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( " 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;}
如果缓存的m_alphaTex为空，则会先调用RenderCombinedAlpha渲染出合并alpha纹理，并缓存进m_alphaTex。
然后再调用GenerateAutomaticMesh生成Mesh点。
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 ); return float4( 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。
顶点数据会给接下来的GenerateMesh使用。
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++) //横向图片数量，例如hframes = 8{ for (int y = 0; y <= vframes; y++) //纵向图片数量 { 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，最后在外部执行一次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 occlusionRenderTexture 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.bpackerMat.SetTexture("_A", m_rtGBuffers[0]);Graphics.Blit(m_rtGBuffers[3], tempTex2, packerMat, 4); //B.aGraphics.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)); return finalColor; } ENDCG}
这一步会将RT的alpha合并至RT，将RT的alpha合并至RT
接下来PackDepth，将深度信息写入RT的A通道：
// Pack DepthPackingRemapping(ref m_rtGBuffers[2], ref m_rtGBuffers[2], 0, packerMat, m_trueDepth);m_trueDepth.Release();m_trueDepth = null;
RT存的是法线，a通道存深度后：
RT的alpha：
FixAlbedo，m_rtGBuffers对应extraTex参数，若传参会被设置到_A采样器。
// Fix AlbedoPackingRemapping(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); return alb; } ENDCG}
存TGA（如果预设里勾选了TGA则调用该处，否则存PNG）：
// TGAfor (int i = 0; i < outputList.Count; i++){ if (outputList.ImageFormat == ImageFormat.TGA) PackingRemapping(ref m_rtGBuffers, ref m_rtGBuffers, 6, packerMat);}
DilateShader边缘膨胀处理：
Shader dilateShader = AssetDatabase.LoadAssetAtPath<Shader>(AssetDatabase.GUIDToAssetPath(DilateGUID));Debug.Log(dilateShader, dilateShader);Material dilateMat = new Material(dilateShader);// Dilationfor (int i = 0; i < outputList.Count; i++){ if (outputList.Active) DilateRenderTextureUsingMask(ref m_rtGBuffers, ref m_rtGBuffers, 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, bool alpha ){ 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：
0, 64 )]public int PixelPadding = 32;
3.Shader渲染部分
Octahedron八面体和球面方案分别使用2种不同的对外Shader，
八面体方案利用了它的特性，实现任何UV向量上可以tiling及沿着向量插值，而球面实现起来则会更耗性能，
所以AmpImpostor的球面方案没有做插值功能。
时间原因，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 originfloat3 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 rotationfloat2 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 float outDepth : 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 float outDepth : 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。
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