Unreal Engine MobileFSR插件实现机制分析

一、源码分析

第一部分：插件启用检查

控制台变量定义

Engine/Plugins/Runtime/MobileFSR/Source/MobileFSR/Private/MobileFSRViewExtension.cpp

cpp 复制代码

static TAutoConsoleVariable<int32> CVarEnableFSR(
    TEXT("r.Mobile.FSR.Enabled"),
    1,
    TEXT("Enable Mobile FSR for Primary Upscale"),
    ECVF_RenderThreadSafe);

TAutoConsoleVariable<int32> CVarMobileFSRCASEnabled(
    TEXT("r.Mobile.FSR.RCAS.Enabled"),
    1,
    TEXT("FidelityFX FSR/RCAS : Robust Contrast Adaptive Sharpening Filter"),
    ECVF_RenderThreadSafe);

TAutoConsoleVariable<int32> CVarMobileFSRUpsamplingEnabled(
    TEXT("r.Mobile.FSR.Upsampling.Enabled"),
    1,
    TEXT("FidelityFX FSR/RCAS : Robust Contrast Adaptive Sharpening Filter"),
    ECVF_Default);

定义了三个控制台变量，分别控制主开关、锐化功能和放大功能。

ViewExtension 设置接口

Engine/Plugins/Runtime/MobileFSR/Source/MobileFSR/Private/MobileFSRViewExtension.cpp

cpp 复制代码

void FMobileFSRViewExtension::BeginRenderViewFamily(FSceneViewFamily& InViewFamily)
{
    if (InViewFamily.GetFeatureLevel() == ERHIFeatureLevel::ES3_1 && CVarEnableFSR.GetValueOnAnyThread() > 0)
    {
        if (CVarMobileFSRUpsamplingEnabled.GetValueOnAnyThread())
        {
            InViewFamily.SetPrimarySpatialUpscalerInterface(new FMobileFSRUpscaler(true));
        }
        if (CVarMobileFSRCASEnabled.GetValueOnAnyThread())
        {
            InViewFamily.SetSecondarySpatialUpscalerInterface(new FMobileFSRUpscaler(false));
        }
    }
}

检查平台是否为 ES3_1（移动平台）
检查主开关是否启用
根据子开关设置主放大器和次级锐化器

第二部分：平台支持检查

平台支持验证

Engine/Plugins/Runtime/MobileFSR/Source/MobileFSR/Private/MobileFSRUpscaler.cpp

cpp 复制代码

bool FMobileFSRUpscaler::IsSupported(EShaderPlatform Platform)
{
    return IsMobilePlatform(Platform) || IsMetalMobilePlatform(Platform);
}

MobileFSR 只支持移动平台或 Metal 移动平台。

第三部分：FSceneRenderer 接口处理

FSceneRenderer 构造函数中的接口处理流程

Engine/Source/Runtime/Renderer/Private/SceneRendering.cpp

cpp 复制代码

FSceneRenderer::FSceneRenderer(const FSceneViewFamily* InViewFamily, ...)
{
    // 步骤1：检查时序放大器冲突
    check(ViewFamily->GetTemporalUpscalerInterface() == nullptr);
    
    // 步骤2：Fork 游戏线程的空间放大器接口
    if (InViewFamily->PrimarySpatialUpscalerInterface)
    {
        ViewFamily.SetPrimarySpatialUpscalerInterface(InViewFamily->PrimarySpatialUpscalerInterface->Fork_GameThread(ViewFamily));
    }
    
    // 步骤3：调用所有 ViewExtension
    for (int32 ViewExt = 0; ViewExt < ViewFamily->ViewExtensions.Num(); ++ViewExt)
    {
        ViewFamily->ViewExtensions[ViewExt]->BeginRenderViewFamily(*ViewFamily);
    }
    
