光源

光照計(jì)算模型說明圖形系統(tǒng)以什么樣的方法計(jì)算燈光照射在物體上的顏色值，它是一種計(jì)算方法，而不是具體的燈光。光源則定義了三維場景中具體的燈光，包括位置、方向、強(qiáng)度等信息。相同的光源可以根據(jù)物體表面材質(zhì)的不同，通過不同的光照模型顯示。比如，在臥室中的一盞臺燈，照射在布質(zhì)床單上時(shí)，應(yīng)使用漫反射模型；而照射在玻璃表面的桌子上時(shí)，則應(yīng)使用鏡面反射模型計(jì)算光照效果。

光源類型

在Direct3D中支持三種光源類型：方向光（directional light）、點(diǎn)光源（point light）和聚光燈（spot light）。它們在Direct3D中以枚舉類型D3DLIGHTTYPE表示，枚舉類型D3DLIGHTTYPE定義如下：

typedef enum _D3DLIGHTTYPE {
    D3DLIGHT_POINT          = 1,
    D3DLIGHT_SPOT           = 2,
    D3DLIGHT_DIRECTIONAL    = 3,
    D3DLIGHT_FORCE_DWORD    = 0x7fffffff, /* force 32-bit size enum */
} D3DLIGHTTYPE;

Constants

D3DLIGHT_POINT

Light is a point source. The light has a position in space and radiates light in all directions.

D3DLIGHT_SPOT

Light is a spotlight source. This light is like a point light, except that the illumination is limited to a cone. This light type has a direction and several other parameters that determine the shape of the cone it produces. For information about these parameters, see the D3DLIGHT9 structure.

D3DLIGHT_DIRECTIONAL

Light is a directional light source. This is equivalent to using a point light source at an infinite distance.

D3DLIGHT_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

Directional lights are slightly faster than point light sources, but point lights look a little better. Spotlights offer interesting visual effects but are computationally time-consuming.

1、點(diǎn)光源

點(diǎn)光源有顏色和位置，但沒有方向，它向所有方向發(fā)射的光都一樣。

電燈泡是點(diǎn)光源的一個(gè)例子，這種光的強(qiáng)度隨著距離物體的遠(yuǎn)近而衰減，在程序中需要提供點(diǎn)光源的位置、顏色和衰減系數(shù)等參數(shù)。在進(jìn)行光照計(jì)算時(shí)，根據(jù)點(diǎn)光源在世界空間中的位置和物體頂點(diǎn)坐標(biāo)計(jì)算出燈光的方向和燈光傳播的距離（用于計(jì)算燈光的衰減），然后再同頂點(diǎn)法向量一起，根據(jù)燈光和物體表面材質(zhì)屬性計(jì)算物體表面每個(gè)頂點(diǎn)的顏色值。

2、方向光

方向光有顏色和方向，沒有位置。它們發(fā)出的是平行光，這意味著由方向光生成的燈光以同一個(gè)方向照向場景。可以把方向光想象成無窮遠(yuǎn)處的一個(gè)光源，如太陽光發(fā)出的光。方向光不會衰竭，當(dāng)Direct3D用這種光源計(jì)算頂點(diǎn)顏色時(shí)，只考慮指定的方向和顏色。因?yàn)椴恍枰紤]照亮范圍和位置的影響，所以這種光的計(jì)算量最少。有策略的應(yīng)用方向光能夠提高程序的執(zhí)行速度。

3、聚光燈

聚光燈有顏色、位置和方向，類似于現(xiàn)實(shí)世界中的探照燈。聚光燈發(fā)出的光線由一個(gè)明亮的內(nèi)錐體（inner cone）和大一點(diǎn)的外錐體（outer cone）組成。光線強(qiáng)度從內(nèi)椎體到外椎體逐漸衰減，通過聚光燈的falloff、theta和phi這三個(gè)屬性共同來控制其衰減規(guī)律，其中theta為內(nèi)錐角，phi為外錐角。

