AmplifyImpostors源码阅读
首先看一下点击Bake按钮后的执行流程：
1.AmplifyImpostorInspector部分
首先点击按钮设置了bakeTexture = true
if( GUILayout.Button( TextureIcon, "buttonright", GUILayout.Height( 24) ) )
{//now recalculates texture and mesh every time because mesh might have changed//if( m_instance.m_alphaTex == null )//{ m_outdatedTexture = true;
m_recalculatePreviewTexture= true;//} bakeTextures= true;
}
如果展开了BillboardMesh选项或是bakeTextures为true，则都会执行下面部分：
if( ( ( m_billboardMesh || m_recalculatePreviewTexture ) && m_instance.m_alphaTex == null ) || ( bakeTextures &&m_recalculatePreviewTexture ) )
{try{
m_instance.RenderCombinedAlpha( m_currentData );
}catch( Exception e )
{
Debug.LogWarning("[AmplifyImpostors] Something went wrong with the mesh preview process, please contact support@amplify.pt with this log message.\n" + e.Message +e.StackTrace );
}if( m_instance.m_cutMode ==CutMode.Automatic )
m_recalculateMesh= true;
m_recalculatePreviewTexture= false;
}
1.1 RenderCombinedAlpha
该函数会遍历一遍所有视角的模型，生成出覆盖范围最大的Bounds，并更新到这2个变量中：
m_xyFitSize =Mathf.Max(m_xyFitSize, frameBounds.size.x, frameBounds.size.y);
m_depthFitSize= Mathf.Max(m_depthFitSize, frameBounds.size.z);通过RenderImpostor函数的combinedAlphas变量，将所有视角模型的alpha叠加在一张RT上，再通过这张叠加RT
修正原有Bounds：
m_xyFitSize *=maxBound;
m_depthFitSize*= maxBound;接着得到哪张材质的索引对应传入RT集合的alpha材质：
bool standardRendering = m_data.Preset.BakeShader == null;int alphaIndex =m_data.Preset.AlphaIndex;if (standardRendering && m_renderPipelineInUse ==RenderPipelineInUse.HDRP)
alphaIndex= 3;else if(standardRendering)
alphaIndex= 2;
用深度图的边缘生成alpha：
RenderTexture tempTex = RenderTextureEx.GetTemporary(m_alphaGBuffers[3]);
Graphics.Blit(m_alphaGBuffers[3], tempTex);
packerMat.SetTexture("_A", tempTex);
Graphics.Blit(m_trueDepth, m_alphaGBuffers[3], packerMat, 11);
RenderTexture.ReleaseTemporary(tempTex);
shader:
Pass //copy depth 11 {
ZTest Always Cull Off ZWrite Off
CGPROGRAM#pragma target 3.0 #pragma vertex vert_img #pragma fragment frag#include"UnityCG.cginc"uniform sampler2D _MainTex;
uniform sampler2D _A;
float4 frag( v2f_img i ) : SV_Target
{float depth =SAMPLE_RAW_DEPTH_TEXTURE( _MainTex, i.uv ).r;
float3 color=tex2D( _A, i.uv ).rgb;float alpha = 1 - step( depth, 0);returnfloat4( color, alpha );
}
ENDCG
}
合并后的alpha会单独存下来，也就是每一个sheet格子的alpha叠在一起，这样做可以让最终生成面片的顶点合理覆盖：
1.2 GenerateAutomaticMesh
这个函数主要生成顶点，会存到AmplifyImpostorAsset的ShapePoints中。
这一步一定会设上triangulateMesh = true;
if (m_recalculateMesh && m_instance.m_alphaTex != null)
{
m_recalculateMesh= false;
m_instance.GenerateAutomaticMesh(m_currentData);
