3D编程：Displacement Mapping（移位贴图）

Displacement Mapping（移位贴图）

Normal maps的另一个应用是displacement mapping，在这个应用中，细节的增加不再是通过虚拟光照得到的，而是真正的改变模型的vertices。使用displacement mapping时，vertex沿着它的法线向量平移，该法线的值由一个displacement map指定。一个displacement map就是一个高度纹理图，只有一个通道值，并且存储了高度的大小。图9.4就是本章一直使用的stone wall texture，对应的一种displacement map。

因此displacement只一个8位的数据通道，因此显示为一块灰色区域。

图9.4 A color map (left) and displacement map (right) for a stone wall. (Textures by Nick Zuccarello, Florida Interactive Entertainment Academy.)

根据从displacement map中采样的结果，对一个vertex移位时，要么沿着法线向内，要么向外。使用如下的公式执行向外移位： Position = Position0 + (Normal * Scale * DisplacementMagnitude)

其中，Scale是一个shader constant，用于调整存储在displacement map中的magnitudes。对于向内移位，使用下面的公式：

Position = Position0 + (Normal * Scale * DisplacementMagnitude 1)

A Displacement Mapping Effect

列表9.2列出了displacement map effect的部分代码，主要是vertex shader部分的代码。

列表9.2 An Abbreviated Displacement Mapping Effect（注：这里提供了完整的displacement map effect代码，也是练习第2题的要求）

#include "include\\Common.fxh"

cbuffer CBufferPerFrame
{
	float4 AmbientColor : AMBIENT <
		string UIName =  "Ambient Light";
		string UIWidget = "Color";
	> = {1.0f, 1.0f, 1.0f, 1.0f};

	float4 LightColor : COLOR <
		string Object = "LightColor0";
		string UIName =  "Light Color";
		string UIWidget = "Color";
	> = {1.0f, 1.0f, 1.0f, 1.0f};

	float3 LightPosition : POSITION <
		string Object = "PointLight0";
		string UIName = "Light Position";
		string Space = "World";
	> = { 0.0f, 0.0f, 0.0f };

	float LightRadius <
		string UIName = "Light Radius";
		string UIWidget = "slider";
		float UIMin = 0.0;
		float UIMax = 100.0;
		float UIStep = 1.0;
	> = { 10.0f };

	float3 CameraPosition : CAMERAPOSITION < string UIWidget="None"; >;
}

cbuffer CBufferPerObject
{
	float4x4 WorldViewProjection : WORLDVIEWPROJECTION < string UIWidget="None"; >;
	float4x4 World : WORLD < string UIWidget="None"; >;

	float4 SpecularColor : SPECULAR <
		string UIName =  "Specular Color";
		string UIWidget = "Color";
	> = {1.0f, 1.0f, 1.0f, 1.0f};

	float SpecularPower : SPECULARPOWER <
		string UIName =  "Specular Power";
		string UIWidget = "slider";
		float UIMin = 1.0;
		float UIMax = 255.0;
		float UIStep = 1.0;
	> = {25.0f};	

	float DisplacementScale <
		string UIName = "Displacement Scale";
		string UIWidget = "slider";
		float UIMin = 0.0;
		float UIMax = 2.0;
		float UIStep = 0.01;
	> = { 0.0f };
}

Texture2D ColorTexture <
	string ResourceName = "default_color.dds";
	string UIName =  "Color Texture";
	string ResourceType = "2D";
>;

Texture2D DisplacementMap <
	string UIName = "Displacement Map";
	string ResourceType = "2D";
>;

SamplerState TrilinearSampler
{
	Filter = MIN_MAG_MIP_LINEAR;
	AddressU = WRAP;
	AddressV = WRAP;
};

RasterizerState DisableCulling
{
	CullMode = NONE;
};

/************* Data Structures *************/

struct VS_INPUT
{
	float4 ObjectPosition : POSITION;
	float2 TextureCoordinate : TEXCOORD;
	float3 Normal : NORMAL;
	float3 Tangent : TANGENT;
};

struct VS_OUTPUT
{
	float4 Position : SV_Position;
	float3 Normal : NORMAL;
	float3 Tangent : TANGENT;
	float3 Binormal : BINORMAL;
	float2 TextureCoordinate : TEXCOORD0;
	float3 LightDirection : TEXCOORD1;
	float3 ViewDirection : TEXCOORD2;
};

/************* Vertex Shader *************/

VS_OUTPUT vertex_shader(VS_INPUT IN)
{
	VS_OUTPUT OUT = (VS_OUTPUT)0;
	float2 textureCoordinate = get_corrected_texture_coordinate(IN.
		TextureCoordinate);
	if (DisplacementScale > 0.0f)
	{
		float displacement = DisplacementMap.
			SampleLevel(TrilinearSampler, textureCoordinate, 0);
		IN.ObjectPosition.xyz += IN.Normal * DisplacementScale *
			(displacement - 1);
	}
	OUT.Position = mul(IN.ObjectPosition, WorldViewProjection);
	OUT.TextureCoordinate = textureCoordinate;
	OUT.Normal = normalize(mul(float4(IN.Normal, 0), World).xyz);
	float3 worldPosition = normalize(mul(IN.ObjectPosition, World)).
		xyz;
	OUT.ViewDirection = normalize(CameraPosition - worldPosition);
	OUT.LightDirection = get_light_data(LightPosition, worldPosition,
		LightRadius);
	return OUT;
}

/************* Pixel Shader *************/

float4 pixel_shader(VS_OUTPUT IN) : SV_Target
{
	float4 OUT = (float4)0;

	float3 normal = normalize(IN.Normal);

	float3 viewDirection = normalize(IN.ViewDirection);
	float4 color = ColorTexture.Sample(TrilinearSampler, IN.TextureCoordinate);
	float3 ambient = get_vector_color_contribution(AmbientColor, color.rgb);

	LIGHT_CONTRIBUTION_DATA lightContributionData;
	lightContributionData.Color = color;
	lightContributionData.Normal = normal;
	lightContributionData.ViewDirection = viewDirection;
	lightContributionData.LightDirection = float4(IN.LightDirection, 1);
	lightContributionData.SpecularColor = SpecularColor;
	lightContributionData.SpecularPower = SpecularPower;
	lightContributionData.LightColor = LightColor;
	float3 light_contribution = get_light_contribution(lightContributionData);

	OUT.rgb = ambient + light_contribution;
	OUT.a = 1.0f;

	return OUT;
}

/************* Techniques *************/

technique10 main10
{
	pass p0
	{
		SetVertexShader(CompileShader(vs_4_0, vertex_shader()));
		SetGeometryShader(NULL);
		SetPixelShader(CompileShader(ps_4_0, pixel_shader()));

		SetRasterizerState(DisableCulling);
	}
}

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