Add support for italics (#1009)

Co-authored-by: Ken Reneris <ms/devops kreneris@microsoft.com>
Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

CHANGELOG.md

@@ -49,6 +49,14 @@ current (development)
           mistake. See #998.
 
 ### Dom
+- Feature: Add `italic` decorator. For instance:
+  ```cpp
+  auto italic_text = text("Italic text") | italic;
+  ```
+  ```cpp
+  auto italic_text = italic(text("Italic text"));
+  ```
+  Proposed by @kenReneris in #1009.
 - Feature: Add `hscroll_indicator`. It display an horizontal indicator
   reflecting the current scroll position. Proposed by @ibrahimnasson in
   [issue 752](https://github.com/ArthurSonzogni/FTXUI/issues/752)

CMakeLists.txt

@@ -84,6 +84,7 @@ add_library(dom
   src/ftxui/dom/hbox.cpp
   src/ftxui/dom/hyperlink.cpp
   src/ftxui/dom/inverted.cpp
+  src/ftxui/dom/italic.cpp
   src/ftxui/dom/linear_gradient.cpp
   src/ftxui/dom/node.cpp
   src/ftxui/dom/node_decorator.cpp

README.md

@@ -109,6 +109,7 @@ Element can become flexible using the the `flex` decorator.
 
 An element can be decorated using the functions:
   - `bold`
+  - `italic`
   - `dim`
   - `inverted`
   - `underlined`

cmake/ftxui_test.cmake

@@ -39,6 +39,7 @@ add_executable(ftxui-tests
   src/ftxui/dom/gridbox_test.cpp
   src/ftxui/dom/hbox_test.cpp
   src/ftxui/dom/hyperlink_test.cpp
+  src/ftxui/dom/italic_test.cpp
   src/ftxui/dom/linear_gradient_test.cpp
   src/ftxui/dom/scroll_indicator_test.cpp
   src/ftxui/dom/selection_test.cpp

doc/mainpage.md

@@ -123,8 +123,8 @@ The project is comprised of 3 modules:
 
 This is the visual element of the program. It defines a `ftxui::Screen`, which
 is a grid of `ftxui::Pixel`. A Pixel represents a Unicode character and its
-associated style (bold, colors, etc.). The screen can be printed as a string
+associated style (bold, italic, colors, etc.). The screen can be printed as a
-using `ftxui::Screen::ToString()`. The following example highlights this
+string using `ftxui::Screen::ToString()`. The following example highlights this
 process:
 
 ```cpp
@@ -476,10 +476,11 @@ See [demo](https://arthursonzogni.github.io/FTXUI/examples/?file=component/linea
 
 ## Style {#dom-style}
 In addition to colored text and colored backgrounds. Many terminals support text
-effects such as: `bold`, `dim`, `underlined`, `inverted`, `blink`.
+effects such as: `bold`, `italic`, `dim`, `underlined`, `inverted`, `blink`.
 
 ```cpp
 Element bold(Element);
+Element italic(Element);
 Element dim(Element);
 Element inverted(Element);
 Element underlined(Element);

examples/dom/CMakeLists.txt

@@ -29,6 +29,7 @@ example(style_dim)
 example(style_gallery)
 example(style_hyperlink)
 example(style_inverted)
+example(style_italic)
 example(style_strikethrough)
 example(style_underlined)
 example(style_underlined_double)

examples/dom/style_gallery.cpp

@@ -15,6 +15,7 @@ int main() {
     hbox({
       text("normal")                                    , text(" ") ,
       text("bold")               | bold                 , text(" ") ,
+      text("italic")             | italic               , text(" ") ,
       text("dim")                | dim                  , text(" ") ,
       text("inverted")           | inverted             , text(" ") ,
       text("underlined")         | underlined           , text(" ") ,

examples/dom/style_italic.cpp

@@ -0,0 +1,23 @@
+// Copyright 2025 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.
+#include <ftxui/dom/elements.hpp>  // for text, operator|, inverted, Fit, hbox, Element
+#include <ftxui/screen/screen.hpp>  // for Full, Screen
+#include <memory>                   // for allocator
+
+#include "ftxui/dom/node.hpp"      // for Render
+#include "ftxui/screen/color.hpp"  // for ftxui
+
+int main() {
+  using namespace ftxui;
+  auto document = hbox({
+      text("This text is "),
+      text("italic") | italic,
+      text(". Do you like it?"),
+  });
+  auto screen = Screen::Create(Dimension::Full(), Dimension::Fit(document));
+  Render(screen, document);
+  screen.Print();
+
+  return 0;
+}

