當(dāng)Direct3D渲染一個(gè)圖元時(shí)，必須將它通過(guò)坐標(biāo)變換映射到二維屏幕上。如果圖元有紋理，Direct3D就需要用紋理來(lái)產(chǎn)生圖元的二維渲染圖像上每個(gè)像素的顏色。對(duì)于圖元在二維屏幕上圖像的每個(gè)像素來(lái)說(shuō)，都必須從紋理中獲得一個(gè)顏色，從紋理中為每個(gè)像素獲取顏色的過(guò)程稱(chēng)為紋理過(guò)濾（texture filtering）。

大多數(shù)情況下，屏幕顯示的圖形大小與紋理貼圖大小不相同，換句話(huà)說(shuō)，這個(gè)紋理將被映射到一個(gè)比它大或小的圖元的圖像上，這樣紋理常常會(huì)被放大或縮小。對(duì)紋理的放大會(huì)造成許多像素被映射到同一個(gè)紋理元素，圖形渲染結(jié)果會(huì)有色塊的感覺(jué)。縮小一個(gè)紋理意味著一個(gè)像素被映射到許多紋理元素，圖形看上去會(huì)閃爍失真或有鋸齒。為了解決這些問(wèn)題，可以將相關(guān)紋理元素的顏色融合到一個(gè)像素上，如何將多個(gè)紋理元素的顏色融合到一個(gè)像素上取決于紋理過(guò)濾方式。

Direct3D支持4種紋理過(guò)濾方式，分別是：最近點(diǎn)采樣（nearest-point sampling）、線(xiàn)性紋理過(guò)濾（linear texture filtering）、各向異性紋理過(guò)濾（anisotropic texture filtering）和多級(jí)漸進(jìn)紋理過(guò)濾（texture filtering with mipmaps）。不同的紋理過(guò)濾方式產(chǎn)生的圖像效果差別可能很大。

4種紋理過(guò)濾方法各有優(yōu)缺點(diǎn)。例如，線(xiàn)性過(guò)濾生成的圖像較粗糙，但計(jì)算量小。多級(jí)漸進(jìn)紋理過(guò)濾的效果通常最好，特別是和各項(xiàng)異性過(guò)濾聯(lián)合使用時(shí)效果更好，但是它需要Direct3D提供的內(nèi)存空間最多，計(jì)算量也最大。

紋理采樣屬性設(shè)置函數(shù)IDirect3DDevice9::SetSamplerState()可以用來(lái)設(shè)置紋理過(guò)濾方式。該函數(shù)的聲明如下：

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.

參數(shù)Type屬于D3DSAMPLERSTATETYPE類(lèi)型，用來(lái)指定對(duì)哪種紋理采樣屬性賦值。

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用來(lái)控制紋理過(guò)濾方式。

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.