triangulateMesh= true;
EditorUtility.SetDirty(m_instance);
}
接着设置previewMesh：
if(triangulateMesh)
m_previewMesh= GeneratePreviewMesh(m_currentData.ShapePoints, true);
然后会将CutMode改为手动，允许用户二次修改：
if (autoChangeToManual /*&& Event.current.type == EventType.Layout*/)
{
autoChangeToManual= false;
m_instance.m_cutMode=CutMode.Manual;
Event.current.Use();
}
最后进入DelayedBake，调用AmplifyImpostor的RenderAllDeferredGroups函数。
2.AmplifyImpostor部分
进入函数RenderAllDeferredGroups，前面都和之前操作差不多，直到调用到RenderImpostor：
if(impostorMaps)
{
commandBuffer.SetViewProjectionMatrices(V, P);
commandBuffer.SetViewport(new Rect((m_data.TexSize.x / hframes) * x, (m_data.TexSize.y / (vframes + (impostorType == ImpostorType.Spherical ? 1 : 0))) * y, (m_data.TexSize.x / m_data.HorizontalFrames), (m_data.TexSize.y / m_data.VerticalFrames)));
绘制时每个sheet的格子都存放对应角度的模型图片，通过SetViewport进行绘制目标区域的裁剪。
不同的ImpostorType对应绘制hframes、vframes的排布方式也不一样。
绘制代码基本的逻辑结构如下：
for (int x = 0; x < hframes; x++)
{for (int y = 0; y <= vframes; y++)
{if(impostorMaps)
{
commandBuffer.SetViewProjectionMatrices(V, P);
commandBuffer.SetViewport(new Rect((m_data.TexSize.x / hframes) * x, (m_data.TexSize.y / (vframes + (impostorType == ImpostorType.Spherical ? 1 : 0))) * y, (m_data.TexSize.x / m_data.HorizontalFrames), (m_data.TexSize.y /m_data.VerticalFrames)));if (standardrendering && m_renderPipelineInUse ==RenderPipelineInUse.HDRP)
{
commandBuffer.SetGlobalMatrix("_ViewMatrix", V);
commandBuffer.SetGlobalMatrix("_InvViewMatrix", V.inverse);
commandBuffer.SetGlobalMatrix("_ProjMatrix", P);
commandBuffer.SetGlobalMatrix("_ViewProjMatrix", P *V);
commandBuffer.SetGlobalVector("_WorldSpaceCameraPos", Vector4.zero);
}
}for (int j = 0; j < validMeshesCount; j++)
{
commandBuffer.DrawRenderer...
}
}
}
Graphics.ExecuteCommandBuffer(commandAlphaBuffer);
优先绘制Y轴，其次X轴，每次绘制写入commandBuffer，最后统一在外部执行ExecuteCommandBuffer。
附一张测试例图方便参考：
2.1 Remapping
这一步工作主要是将深度通道塞进去。
合并Alpha：
//Switch alpha with occlusion RenderTexture tempTex = RenderTexture.GetTemporary(m_rtGBuffers[0].width, m_rtGBuffers[0].height, m_rtGBuffers[0].depth, m_rtGBuffers[0].format);
RenderTexture tempTex2= RenderTexture.GetTemporary(m_rtGBuffers[3].width, m_rtGBuffers[3].height, m_rtGBuffers[3].depth, m_rtGBuffers[3].format);
packerMat.SetTexture("_A", m_rtGBuffers[2]);
Graphics.Blit(m_rtGBuffers[0], tempTex, packerMat, 4); //A.b packerMat.SetTexture("_A", m_rtGBuffers[0]);
Graphics.Blit(m_rtGBuffers[3], tempTex2, packerMat, 4); //B.a Graphics.Blit(tempTex, m_rtGBuffers[0]);
Graphics.Blit(tempTex2, m_rtGBuffers[3]);