    // 步骤4：冲突检查
    checkf(!(ViewFamily->GetTemporalUpscalerInterface() != nullptr && ViewFamily->GetPrimarySpatialUpscalerInterface() != nullptr),
        TEXT("Conflict setting up a third party primary spatial upscaler or temporal upscaler."));
}

确保游戏线程没有设置TemporalUpscaler
如果游戏线程已设置SpatialUpscaler，则 Fork 到渲染线程
调用所有 ViewExtension（包括 MobileFSR）
检查TemporalUpscaler和PrimarySpatialUpscaler不能同时存在

冲突检查的含义

cpp 复制代码

checkf(!(ViewFamily->GetTemporalUpscalerInterface() != nullptr && ViewFamily->GetPrimarySpatialUpscalerInterface() != nullptr),
    TEXT("Conflict setting up a third party primary spatial upscaler or temporal upscaler."));

这个检查确保时序放大器和空间放大器不能同时启用。如果同时存在，会触发断言失败。这是为了防止渲染逻辑混乱，因为TemporalUpscaler（如 TAAU、TSR）在时域中进行放大，PrimarySpatialUpscaler（如 MobileFSR）在空间域中进行放大，两者不能同时启用。

SetPrimarySpatialUpscalerInterface 的冲突保护

Engine/Source/Runtime/Engine/Public/SceneView.h

cpp 复制代码

FORCEINLINE void SetPrimarySpatialUpscalerInterface(ISpatialUpscaler* InSpatialUpscalerInterface)
{
    check(InSpatialUpscalerInterface);
    checkf(PrimarySpatialUpscalerInterface == nullptr, TEXT("View family already had a primary spatial upscaler assigned."));
    PrimarySpatialUpscalerInterface = InSpatialUpscalerInterface;
}

确保只能有一个PrimarySpatialUpscaler，防止多个PrimarySpatialUpscaler冲突，确保每个ViewFamily只有一个PrimarySpatialUpscaler 。

第四部分：PostProcessing 中的执行逻辑

bShouldPrimaryUpscale 计算逻辑

Engine/Source/Runtime/Renderer/Private/PostProcess/PostProcessing.cpp

cpp 复制代码

void AddMobilePostProcessingPasses(FRDGBuilder& GraphBuilder, FScene* Scene, const FViewInfo& View, ...)
{
    // 基础条件：传统空间放大或镜头畸变
    bool bShouldPrimaryUpscale = (View.PrimaryScreenPercentageMethod == EPrimaryScreenPercentageMethod::SpatialUpscale && View.UnscaledViewRect != View.ViewRect) || View.LensDistortionLUT.IsEnabled();
    
    // 满足任一条件即可启用
    bShouldPrimaryUpscale |= View.Family->GetPrimarySpatialUpscalerInterface() != nullptr;
    
    // 启用 PrimaryUpscale Pass
    PassSequence.SetEnabled(EPass::PrimaryUpscale, bShouldPrimaryUpscale);
}

只要设置了自定义空间放大器（如 MobileFSR），就会启用 PrimaryUpscale Pass，不管是否需要放大。

PrimaryUpscale Pass 执行逻辑

Engine/Source/Runtime/Renderer/Private/PostProcess/PostProcessing.cpp

cpp 复制代码

void AddMobilePostProcessingPasses(...)
{
    if (PassSequence.IsEnabled(EPass::PrimaryUpscale))
    {
        ISpatialUpscaler::FInputs PassInputs;
        PassInputs.Stage = EUpscaleStage::PrimaryToOutput;
        PassInputs.SceneColor = SceneColor;
        
        // 检查是否有自定义放大器
        if (const ISpatialUpscaler* CustomUpscaler = View.Family->GetPrimarySpatialUpscalerInterface())
        {
            RDG_EVENT_SCOPE(GraphBuilder, "ThirdParty PrimaryUpscale %s", CustomUpscaler->GetDebugName());
            