include/ftxui/dom/elements.hpp

@@ -95,6 +95,7 @@ Element canvas(std::function<void(Canvas&)>);
 // -- Decorator ---
 Element bold(Element);
 Element dim(Element);
+Element italic(Element);
 Element inverted(Element);
 Element underlined(Element);
 Element underlinedDouble(Element);

include/ftxui/screen/pixel.hpp

@@ -17,6 +17,7 @@ struct Pixel {
       : blink(false),
         bold(false),
         dim(false),
+        italic(false),
         inverted(false),
         underlined(false),
         underlined_double(false),
@@ -27,6 +28,7 @@ struct Pixel {
   bool blink : 1;
   bool bold : 1;
   bool dim : 1;
+  bool italic : 1;
   bool inverted : 1;
   bool underlined : 1;
   bool underlined_double : 1;

src/ftxui/dom/dbox.cpp

@@ -75,6 +75,7 @@ class DBox : public Node {
             acc->bold = pixel.bold;
             acc->dim = pixel.dim;
             acc->inverted = pixel.inverted;
+            acc->italic = pixel.italic;
             acc->underlined = pixel.underlined;
             acc->underlined_double = pixel.underlined_double;
             acc->strikethrough = pixel.strikethrough;

src/ftxui/dom/italic.cpp

@@ -0,0 +1,35 @@
+// Copyright 2025 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.
+#include <memory>   // for make_shared
+#include <utility>  // for move
+
+#include "ftxui/dom/elements.hpp"        // for Element, underlinedDouble
+#include "ftxui/dom/node.hpp"            // for Node
+#include "ftxui/dom/node_decorator.hpp"  // for NodeDecorator
+#include "ftxui/screen/box.hpp"          // for Box
+#include "ftxui/screen/screen.hpp"       // for Pixel, Screen
+
+namespace ftxui {
+
+/// @brief Apply a underlinedDouble to text.
+/// @ingroup dom
+Element italic(Element child) {
+  class Impl : public NodeDecorator {
+   public:
+    using NodeDecorator::NodeDecorator;
+
+    void Render(Screen& screen) override {
+      for (int y = box_.y_min; y <= box_.y_max; ++y) {
+        for (int x = box_.x_min; x <= box_.x_max; ++x) {
+          screen.PixelAt(x, y).italic = true;
+        }
+      }
+      Node::Render(screen);
+    }
+  };
+
+  return std::make_shared<Impl>(std::move(child));
+}
+
+}  // namespace ftxui

src/ftxui/dom/italic_test.cpp

@@ -0,0 +1,22 @@
+// Copyright 2025 Arthur Sonzogni. All rights reserved.
+// Use of this source code is governed by the MIT license that can be found in
+// the LICENSE file.
+#include <string>  // for allocator, string
+
+#include "ftxui/dom/elements.hpp"   // for operator|, text, bold, Element
+#include "ftxui/dom/node.hpp"       // for Render
+#include "ftxui/screen/screen.hpp"  // for Screen, Pixel
+#include "gtest/gtest.h"  // for Test, AssertionResult, EXPECT_TRUE, Message, TEST, TestPartResult
+
+// NOLINTBEGIN
+namespace ftxui {
+
+TEST(ItalicTest, Basic) {
+  auto element = text("text") | italic;
+  Screen screen(5, 1);
+  Render(screen, element);
+  EXPECT_TRUE(screen.PixelAt(0, 0).italic);
+}
+
+}  // namespace ftxui
+// NOLINTEND

src/ftxui/screen/screen.cpp

@@ -106,6 +106,12 @@ void UpdatePixelStyle(const Screen* screen,
                          : "\x1B[27m");  // INVERTED_RESET
   }
 
+  // Italics
+  if (FTXUI_UNLIKELY(next.italic != prev.italic)) {
+    ss << (next.italic ? "\x1B[3m"     // ITALIC_SET
+                       : "\x1B[23m");  // ITALIC_RESET
+  }
+
   // StrikeThrough
   if (FTXUI_UNLIKELY(next.strikethrough != prev.strikethrough)) {
     ss << (next.strikethrough ? "\x1B[9m"     // CROSSED_OUT