AtlasEngine: Minor bug fixes (#16219)

This commit fixes 4 minor bugs:
* Forgot to set the maximum swap chain latency. Without it, it defaults
  to up to 3 frames of latency. We don't need this, because our renderer
  is simple and fast and is expected to draw frames within <1ms.
* ClearType treats the alpha channel as ignored, whereas custom shaders
  can manipulate the alpha channel freely. This meant that using both
  simultaneously would produce weird effects, like text having black
  background. We now force grayscale AA instead.
* The builtin retro shader should not be effected by the previous point.
* When the cbuffer is entirely unused in a custom shader, it has so far
  resulted in constant redraws. This happened because the D3D reflection
  `GetDesc` call will then return `E_FAIL` in this situation.
  The new code on the other hand will now assume that a failure
  to get the description is equal to the variable being unused.

Closes #15960

## Validation Steps Performed
* A custom passthrough shader works with grayscale and ClearType AA
  while also changing the opacity with Ctrl+Shift+Scroll ✅
* Same for the builtin retro shader, but ClearType works ✅
* The passthrough shader doesn't result in constant redrawing ✅

src/renderer/atlas/AtlasEngine.api.cpp

@@ -490,20 +490,20 @@ void AtlasEngine::UpdateHyperlinkHoveredId(const uint16_t hoveredId) noexcept
 
 void AtlasEngine::_resolveTransparencySettings() noexcept
 {
+    // An opaque background allows us to use true "independent" flips. See AtlasEngine::_createSwapChain().
+    // We can't enable them if custom shaders are specified, because it's unknown, whether they support opaque inputs.
+    const bool useAlpha = _api.enableTransparentBackground || !_api.s->misc->customPixelShaderPath.empty();
     // If the user asks for ClearType, but also for a transparent background
     // (which our ClearType shader doesn't simultaneously support)
     // then we need to sneakily force the renderer to grayscale AA.
-    const auto antialiasingMode = _api.enableTransparentBackground && _api.antialiasingMode == AntialiasingMode::ClearType ? AntialiasingMode::Grayscale : _api.antialiasingMode;
-    const bool enableTransparentBackground = _api.enableTransparentBackground || !_api.s->misc->customPixelShaderPath.empty() || _api.s->misc->useRetroTerminalEffect;
+    const auto antialiasingMode = useAlpha && _api.antialiasingMode == AntialiasingMode::ClearType ? AntialiasingMode::Grayscale : _api.antialiasingMode;
 
-    if (antialiasingMode != _api.s->font->antialiasingMode || enableTransparentBackground != _api.s->target->enableTransparentBackground)
+    if (antialiasingMode != _api.s->font->antialiasingMode || useAlpha != _api.s->target->useAlpha)
     {
         const auto s = _api.s.write();
         s->font.write()->antialiasingMode = antialiasingMode;
-        // An opaque background allows us to use true "independent" flips. See AtlasEngine::_createSwapChain().
-        // We can't enable them if custom shaders are specified, because it's unknown, whether they support opaque inputs.
-        s->target.write()->enableTransparentBackground = enableTransparentBackground;
-        _api.backgroundOpaqueMixin = enableTransparentBackground ? 0x00000000 : 0xff000000;
+        s->target.write()->useAlpha = useAlpha;
+        _api.backgroundOpaqueMixin = useAlpha ? 0x00000000 : 0xff000000;
     }
 }
 

src/renderer/atlas/AtlasEngine.r.cpp

@@ -329,7 +329,7 @@ void AtlasEngine::_createSwapChain()
         .SwapEffect = DXGI_SWAP_EFFECT_FLIP_SEQUENTIAL,
         // If our background is opaque we can enable "independent" flips by setting DXGI_ALPHA_MODE_IGNORE.
         // As our swap chain won't have to compose with DWM anymore it reduces the display latency dramatically.
-        .AlphaMode = _p.s->target->enableTransparentBackground ? DXGI_ALPHA_MODE_PREMULTIPLIED : DXGI_ALPHA_MODE_IGNORE,
+        .AlphaMode = _p.s->target->useAlpha ? DXGI_ALPHA_MODE_PREMULTIPLIED : DXGI_ALPHA_MODE_IGNORE,
         .Flags = swapChainFlags,
     };
 
