DTEXF_NONE = 0,
    D3DTEXF_POINT = 1,
    D3DTEXF_LINEAR = 2,
    D3DTEXF_ANISOTROPIC = 3,
    D3DTEXF_PYRAMIDALQUAD = 6,
    D3DTEXF_GAUSSIANQUAD = 7,
    D3DTEXF_CONVOLUTIONMONO = 8,
    D3DTEXF_FORCE_DWORD = 0x7fffffff,
} D3DTEXTUREFILTERTYPE, *LPD3DTEXTUREFILTERTYPE;

Constants

D3DTEXF_NONE

Mipmapping disabled. The rasterizer should use the magnification filter instead.

D3DTEXF_POINT

Point filtering used as a texture magnification or minification filter. The texel with coordinates nearest to the desired pixel value is used. The texture filter to be used between mipmap levels is nearest-point mipmap filtering. The rasterizer uses the color from the texel of the nearest mipmap texture.

D3DTEXF_LINEAR

Bilinear interpolation filtering used as a texture magnification or minification filter. A weighted average of a 2 x 2 area of texels surrounding the desired pixel is used. The texture filter to use between mipmap levels is trilinear mipmap interpolation. The rasterizer linearly interpolates pixel color, using the texels of the two nearest mipmap textures.

D3DTEXF_ANISOTROPIC

Anisotropic texture filtering used as a texture magnification or minification filter. Compensates for distortion caused by the difference in angle between the texture polygon and the plane of the screen.

D3DTEXF_PYRAMIDALQUAD

A 4-sample tent filter used as a texture magnification or minification filter.

D3DTEXF_GAUSSIANQUAD

A 4-sample Gaussian filter used as a texture magnification or minification filter.

D3DTEXF_CONVOLUTIONMONO

Convolution filter for monochrome textures. See D3DFMT_A1.

Differences between Direct3D 9 and Direct3D 9Ex: This flag is available in Direct3D 9Ex only.

D3DTEXF_FORCE_DWORD

Forces this enumeration to compile to 32 bits in size. Without this value, some compilers would allow this enumeration to compile to a size other than 32 bits. This value is not used.

Remarks

To check if a format supports texture filter types other than D3DTEXF_POINT (which is always supported), call IDirect3D9::CheckDeviceFormat with D3DUSAGE_QUERY_FILTER.

Set a texture stage's magnification filter by calling IDirect3DDevice9::SetSamplerState with the D3DSAMP_MAGFILTER value as the second parameter and one member of this enumeration as the third parameter.

Set a texture stage's minification filter by calling IDirect3DDevice9::SetSamplerState with the D3DSAMP_MINFILTER value as the second parameter and one member of this enumeration as the third parameter.

Set the texture filter to use between-mipmap levels by calling IDirect3DDevice9::SetSamplerState with the D3DSAMP_MIPFILTER value as the second parameter and one member of this enumeration as the third parameter.

Not all valid filtering modes for a device will apply to volume maps. In general, D3DTEXF_POINT and D3DTEXF_LINEAR magnification filters will be supported for volume maps. If D3DPTEXTURECAPS_MIPVOLUMEMAP is set, then the D3DTEXF_POINT mipmap filter and D3DTEXF_POINT and D3DTEXF_LINEAR minification filters will be supported for volume maps. The device may or may not support the D3DTEXF_LINEAR mipmap filter for volume maps. Devices that support anisotropic filtering for 2D maps do not necessarily support anisotropic filtering for volume maps. However, applications that enable anisotropic filtering will receive the best available filtering (probably linear) if anisotropic filtering is not supported.

Unsigned Formats

Data in an unsigned format must be positive. Unsigned formats use combinations of (R)ed, (G)reen, (B)lue, (A)lpha, (L)uminance, and (P)alette data. Palette data is also referred to as color indexed data because the data is used to index a color palette.