一個(gè)頂點(diǎn)受到的聚光燈光照總量以該頂點(diǎn)在內(nèi)椎體或外椎體中的位置為基礎(chǔ)。Direct3D計(jì)算聚光燈方向向量L與從光源到頂點(diǎn)的向量D的點(diǎn)積，這個(gè)點(diǎn)積的值等于兩個(gè)向量夾角alpha的余弦。Direct3D將該值與角度theta和phi的余弦作比較。如果向量的點(diǎn)積小于或等于外椎體的余弦，該頂點(diǎn)超出了外椎體，不受光照；若這個(gè)點(diǎn)積大于內(nèi)椎體角的余弦，則該頂點(diǎn)在內(nèi)椎體內(nèi)，受到的光照總量較多，仍需考慮在距離上的衰減。如果頂點(diǎn)在兩個(gè)區(qū)域之間，用下面的公式計(jì)算衰減后的光照系數(shù)。

I_f = ((cos(alpha) - cos(phi/2)) / (cos(theta/2) - cos(phi/2)))^p

其中，I_f為衰減后的光強(qiáng)系數(shù)，alpha表示向量L和D間的夾角，theta表示內(nèi)錐角，phi表示外錐角，p表示衰減系數(shù)falloff。

該計(jì)算公式得到的I_f值在0.0~1.0之間，用來縮放光線強(qiáng)度以實(shí)現(xiàn)衰減。當(dāng)然還要考慮從具體每個(gè)頂點(diǎn)到光源的距離衰減（attenuation）。在Direct3D中，聚光燈是所有光源類型中最耗時(shí)的光源，因此在系統(tǒng)性能不高的情況下應(yīng)該謹(jǐn)慎采用聚光燈。

光源屬性

三維場景的一個(gè)光源通過其各種屬性來描述，光源屬性包括光的發(fā)光位置、方向、顏色和衰減范圍等，這些屬性可控制光線對物體的照明程度，各種類型光源的屬性由結(jié)構(gòu)D3DLIGHT9定義。

Defines a set of lighting properties.

typedef struct D3DLIGHT9 {
    D3DLIGHTTYPE Type;
    D3DCOLORVALUE Diffuse;
    D3DCOLORVALUE Specular;
    D3DCOLORVALUE Ambient;
    D3DVECTOR Position;
    D3DVECTOR Direction;
    float Range;
    float Falloff;
    float Attenuation0;
    float Attenuation1;
    float Attenuation2;
    float Theta;
    float Phi;
} D3DLIGHT9, *LPD3DLIGHT;

Members

Type

Type of the light source. This value is one of the members of the D3DLIGHTTYPE enumerated type.

Diffuse

Diffuse color emitted by the light. This member is a D3DCOLORVALUE structure.

Specular

Specular color emitted by the light. This member is a D3DCOLORVALUE structure.

Ambient

Ambient color emitted by the light. This member is a D3DCOLORVALUE structure.

Position

Position of the light in world space, specified by a D3DVECTOR structure. This member has no meaning for directional lights and is ignored in that case.

Direction

Direction that the light is pointing in world space, specified by a D3DVECTOR structure. This member has meaning only for directional and spotlights. This vector need not be normalized, but it should have a nonzero length.

Range

Distance beyond which the light has no effect. The maximum allowable value for this member is the square root of FLT_MAX. This member does not affect directional lights.

Falloff

Decrease in illumination between a spotlight's inner cone (the angle specified by Theta) and the outer edge of the outer cone (the angle specified by Phi).

The effect of falloff on the lighting is subtle. Furthermore, a small performance penalty is incurred by shaping the falloff curve. For these reasons, most developers set this value to 1.0.

 

Attenuation0

Value specifying how the light intensity changes over distance. Attenuation values are ignored for directional lights. This member represents an attenuation constant. For information about attenuation, see Light Properties (Direct3D 9). Valid values for this member range from 0.0 to infinity. For non-directional lights, all three attenuation values should not be set to 0.0 at the same time.

Attenuation1

Value specifying how the light intensity changes over distance. Attenuation values are ignored for directional lights. This member represents an attenuation constant. For information about attenuation, see Light Properties (Direct3D 9). Valid values for this member range from 0.0 to infinity. For non-directional lights, all three attenuation values should not be set to 0.0 at the same time.

Attenuation2

Value specifying how the light intensity changes over distance. Attenuation values are ignored for directional lights. This member represents an attenuation constant. For information about attenuation, see Light Properties (Direct3D 9). Valid values for this member range from 0.0 to infinity. For non-directional lights, all three attenuation values should not be set to 0.0 at the same time.

