137 lines
4.1 KiB
Markdown
137 lines
4.1 KiB
Markdown
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---
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title: 正交投影混合
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date: 2026-05-03 00:00:00
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excerpt: OrthoBlend 透视校正技术，实现动画风格的透视扁平化效果
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tags:
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- ARC
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- Rendering
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- VertexFactory
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- Toon
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rating: ⭐
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---
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# 正交投影混合
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返回 [[Rendering]]
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## 概述
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正交投影混合（OrthoBlend）是 ARC 引擎实现**动画风格透视扁平化**的核心技术。在动画/格斗游戏中，角色需要减少透视畸变以保持美术控制的外观，这通过在透视投影和正交投影之间进行混合来实现。
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引擎提供了两个版本：
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- **V1** (`USES_ORTHO_BLEND_POSITION`)：简单的 X 轴正交混合
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- **V2** (`USES_ORTHO_BLEND_POSITION2`)：基于物体中心距离的全透视校正（"Purse correction"）
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## 实现细节
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### V1：X 轴正交混合
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在 `Common.ush` 中定义：
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```hlsl
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float4 GetOrthoBlendPosition(
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float4 WorldPosition,
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float4x4 ViewProjectionMatrix,
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float weight)
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{
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float4 ScreenPosition = mul(WorldPosition, ViewProjectionMatrix);
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// 计算正交投影的 X 坐标
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float OrthoScreenPositionX = dot(
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ResolvedView.OrthoViewProjectionX, WorldPosition);
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// 在透视和正交之间混合 X 轴
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ScreenPosition.x = lerp(
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ScreenPosition.x,
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OrthoScreenPositionX * ScreenPosition.w,
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weight * ResolvedView.OrthoBlendParameter);
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return ScreenPosition;
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}
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```
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`OrthoBlendParameter` 从 C++ 端通过 `ViewUniformShaderParameters` 传入，全局控制混合强度。`weight` 来自材质属性 `MP_OrthoBlendWeight`，允许逐材质控制。
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### V2：全透视校正
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```hlsl
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float4 GetOrthoBlendPosition2(
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float4 WorldPosition,
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float4x4 ViewProjectionMatrix,
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float weight,
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float3 ObjWorldPosition)
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{
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// 计算顶点相对于物体中心的偏移
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float3 offset = WorldPosition.xyz - ObjWorldPosition.xyz;
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// 物体中心到相机的距离（沿视线方向）
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float dist = abs(dot(
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ObjWorldPosition.xyz - ResolvedView.WorldCameraOrigin.xyz,
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ResolvedView.ViewForward.xyz));
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// 将顶点"拍平"到与物体中心相同距离的平面
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float3 origin = ResolvedView.WorldCameraOrigin.xyz
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+ ResolvedView.ViewForward.xyz * dist;
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WorldPosition.xy = origin.xy + offset.xy;
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// ... 后续投影变换
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}
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```
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V2 的原理：将所有顶点投影到与物体中心等距的平面上，消除了前后肢体因透视导致的大小差异。
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### 全 VertexFactory 覆盖
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两个版本均在所有顶点着色器中实现：
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| 顶点着色器 | 文件 |
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|-----------|------|
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| BasePass | `BasePassVertexShader.usf` |
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| DepthOnly | `DepthOnlyVertexShader.usf` |
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| PositionOnlyDepth | `PositionOnlyDepthVertexShader.usf` |
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| HitProxy | `HitProxyVertexShader.usf` |
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| DebugViewMode | `DebugViewModeVertexShader.usf` |
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覆盖的 VertexFactory：
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- `LocalVertexFactory.ush`
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- `GpuSkinVertexFactory.ush`
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- `LandscapeVertexFactory.ush`
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- `MeshParticleVertexFactory.ush`
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- 所有 Particle VertexFactory
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## 材质属性
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通过 `MP_OrthoBlendWeight` 材质属性控制每个材质的混合权重：
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```cpp
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// MaterialTemplate.ush
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float GetMaterialOrthoBlendWeight(FMaterialVertexParameters Parameters)
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{
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%s; // 由材质图生成
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}
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```
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## C++ 端支持
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- `SceneRendering.cpp`：计算 `OrthoBlendParameter` 并写入 ViewUniformBuffer
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- `SceneTypes.h`：`MP_OrthoBlendWeight` 枚举值
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- `HLSLMaterialTranslator.cpp`：材质编译支持
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## 关联文档
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- [[自定义材质属性]] — `MP_OrthoBlendWeight` 的定义
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- [[屏幕对齐网格]] — 另一种屏幕空间变换方案
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- [[局部位置缩放]] — 局部空间的顶点变换
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## 修改文件列表
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| 文件 | 修改类型 |
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|------|---------|
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| `Shaders/Private/Common.ush` | 新增 `GetOrthoBlendPosition` / `GetOrthoBlendPosition2` |
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| `Shaders/Private/BasePassVertexShader.usf` | 调用正交混合 |
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| `Shaders/Private/DepthOnlyVertexShader.usf` | 调用正交混合 |
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| `Shaders/Private/PositionOnlyDepthVertexShader.usf` | 调用正交混合 |
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| `Shaders/Private/HitProxyVertexShader.usf` | 调用正交混合 |
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| `Shaders/Private/DebugViewModeVertexShader.usf` | 调用正交混合 |
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| 所有 VertexFactory `.ush` | 提供 ObjWorldPosition |
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