AmplifyImpostors源碼閱讀
首先看一下點擊Bake按鈕后的執行流程：
1.AmplifyImpostorInspector部分
首先點擊按鈕設置了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; }
如果展開了BillboardMesh選項或是bakeTextures為true，則都會執行下面部分：
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; }
如果緩存的m_alphaTex為空，則會先調用RenderCombinedAlpha渲染出合并alpha紋理，并緩存進m_alphaTex。
然后再調用GenerateAutomaticMesh生成Mesh點。
1.1 RenderCombinedAlpha
該函數會遍歷一遍所有視角的模型，生成出覆蓋范圍最大的Bounds，并更新到這2個變量中：
m_xyFitSize = Mathf.Max(m_xyFitSize, frameBounds.size.x, frameBounds.size.y);
m_depthFitSize = Mathf.Max(m_depthFitSize, frameBounds.size.z);
通過RenderImpostor函數的combinedAlphas變量，將所有視角模型的alpha疊加在一張RT上，再通過這張疊加RT
修正原有Bounds：
m_xyFitSize *= maxBound;
m_depthFitSize *= maxBound;
接著得到哪張材質的索引對應傳入RT集合的alpha材質：
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;
用深度圖的邊緣生成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會單獨存下來，也就是每一個sheet格子的alpha疊在一起，這樣做可以讓最終生成面片的頂點合理覆蓋：
1.2 GenerateAutomaticMesh
這個函數主要生成頂點，會存到AmplifyImpostorAsset的ShapePoints中。
頂點數據會給接下來的GenerateMesh使用。
這一步一定會設上triangulateMesh = true;
if (m_recalculateMesh && m_instance.m_alphaTex != null) { m_recalculateMesh = false; m_instance.GenerateAutomaticMesh(m_currentData); triangulateMesh = true; EditorUtility.SetDirty(m_instance); }
接著設置previewMesh：
if (triangulateMesh) m_previewMesh = GeneratePreviewMesh(m_currentData.ShapePoints, true);
然后會將CutMode改為手動，允許用戶二次修改：
if (autoChangeToManual /*&& Event.current.type == EventType.Layout*/ ) { autoChangeToManual = false; m_instance.m_cutMode = CutMode.Manual; Event.current.Use(); }
最后進入DelayedBake，調用AmplifyImpostor的RenderAllDeferredGroups函數。
2.AmplifyImpostor部分
進入函數RenderAllDeferredGroups，前面都和之前操作差不多，直到調用到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)));
繪制時每個sheet的格子都存放對應角度的模型圖片，通過SetViewport進行繪制目標區域的裁剪。
不同的ImpostorType對應繪制hframes、vframes的排布方式也不一樣。
繪制代碼基本的邏輯結構如下：
for (int x = 0; x < hframes; x++) //橫向圖片數量，例如hframes = 8 { for (int y = 0; y <= vframes; y++) //縱向圖片數量 { 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);
優先繪制Y軸，其次X軸，每次繪制寫入commandBuffer，最后在外部執行一次ExecuteCommandBuffer。
附一張測試例圖方便參考：
2.1 Remapping
這一步工作主要是將深度通道塞進去。
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)); return finalColor; } ENDCG }
這一步會將RT[2]的alpha合并至RT[0]，將RT[0]的alpha合并至RT[3]
接下來PackDepth，將深度信息寫入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]存的是法線，a通道存深度后：
RT[0]的alpha：
FixAlbedo，m_rtGBuffers[1]對應extraTex參數，若傳參會被設置到_A采樣器。
// 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); return alb; } ENDCG }
存TGA（如果預設里勾選了TGA則調用該處，否則存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邊緣膨脹處理：
Shader dilateShader = AssetDatabase.LoadAssetAtPath<Shader>(AssetDatabase.GUIDToAssetPath(DilateGUID)); Debug.Log(dilateShader, dilateShader); Material dilateMat = new Material(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是沿著周圍8個方向外拓一圈：
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 ) };
函數中會根據pixelBlend將這個shader調用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); }
默認值是調用32次：
[SerializeField] [Range( 0, 64 )] public int PixelPadding = 32;
3.Shader渲染部分
Octahedron八面體和球面方案分別使用2種不同的對外Shader，
八面體方案利用了它的特性，實現任何UV向量上可以tiling及沿著向量插值，而球面實現起來則會更耗性能，
所以AmpImpostor的球面方案沒有做插值功能。
時間原因，接下來只看球面部分。
3.1 SphereImpostorVertex
先看ForwardBase的pass：
頂點部分執行SphereImpostorVertex( v.vertex, v.normal, o.frameUVs, o.viewPos );
這個函數會處理Billboard的位置信息，并返回常規頂點信息和frameUVs信息。
得到相對相機位置，并轉換至object空間，_Offset是實際模型中心偏移量，通過像素轉頂點的方式離線計算得到
float3 objectCameraPosition = mul( ai_WorldToObject, float4( worldCameraPos, 1 ) ).xyz - _Offset.xyz; //ray origin float3 objectCameraDirection = normalize( objectCameraPosition );
構建一組基向量：
float3 upVector = float3( 0,1,0 ); float3 objectHorizontalVector = normalize( cross( objectCameraDirection, upVector ) ); float3 objectVerticalVector = cross( objectHorizontalVector, objectCameraDirection );
橫向信息用arctan2，變量名作者寫錯了
float verticalAngle = frac( atan2( -objectCameraDirection.z, -objectCameraDirection.x ) * AI_INV_TWO_PI ) * sizeX + 0.5;
縱向信息用acos將點乘轉線性
float verticalDot = dot( objectCameraDirection, upVector ); float upAngle = ( acos( -verticalDot ) * AI_INV_PI ) + axisSizeFraction * 0.5f;
yRot構建的旋轉矩陣用作細節修正
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用于將坐標縮放為對應sheet內格子大小
float2 frameUV = ( ( uvExpansion * fractionsUVscale + 0.5 ) + relativeCoords ) * sizeFraction;
3.2 SphereImpostorFragment
frag一些邏輯都是常規操作，看下深度部分的處理，
離近了看會有真實深度的遮擋：
因為是正交相機拍攝，不存在DeviceDepth轉線性EyeDepth。
深度賦值取的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讀的是csharp變量m_depthFitSize，在烘焙時這個值是正交相機的遠截面：
Matrix4x4 P = Matrix4x4.Ortho(-fitSize + m_pixelOffset.x, fitSize + m_pixelOffset.x, -fitSize + m_pixelOffset.y, fitSize + m_pixelOffset.y, 0, zFar: -m_depthFitSize);
最后深度計算這里，_DepthSize*0.5猜測是物體中心是z=0.5，是基于物體中心增加偏移深度，并且remapNormal.a之前已經隨著法線做了-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 )其實是乘以Z軸的縮放，如果沒有縮放改成1結果不變：
float depth = remapNormal.a * _DepthSize * 0.5 * length( ai_ObjectToWorld[ 2 ].xyz );
計算完后再將顏色和深度輸出：
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 ); }
陰影部分ShadowCaster pass用了同樣的代碼，因此impostor也有陰影。
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