            // 执行自定义放大器（MobileFSR）
            SceneColor = CustomUpscaler->AddPasses(GraphBuilder, View, PassInputs);
        }
        else
        {
            // 执行默认放大算法
            SceneColor = ISpatialUpscaler::AddDefaultUpscalePass(GraphBuilder, View, PassInputs, EUpscaleMethod::Bilinear, View.LensDistortionLUT);
        }
    }
}

如果存在自定义放大器，则调用其 AddPasses 方法；否则使用默认的双线性放大。

第五部分：MobileFSR 具体实现

Fork_GameThread 实现

Engine/Plugins/Runtime/MobileFSR/Source/MobileFSR/Private/MobileFSRUpscaler.cpp

cpp 复制代码

class FMobileFSRUpscaler : public ISpatialUpscaler
{
public:
    virtual ISpatialUpscaler* Fork_GameThread(const class FSceneViewFamily& ViewFamily) const override
    {
        return new FMobileFSRUpscaler();
    }
    
    virtual const TCHAR* GetDebugName() const override
    {
        return TEXT("MobileFSR");
    }
    
    virtual FScreenPassTexture AddPasses(
        FRDGBuilder& GraphBuilder,
        const FViewInfo& View,
        const FInputs& PassInputs) const override;
};

Fork_GameThread 创建渲染线程专用的 MobileFSR 实例，确保线程安全。

第六部分：强制测试机制

测试覆盖机制

Engine/Source/Runtime/Renderer/Private/SceneRendering.cpp

cpp 复制代码

static TAutoConsoleVariable<int32> CVarTestPrimaryScreenPercentageMethodOverride(
    TEXT("r.Test.PrimaryScreenPercentageMethodOverride"),
    0,
    TEXT("Override the screen percentage method for all view family.\n")
    TEXT(" 0: view family's screen percentage interface choose; (default)\n")
    TEXT(" 1: old fashion upscaling pass at the very end right before before UI;\n")
    TEXT(" 2: TemporalAA upsample."));

PrepareViewRectsForRendering 有依据 CVarTestPrimaryScreenPercentageMethodOverride 设置的强制测试覆盖机制。

cpp 复制代码

 static TAutoConsoleVariable<int32> CVarTestPrimaryScreenPercentageMethodOverride(
            TEXT("r.Test.PrimaryScreenPercentageMethodOverride"),
            0,
            TEXT("Override the screen percentage method for all view family.\n")
            TEXT(" 0: view family's screen percentage interface choose; (default)\n")
            TEXT(" 1: old fashion upscaling pass at the very end right before before UI;\n")
            TEXT(" 2: TemporalAA upsample.\n")
            TEXT(" 3: Raw output."));

cpp 复制代码

void FSceneRenderer::PrepareViewRectsForRendering(FRHICommandListImmediate& RHICmdList)
{
    // ... 前面的初始化代码 ...
    // Checks that view rects are correctly initialized.
	for (int32 i = 0; i < Views.Num(); i++)
	{
    	#if !UE_BUILD_SHIPPING
		// For testing purpose, override the screen percentage method.
		{
			switch (CVarTestPrimaryScreenPercentageMethodOverride.GetValueOnRenderThread())
			{
			case 1: View.PrimaryScreenPercentageMethod = EPrimaryScreenPercentageMethod::SpatialUpscale; break;
			case 2: View.PrimaryScreenPercentageMethod = EPrimaryScreenPercentageMethod::TemporalUpscale; break;
			case 3: View.PrimaryScreenPercentageMethod = EPrimaryScreenPercentageMethod::RawOutput; break;
			}
		}
		#endif
    }
    // switch 后面的重要逻辑开始
    // 这里是 FSceneRenderer 构造函数的继续执行部分
    // 包括：- View 初始化/渲染状态设置/场景数据准备/渲染器初始化/...
    