@@ -360,6 +360,8 @@ void AtlasEngine::_createSwapChain()
     _p.swapChain.targetSize = _p.s->targetSize;
     _p.swapChain.waitForPresentation = true;
 
+    LOG_IF_FAILED(_p.swapChain.swapChain->SetMaximumFrameLatency(1));
+
     WaitUntilCanRender();
 
     if (_p.swapChainChangedCallback)

src/renderer/atlas/BackendD3D.cpp

@@ -403,23 +403,24 @@ void BackendD3D::_recreateCustomShader(const RenderingPayload& p)
             /* ppCode      */ blob.addressof(),
             /* ppErrorMsgs */ error.addressof());
 
-        // Unless we can determine otherwise, assume this shader requires evaluation every frame
-        _requiresContinuousRedraw = true;
-
         if (SUCCEEDED(hr))
         {
-            THROW_IF_FAILED(p.device->CreatePixelShader(blob->GetBufferPointer(), blob->GetBufferSize(), nullptr, _customPixelShader.put()));
+            THROW_IF_FAILED(p.device->CreatePixelShader(blob->GetBufferPointer(), blob->GetBufferSize(), nullptr, _customPixelShader.addressof()));
 
             // Try to determine whether the shader uses the Time variable
             wil::com_ptr<ID3D11ShaderReflection> reflector;
-            if (SUCCEEDED_LOG(D3DReflect(blob->GetBufferPointer(), blob->GetBufferSize(), IID_PPV_ARGS(reflector.put()))))
+            if (SUCCEEDED_LOG(D3DReflect(blob->GetBufferPointer(), blob->GetBufferSize(), IID_PPV_ARGS(reflector.addressof()))))
             {
+                // Depending on the version of the d3dcompiler_*.dll, the next two functions either return nullptr
+                // on failure or an instance of CInvalidSRConstantBuffer or CInvalidSRVariable respectively,
+                // which cause GetDesc() to return E_FAIL. In other words, we have to assume that any failure in the
+                // next few lines indicates that the cbuffer is entirely unused (--> _requiresContinuousRedraw=false).
                 if (ID3D11ShaderReflectionConstantBuffer* constantBufferReflector = reflector->GetConstantBufferByIndex(0)) // shader buffer
                 {
                     if (ID3D11ShaderReflectionVariable* variableReflector = constantBufferReflector->GetVariableByIndex(0)) // time
                     {
                         D3D11_SHADER_VARIABLE_DESC variableDescriptor;
-                        if (SUCCEEDED_LOG(variableReflector->GetDesc(&variableDescriptor)))
+                        if (SUCCEEDED(variableReflector->GetDesc(&variableDescriptor)))
                         {
                             // only if time is used
                             _requiresContinuousRedraw = WI_IsFlagSet(variableDescriptor.uFlags, D3D_SVF_USED);
@@ -427,6 +428,11 @@ void BackendD3D::_recreateCustomShader(const RenderingPayload& p)
                     }
                 }
             }
+            else
+            {
+                // Unless we can determine otherwise, assume this shader requires evaluation every frame
+                _requiresContinuousRedraw = true;
+            }
         }
         else
         {
@@ -447,8 +453,6 @@ void BackendD3D::_recreateCustomShader(const RenderingPayload& p)
     else if (p.s->misc->useRetroTerminalEffect)
     {
         THROW_IF_FAILED(p.device->CreatePixelShader(&custom_shader_ps[0], sizeof(custom_shader_ps), nullptr, _customPixelShader.put()));
-        // We know the built-in retro shader doesn't require continuous redraw.
-        _requiresContinuousRedraw = false;
     }
 
     if (_customPixelShader)

src/renderer/atlas/common.h

@@ -313,7 +313,7 @@ namespace Microsoft::Console::Render::Atlas
     struct TargetSettings
     {
         HWND hwnd = nullptr;
-        bool enableTransparentBackground = false;
+        bool useAlpha = false;
         bool useSoftwareRendering = false;
     };