RenderTexture.ReleaseTemporary(tempTex);
RenderTexture.ReleaseTemporary(tempTex2);
shader:
Pass //Copy Alpha 4 {
CGPROGRAM#pragma target 3.0 #pragma vertex vert_img #pragma fragment frag#include"UnityCG.cginc"uniform sampler2D _MainTex;
uniform sampler2D _A;
fixed4 frag (v2f_img i ) : SV_Target
{float alpha =tex2D( _A, i.uv ).a;
fixed4 finalColor=(float4(tex2D( _MainTex, i.uv ).rgb , alpha));returnfinalColor;
}
ENDCG
}
这一步会将RT[2]的alpha合并至RT[0]，将RT[0]的alpha合并至RT[3]
接下来PackDepth，将深度信息写入RT[2]的A通道：
//Pack Depth PackingRemapping(ref m_rtGBuffers[2], ref m_rtGBuffers[2], 0, packerMat, m_trueDepth);
m_trueDepth.Release();
m_trueDepth= null;
RT[2]存的是法线，a通道存深度后：
RT[0]的alpha：
FixAlbedo，m_rtGBuffers[1]对应extraTex参数，若传参会被设置到_A采样器。
//Fix Albedo PackingRemapping(ref m_rtGBuffers[0], ref m_rtGBuffers[0], 5, packerMat, m_rtGBuffers[1]);
alb.rgb / (1-spec)不太清楚。
Pass //Fix albedo 5 {
CGPROGRAM#pragma target 3.0 #pragma vertex vert_img #pragma fragment frag#include"UnityCG.cginc"uniform sampler2D _MainTex;
uniform sampler2D _A;//specular fixed4 frag (v2f_img i ) : SV_Target
{
float3 spec=tex2D( _A, i.uv ).rgb;
float4 alb=tex2D( _MainTex, i.uv );
alb.rgb= alb.rgb / (1-spec);returnalb;
}
ENDCG
}
存TGA（如果预设里勾选了TGA则调用该处，否则存PNG）：
//TGA for (int i = 0; i < outputList.Count; i++)
{if (outputList[i].ImageFormat ==ImageFormat.TGA)
PackingRemapping(ref m_rtGBuffers[i], ref m_rtGBuffers[i], 6, packerMat);
}
DilateShader边缘膨胀处理：
Shader dilateShader = AssetDatabase.LoadAssetAtPath<Shader>(AssetDatabase.GUIDToAssetPath(DilateGUID));
Debug.Log(dilateShader, dilateShader);
Material dilateMat= newMaterial(dilateShader);//Dilation for (int i = 0; i < outputList.Count; i++)
{if(outputList[i].Active)
DilateRenderTextureUsingMask(ref m_rtGBuffers[i], ref m_rtGBuffers[alphaIndex], m_data.PixelPadding, alphaIndex !=i, dilateMat);
}
shader是沿着周围8个方向外拓一圈：
float4 frag_dilate( v2f_img i, boolalpha )
{
float2 offsets[8 ] ={
float2(-1, -1),
float2(0, -1),
float2(+1, -1),
float2(-1, 0),
float2(+1, 0),
float2(-1, +1),
float2(0, +1),
float2(+1, +1)
};
函数中会根据pixelBlend将这个shader调用N次：
for (int i = 0; i < pixelBleed; i++)
{
dilateMat.SetTexture("_MaskTex", dilatedMask);
Graphics.Blit(mainTex, tempTex, dilateMat, alpha? 1 : 0);
Graphics.Blit(tempTex, mainTex);
Graphics.Blit(dilatedMask, tempMask, dilateMat,1);
Graphics.Blit(tempMask, dilatedMask);
}
默认值是调用32次：
[SerializeField]
[Range(0, 64)]public int PixelPadding = 32;
3.Shader渲染部分
Octahedron八面体方案和球面分别使用2个对外Shader，
八面体方案会采样3次做插值，球面则代码稍少，接下来只看球面部分。
3.1 SphereImpostorVertex
先看ForwardBase的pass：
