ImGuiIntegration: test text rendering.

Mainly to test that font atlas upload and sampling works, especially
when using single-channel format + swizzling on OpenGL and GLES3

src/Magnum/ImGuiIntegration/Test/CMakeLists.txt

@@ -5,7 +5,7 @@
 #               2020, 2021, 2022, 2023, 2024
 #             Vladimír Vondruš <[email protected]>
 #   Copyright © 2018 Jonathan Hale <[email protected]>
-#   Copyright © 2022 Pablo Escobar <[email protected]>
+#   Copyright © 2022, 2024 Pablo Escobar <[email protected]>
 #
 #   Permission is hereby granted, free of charge, to any person obtaining a
 #   copy of this software and associated documentation files (the "Software"),
@@ -63,6 +63,7 @@ if(MAGNUM_BUILD_GL_TESTS)
             ContextTestFiles/draw.png
             ContextTestFiles/draw-scissor.png
             ContextTestFiles/draw-texture.png
+            ContextTestFiles/draw-text.png
             ContextTestFiles/texture.png)
     target_include_directories(ImGuiContextGLTest PRIVATE ${CMAKE_CURRENT_BINARY_DIR})
     if(MAGNUM_IMGUIINTEGRATION_BUILD_STATIC)

src/Magnum/ImGuiIntegration/Test/ContextGLTest.cpp

@@ -268,6 +268,7 @@ struct ContextGLTest: GL::OpenGLTester {
     void draw();
     void drawCallback();
     void drawTexture();
+    void drawText();
     void drawScissor();
     void drawVertexOffset();
     void drawIndexOffset();
@@ -313,6 +314,7 @@ ContextGLTest::ContextGLTest() {
     addTests({&ContextGLTest::draw,
               &ContextGLTest::drawCallback,
               &ContextGLTest::drawTexture,
+              &ContextGLTest::drawText,
               &ContextGLTest::drawScissor,
               &ContextGLTest::drawVertexOffset,
               &ContextGLTest::drawIndexOffset},
@@ -1397,6 +1399,51 @@ void ContextGLTest::drawTexture() {
         (DebugTools::CompareImageToFile{_manager, 1.0f, 0.5f}));
 }
 
+void ContextGLTest::drawText() {
+    Context c{{200, 200}};
+
+    /* Scale up default font so large text output is not a complete blurry mess */
+    ImGui::GetIO().Fonts->Clear();
+    auto* font = ImGui::GetIO().Fonts->AddFontDefault();
+    font->Scale = 8.0f;
+
+    c.relayout({200, 200}, {70, 70}, _framebuffer.viewport().size());
+
+    /* ImGui doesn't draw anything the first frame */
+    c.newFrame();
+    c.drawFrame();
+
+    MAGNUM_VERIFY_NO_GL_ERROR();
+
+    Utility::System::sleep(1);
+
+    c.newFrame();
+
+    /* Last drawlist that gets rendered, covers the entire display */
+    ImDrawList* drawList = ImGui::GetForegroundDrawList();
+    const ImVec2& size = ImGui::GetIO().DisplaySize;
+
+    drawList->AddRectFilled({size.x*0.1f, size.y*0.2f}, {size.x*0.9f, size.y*0.8f},
+        IM_COL32(255, 128, 128, 255));
+    drawList->AddText(nullptr, font->FontSize*font->Scale,
+        {size.x*0.3f, size.y*0.3f}, IM_COL32(255, 255, 0, 200), "ABC");
+
+    c.drawFrame();
+
+    MAGNUM_VERIFY_NO_GL_ERROR();
+
+    /* Catch also ABI and interface mismatch errors */
+    if(!(_manager.load("AnyImageImporter") & PluginManager::LoadState::Loaded) ||
+       !(_manager.load("PngImporter") & PluginManager::LoadState::Loaded))
+        CORRADE_SKIP("AnyImageImporter / PngImporter plugin can't be loaded.");
+
+    CORRADE_COMPARE_WITH(
+        /* Dropping the alpha channel, as it's always 1.0 */
+        Containers::arrayCast<Color3ub>(_framebuffer.read(_framebuffer.viewport(), {PixelFormat::RGBA8Unorm}).pixels<Color4ub>()),
+        Utility::Path::join(IMGUIINTEGRATION_TEST_DIR, "ContextTestFiles/draw-text.png"),
+        (DebugTools::CompareImageToFile{_manager, 1.0f, 0.5f}));
+}
+
 void ContextGLTest::drawScissor() {
     Context c{{200, 200}, {70, 70}, _framebuffer.viewport().size()};