UE4 从TextureRenderTargetCube获取六个面的图片（C++实现）

要从TextureRenderTargetCube获取六个面的图片，可以通过以下步骤实现：

1. 创建一个RenderTargetCube，用于渲染。

UTextureRenderTargetCube* RenderTarget = NewObject<UTextureRenderTargetCube>();
RenderTarget->InitAutoFormat(512);

2. 渲染六个面，将结果存储到RenderTarget中。

FVector Location(0.f, 0.f, 0.f);
FRotator Rotation(0.f, 0.f, 0.f);
FSceneViewFamilyContext ViewFamily(FSceneViewFamily::ConstructionValues(
        RenderTarget->GetSizeXY(), FEngineShowFlags(ESFIM_Game))
    .SetRealtimeUpdate(true));
FSceneView* View = CalculateView(Location, Rotation, ViewFamily);
View->ViewMatrices.ApplyPreViewTranslation(-Location);
View->ViewMatrices.SetProjectionMatrix(FMatrix::Identity);
View->ViewState.SetViewRect(FIntRect(0, 0, RenderTarget->GetSurfaceWidth(), RenderTarget->GetSurfaceHeight()));
View->Render(ViewFamily);

3. 从RenderTarget中获取六个面的图片。

UTexture2D* TextureArray[6];
for (int i = 0; i < 6; i++)
{
    TextureArray[i] = NewObject<UTexture2D>();
    TextureArray[i]->Source.Init(TextureSize, TextureSize, 1, 1, ETextureSourceFormat::TSF_RGBA16F);
    TextureArray[i]->UpdateResource();
    ENQUEUE_RENDER_COMMAND(CaptureCommand)(
        [TextureArray, RenderTarget, i](FRHICommandListImmediate& RHICmdList)
        {
            check(IsInRenderingThread());

            FTextureRenderTargetResource* RenderTargetResource = RenderTarget->GameThread_GetRenderTargetResource();
            FReadSurfaceDataFlags ReadPixelFlags(RCM_UNorm, CubeFaceToIndex((ECubeFace)i));
            ReadPixelFlags.SetLinearToGamma(true);
            RHICmdList.ReadSurfaceData(
                RenderTargetResource->GetRenderTargetTexture(),
                FIntRect(0, 0, TextureSize, TextureSize),
                TextureArray[i]->Source.Lock(LOCK_READ_WRITE),
                ReadPixelFlags
            );
            TextureArray[i]->Source.Unlock();
            TextureArray[i]->UpdateResource();
        });
    FlushRenderingCommands();
}

其中，CubeFaceToIndex函数用于将Cube Face转换为数组下标。

int CubeFaceToIndex(ECubeFace CubeFace)
{
    switch (CubeFace)
    {
    case CubeFace_PosX:
        return 0;
    case CubeFace_NegX:
        return 1;
    case CubeFace_PosY:
        return 2;
    case CubeFace_NegY:
        return 3;
    case CubeFace_PosZ:
        return 4;
    case CubeFace_NegZ:
        return 5;
    default:
        check(false);
        return -1;
    }
}

这样就可以从TextureRenderTargetCube获取六个面的图片了。