Optimize rendering runs of spaces when there is no visual change (#4877)

cmatrix is somewhat of a pathological case for our infrastructure: it
prints out a bunch of green and white characters and then updates them a
million times a second.

It also maintains a column of space between every green character. When
it prints this column, it prints it in "default" or "white". This ends
up making runs of text that look like this:

(def: G=green B=bright white W=white *=matrix char  =space)

G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W
G W G W G W G W G W G W G W G W

As characters trickle in:

G*W G*W G*W G*W G*W G*W G*W B*W
G*W G*W G*W G*W G*W G*W G*W G W
G*W G*W G*W B*W G*W G*W G*W G W
G*W B*W G*W G W G*W G*W G*W G*W
G*W G W G*W G W G*W B*W G*W G*W
B*W G W G*W G W G*W G W B*W G*W
G W G W G*W G W G*W G W G W B*W
G W G W B*W G W G*W G W G W G W

Every one of those color transitions causes us to break up the run of
text and start rendering it again. This impacts GDI, Direct2D *and*
ConPTY. In the example above, there are 120 runs.

The problem is, printing a space doesn't **use** the foreground color!

This commit introduces an optimization. When we're about to break a text
cluster because its attributes changed, we make sure that it's not just
filled with spaces and doesn't differ in any visually-meaningful way
(like underline or strikethrough, considering global invert state).

This lets us optimize both the rendering _and_ the PTY output to look
like this:

G*   *   *   *   *   *   *  B*G
G*   *   *   *   *   *   *
G*   *   *  B*G  *   *   *
G*  B*G  *       *   *   *   *
G*       *       *  B*G  *   *
B*G      *       *      B*G  *
G        *       *          B*G
G       B*G      *

Text will be printed at best line-by-line and at worst only when the
visible properties of the screen actually change. In the example
above, there are only 21 runs.

This speeds up cmatrix remarkably.

Refs #1064

src/buffer/out/TextAttribute.cpp

@@ -155,11 +155,6 @@ void TextAttribute::SetColor(const COLORREF rgbColor, const bool fIsForeground)
     }
 }
 
-bool TextAttribute::_IsReverseVideo() const noexcept
-{
-    return WI_IsFlagSet(_wAttrLegacy, COMMON_LVB_REVERSE_VIDEO);
-}
-
 bool TextAttribute::IsLeadingByte() const noexcept
 {
     return WI_IsFlagSet(_wAttrLegacy, COMMON_LVB_LEADING_BYTE);

src/buffer/out/TextAttribute.hpp

@@ -163,12 +163,41 @@ public:
         return _foreground.IsRgb() || _background.IsRgb();
     }
 
+    // This returns whether this attribute, if printed directly next to another attribute, for the space
+    // character, would look identical to the other one.
+    constexpr bool HasIdenticalVisualRepresentationForBlankSpace(const TextAttribute& other, const bool inverted = false) const noexcept
+    {
+        // sneaky-sneaky: I'm using xor here
+        // inverted is whether there's a global invert; Reverse is a local one.
+        // global ^ local == true : the background attribute is actually the visible foreground, so we care about the foregrounds being identical
+        // global ^ local == false: the foreground attribute is the visible foreground, so we care about the backgrounds being identical
+        const auto checkForeground = (inverted != _IsReverseVideo());
+        return !IsAnyGridLineEnabled() && // grid lines have a visual representation
+               // crossed out, doubly and singly underlined have a visual representation
+               WI_AreAllFlagsClear(_extendedAttrs, ExtendedAttributes::CrossedOut | ExtendedAttributes::DoublyUnderlined | ExtendedAttributes::Underlined) &&
+               // all other attributes do not have a visual representation
+               (_wAttrLegacy & META_ATTRS) == (other._wAttrLegacy & META_ATTRS) &&
+               ((checkForeground && _foreground == other._foreground) ||
+                (!checkForeground && _background == other._background)) &&
+               _extendedAttrs == other._extendedAttrs;
+    }
+
+    constexpr bool IsAnyGridLineEnabled() const noexcept
+    {
+        return WI_IsAnyFlagSet(_wAttrLegacy, COMMON_LVB_GRID_HORIZONTAL | COMMON_LVB_GRID_LVERTICAL | COMMON_LVB_GRID_RVERTICAL | COMMON_LVB_UNDERSCORE);
+    }
+
 private:
     COLORREF _GetRgbForeground(std::basic_string_view<COLORREF> colorTable,
                                COLORREF defaultColor) const noexcept;
     COLORREF _GetRgbBackground(std::basic_string_view<COLORREF> colorTable,
                                COLORREF defaultColor) const noexcept;
-    bool _IsReverseVideo() const noexcept;
+
+    constexpr bool _IsReverseVideo() const noexcept
+    {
+        return WI_IsFlagSet(_wAttrLegacy, COMMON_LVB_REVERSE_VIDEO);
+    }
+
     void _SetBoldness(const bool isBold) noexcept;
 