顶点部分执行SphereImpostorVertex( v.vertex, v.normal, o.frameUVs, o.viewPos );
这个函数会处理Billboard的位置信息，并返回常规顶点信息和frameUVs信息。
得到相对相机位置，并转换至object空间，_Offset是实际模型中心偏移量，通过像素转顶点的方式离线计算得到
float3 objectCameraPosition = mul( ai_WorldToObject, float4( worldCameraPos, 1 ) ).xyz - _Offset.xyz; //ray origin float3 objectCameraDirection = normalize( objectCameraPosition );
构建一组基向量：
float3 upVector = float3( 0,1,0);
float3 objectHorizontalVector=normalize( cross( objectCameraDirection, upVector ) );
float3 objectVerticalVector= cross( objectHorizontalVector, objectCameraDirection );
横向信息用arctan2，变量名作者写错了
float verticalAngle = frac( atan2( -objectCameraDirection.z, -objectCameraDirection.x ) * AI_INV_TWO_PI ) * sizeX + 0.5;
纵向信息用acos将点乘转线性
float verticalDot =dot( objectCameraDirection, upVector );float upAngle = ( acos( -verticalDot ) * AI_INV_PI ) + axisSizeFraction * 0.5f;
yRot构建的旋转矩阵用作细节修正
float yRot = sizeFraction.x * AI_PI * verticalDot * ( 2 * frac( verticalAngle ) - 1);//Billboard rotation float2 uvExpansion =vertex.xy;float cosY =cos( yRot );float sinY =sin( yRot );
float2 uvRotator= mul( uvExpansion, float2x2( cosY, -sinY, sinY, cosY ) );
最后sizeFraction用于将坐标缩放为对应sheet内格子大小
float2 frameUV = ( ( uvExpansion * fractionsUVscale + 0.5 ) + relativeCoords ) * sizeFraction;3.2 SphereImpostorFragment
frag一些逻辑都是常规操作，看下深度部分的处理，
离近了看会有真实深度的遮挡：
因为是正交相机拍摄，不存在DeviceDepth转线性EyeDepth。
深度赋值取的clipPos.z：
fixed4 frag_surf (v2f_surf IN, out floatoutDepth : SV_Depth ) : SV_Target {
...
IN.pos.zw=clipPos.zw;
outDepth= IN.pos.z;
_DepthSize读的是csharp变量m_depthFitSize，在烘焙时这个值是正交相机的远截面：
Matrix4x4 P = Matrix4x4.Ortho(-fitSize + m_pixelOffset.x, fitSize + m_pixelOffset.x, -fitSize + m_pixelOffset.y, fitSize + m_pixelOffset.y, 0, zFar: -m_depthFitSize);
最后深度计算这里，_DepthSize*0.5猜测是物体中心是z=0.5，是基于物体中心增加偏移深度，并且remapNormal.a之前已经随着法线做了-1 - 1的映射操作：
float4 remapNormal = normalSample * 2 - 1; //object normal is remapNormal.rgb
最后乘以length( ai_ObjectToWorld[ 2 ].xyz )其实是乘以Z轴的缩放，如果没有缩放改成1结果不变：
float depth = remapNormal.a * _DepthSize * 0.5 * length( ai_ObjectToWorld[ 2 ].xyz );
计算完后再将颜色和深度输出：
fixed4 frag_surf (v2f_surf IN, out floatoutDepth : SV_Depth ) : SV_Target {
UNITY_SETUP_INSTANCE_ID(IN);
SurfaceOutputStandardSpecular o;
UNITY_INITIALIZE_OUTPUT( SurfaceOutputStandardSpecular, o );
float4 clipPos;
float3 worldPos;
SphereImpostorFragment( o, clipPos, worldPos, IN.frameUVs, IN.viewPos );
IN.pos.zw=clipPos.zw;
outDepth=IN.pos.z;
UNITY_APPLY_DITHER_CROSSFADE(IN.pos.xy);return float4( _ObjectId, _PassValue, 1.0, 1.0);
}
阴影部分ShadowCaster pass用了同样的代码，因此impostor也有阴影。