Theta

Angle, in radians, of a spotlight's inner cone - that is, the fully illuminated spotlight cone. This value must be in the range from 0 through the value specified by Phi.

Phi

Angle, in radians, defining the outer edge of the spotlight's outer cone. Points outside this cone are not lit by the spotlight. This value must be between 0 and pi.

Ambient、Diffuse、Specular分別表示光以不同的光照計(jì)算模型計(jì)算時(shí)的光源顏色，例如Diffuse就表示光源的漫反射顏色。因?yàn)橐粋€(gè)光源會照亮場景中的多個(gè)物體，而這些物體可能會使用不同的光照計(jì)算模型進(jìn)行光照計(jì)算，所以這就需要為燈光分別設(shè)置用于不同光照計(jì)算模型的燈光顏色。

光源發(fā)出光的三種顏色，與當(dāng)前材質(zhì)的相應(yīng)部分相作用，生成最終用于渲染的顏色。光源漫反射顏色與當(dāng)前材質(zhì)的漫反射屬性作用，光源鏡面反射顏色與當(dāng)前材質(zhì)的鏡面反射屬性作用等。

這三個(gè)顏色成員都是D3DCOLORVALUE結(jié)構(gòu)體類型的變量，其中R、G、B這三個(gè)分量的取值區(qū)間為0.0f~1.0f。最常見的漫反射顏色是白色（R:1.0 G:1.0 B:1.0），但可根據(jù)需要實(shí)現(xiàn)的效果創(chuàng)建所需的顏色。例如，為一個(gè)壁爐采用紅光。

Describes color values.

typedef struct D3DCOLORVALUE {
    float r;
    float g;
    float b;
    float a;
} D3DCOLORVALUE, *LPD3DCOLORVALUE;

Members

r

Floating-point value specifying the red component of a color. This value generally is in the range from 0.0 through 1.0, with 0.0 being black.

g

Floating-point value specifying the green component of a color. This value generally is in the range from 0.0 through 1.0, with 0.0 being black.

b

Floating-point value specifying the blue component of a color. This value generally is in the range from 0.0 through 1.0, with 0.0 being black.

a

Floating-point value specifying the alpha component of a color. This value generally is in the range from 0.0 through 1.0, with 0.0 being black.

Remarks

You can set the members of this structure to values outside the range of 0 through 1 to implement some unusual effects. Values greater than 1 produce strong lights that tend to wash out a scene. Negative values produce dark lights that actually remove light from a scene.

距離衰減系數(shù)Attenuation0、Attenuation1、Attenuation2控制一個(gè)光源的光強(qiáng)如何向著最遠(yuǎn)距離減弱，最遠(yuǎn)距離是由屬性Range指定的照射范圍。這三個(gè)成員都是浮點(diǎn)型、取值范圍是從0.0到無窮大。Attenuation0、Attenuation1、Attenuation2分別表示光的常量、線性、二次距離衰減系數(shù)。衰減公式為：

Atten = 1 / (Attenuation0 + Attenuation1 * d + Attenuation2 * d²)

其中d為物體頂點(diǎn)和光源之間的距離。物體距離光源越遠(yuǎn)，它的亮度越小。距離衰減系數(shù)僅對點(diǎn)光源和聚光燈有效。一些應(yīng)用程序可將Attenuation1設(shè)為1.0，其他的設(shè)為0.0，從而使光強(qiáng)按1/d的比例改變。在光源處光強(qiáng)最大，在光源的照射范圍邊界處減少到1/Range。也可以分別設(shè)置3個(gè)衰減系數(shù)為不同的數(shù)值，得到更復(fù)雜的衰減效果，這些值可以任意設(shè)置，但是不能為負(fù)數(shù)。

結(jié)構(gòu)類型D3DLIGHT9包含三個(gè)僅有聚光燈使用的成員：內(nèi)外錐體衰減系數(shù)falloff、內(nèi)椎體角度theta、外椎體角度phi，共同控制著一個(gè)聚光燈的內(nèi)外椎體各有多大，以及在它們中間的光線如何減弱。