     WORD _wAttrLegacy;

src/cascadia/TerminalCore/Terminal.hpp

@@ -145,6 +145,7 @@ public:
     CursorType GetCursorStyle() const noexcept override;
     COLORREF GetCursorColor() const noexcept override;
     bool IsCursorDoubleWidth() const noexcept override;
+    bool IsScreenReversed() const noexcept override;
     const std::vector<Microsoft::Console::Render::RenderOverlay> GetOverlays() const noexcept override;
     const bool IsGridLineDrawingAllowed() noexcept override;
 #pragma endregion

src/cascadia/TerminalCore/terminalrenderdata.cpp

@@ -204,3 +204,13 @@ void Terminal::UnlockConsole() noexcept
 {
     _readWriteLock.unlock_shared();
 }
+
+// Method Description:
+// - Returns whether the screen is inverted;
+//   This state is not currently known to Terminal.
+// Return Value:
+// - false.
+bool Terminal::IsScreenReversed() const noexcept
+{
+    return false;
+}

src/host/renderData.cpp

@@ -445,4 +445,16 @@ void RenderData::ColorSelection(const COORD coordSelectionStart, const COORD coo
 {
     Selection::Instance().ColorSelection(coordSelectionStart, coordSelectionEnd, attr);
 }
+
+// Method Description:
+// - Returns true if the screen is globally inverted
+// Arguments:
+// - <none>
+// Return Value:
+// - true if the screen is globally inverted
+bool RenderData::IsScreenReversed() const noexcept
+{
+    const CONSOLE_INFORMATION& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    return gci.IsScreenReversed();
+}
 #pragma endregion

src/host/renderData.hpp

@@ -49,6 +49,8 @@ public:
     COLORREF GetCursorColor() const noexcept override;
     bool IsCursorDoubleWidth() const noexcept override;
 
+    bool IsScreenReversed() const noexcept override;
+
     const std::vector<Microsoft::Console::Render::RenderOverlay> GetOverlays() const noexcept override;
 
     const bool IsGridLineDrawingAllowed() noexcept override;

src/renderer/base/renderer.cpp

@@ -615,11 +615,19 @@ void Renderer::_PaintBufferOutput(_In_ IRenderEngine* const pEngine)
     }
 }
 
+static bool _IsAllSpaces(const std::wstring_view v)
+{
+    // first non-space char is not found (is npos)
+    return v.find_first_not_of(L" ") == decltype(v)::npos;
+}
+
 void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
                                         TextBufferCellIterator it,
                                         const COORD target,
                                         const bool lineWrapped)
 {
+    auto globalInvert{ _pData->IsScreenReversed() };
+
     // If we have valid data, let's figure out how to draw it.
     if (it)
     {
@@ -666,8 +674,14 @@ void Renderer::_PaintBufferOutputHelper(_In_ IRenderEngine* const pEngine,
             {
                 if (color != it->TextAttr())
                 {
-                    color = it->TextAttr();
-                    break;
+                    auto newAttr{ it->TextAttr() };
+                    // foreground doesn't matter for runs of spaces (!)
+                    // if we trick it . . . we call Paint far fewer times for cmatrix
+                    if (!_IsAllSpaces(it->Chars()) || !newAttr.HasIdenticalVisualRepresentationForBlankSpace(color, globalInvert))
+                    {
+                        color = newAttr;
+                        break; // vend this run
+                    }
                 }
 
                 // Walk through the text data and turn it into rendering clusters.

src/renderer/inc/IRenderData.hpp

@@ -60,6 +60,8 @@ namespace Microsoft::Console::Render
         virtual COLORREF GetCursorColor() const noexcept = 0;
         virtual bool IsCursorDoubleWidth() const noexcept = 0;
 
+        virtual bool IsScreenReversed() const noexcept = 0;
+
         virtual const std::vector<RenderOverlay> GetOverlays() const noexcept = 0;
 
         virtual const bool IsGridLineDrawingAllowed() noexcept = 0;