    // 步骤：调用所有 ViewExtension
    for (int32 ViewExt = 0; ViewExt < ViewFamily->ViewExtensions.Num(); ++ViewExt)
    {
        ViewFamily->ViewExtensions[ViewExt]->BeginRenderViewFamily(*ViewFamily);
    }
    
    // 步骤：冲突检查
    checkf(!(ViewFamily->GetTemporalUpscalerInterface() != nullptr && ViewFamily->GetPrimarySpatialUpscalerInterface() != nullptr),
        TEXT("Conflict setting up a third party primary spatial upscaler or temporal upscaler."));
    
    // 更多 FSceneRenderer 初始化逻辑...
}

这里的强制测试机制允许开发者覆盖屏幕百分比方法，用于测试不同的Upsacle。开发者可以通过控制变量 r.Test.PrimaryScreenPercentageMethodOverride 强制切换不同的上采样方法来：

测试性能差异：比较不同方法的GPU使用率和帧率
验证视觉质量：在同一场景中快速切换不同方法，比较图像质量
调试渲染问题：当某个上采样方法出现问题时，可以强制使用其他方法来隔离问题

一、生效条件总结

必要条件：

平台支持 ：InViewFamily.GetFeatureLevel() == ERHIFeatureLevel::ES3_1
主开关启用 ：r.Mobile.FSR.Enabled > 0
放大功能启用 ：r.Mobile.FSR.Upsampling.Enabled > 0
接口设置成功：MobileFSR ViewExtension 成功设置接口
无冲突：没有其他系统已设置空间放大器或时序放大器

执行条件：

只要设置了 PrimarySpatialUpscalerInterface，就会启用 PrimaryUpscale Pass
不检查是否需要放大（UnscaledViewRect != ViewRect）>>> 改进点

如果不生效的检查方法

1. 检查控制台变量

cpp 复制代码

r.Mobile.FSR.Enabled
r.Mobile.FSR.Upsampling.Enabled
// 应该都返回 1

2. 检查接口冲突

cpp 复制代码

// 在 FSceneRenderer 构造函数后添加日志
UE_LOG(LogTemp, Warning, TEXT("TemporalUpscalerInterface = %s"), 
    ViewFamily->GetTemporalUpscalerInterface() ? TEXT("Valid") : TEXT("Null"));
UE_LOG(LogTemp, Warning, TEXT("PrimarySpatialUpscalerInterface = %s"), 
    ViewFamily->GetPrimarySpatialUpscalerInterface() ? TEXT("Valid") : TEXT("Null"));

3. 检查 Pass 启用

cpp 复制代码

// 在 AddMobilePostProcessingPasses 中添加日志
UE_LOG(LogTemp, Warning, TEXT("bShouldPrimaryUpscale = %s"), bShouldPrimaryUpscale ? TEXT("True") : TEXT("False"));
UE_LOG(LogTemp, Warning, TEXT("PrimaryUpscale Pass Enabled = %s"), 
    PassSequence.IsEnabled(EPass::PrimaryUpscale) ? TEXT("True") : TEXT("False"));

4. 强制测试

cpp 复制代码

r.Test.PrimaryScreenPercentageMethodOverride 1
// 强制使用 SpatialUpscale 进行测试

5.检查 ViewExtension 执行顺序

cpp 复制代码

// 在 FSceneRenderer 构造函数中添加调试
for (int32 ViewExt = 0; ViewExt < ViewFamily->ViewExtensions.Num(); ++ViewExt)
{
    UE_LOG(LogTemp, Warning, TEXT("ViewExtension[%d]: %s"), 
        ViewExt, *ViewFamily->ViewExtensions[ViewExt]->GetClass()->GetName());
    
    ViewFamily->ViewExtensions[ViewExt]->BeginRenderViewFamily(*ViewFamily);
    
    UE_LOG(LogTemp, Warning, TEXT("After ViewExtension[%d]: PrimarySpatialUpscalerInterface = %s"), 
        ViewExt, ViewFamily->GetPrimarySpatialUpscalerInterface() ? TEXT("Valid") : TEXT("Null"));
}