Fix banding and discoloration issue in 3D scene

src/plugins/minimal_scene/MinimalScene.cc

@@ -1073,7 +1073,10 @@ void TextureNode::NewTexture(void* _texturePtr, const QSize &_size)
 /////////////////////////////////////////////////
 void TextureNode::PrepareNode()
 {
-  this->rhi->PrepareNode();
+  {
+    std::unique_lock<std::mutex> lock(this->renderSync.mutex);
+    this->rhi->PrepareNode();
+  }
 
   if (this->rhi->HasNewTexture())
   {
@@ -1085,6 +1088,7 @@ void TextureNode::PrepareNode()
     // rendered and it can start rendering to the other one.
     // emit TextureInUse(&this->renderSync); See comment below
   }
+
   // NOTE: The original code from Qt samples only emitted when
   // newId is not null.
   //
@@ -1545,12 +1549,6 @@ void MinimalScene::LoadConfig(const tinyxml2::XMLElement *_pluginElem)
   }
 
   renderWindow->SetEngineName(cmdRenderEngine);
-  // there is a problem with displaying ogre2 render textures that are in
-  // sRGB format. Workaround for now is to apply gamma correction
-  // manually.
-  // There maybe a better way to solve the problem by making OpenGL calls.
-  if (cmdRenderEngine == std::string("ogre2"))
-    this->PluginItem()->setProperty("gammaCorrect", true);
 }
 
 /////////////////////////////////////////////////

src/plugins/minimal_scene/MinimalScene.qml

@@ -54,17 +54,6 @@ Rectangle {
     visible: MinimalScene.loadingError.length == 0
   }
 
-  /*
-   * Gamma correction for sRGB output. Enabled when engine is set to ogre2
-   */
-  GammaAdjust {
-      anchors.fill: renderWindow
-      source: renderWindow
-      gamma: 2.4
-      enabled: gammaCorrect
-      visible: gammaCorrect
-  }
-
   onParentChanged: {
     if (undefined === parent)
       return;

src/plugins/minimal_scene/MinimalSceneRhiOpenGL.cc

@@ -52,8 +52,19 @@ namespace gz::gui::plugins
 
   class TextureNodeRhiOpenGLPrivate
   {
+    // Current texture id from the render engine
     public: GLuint textureId = 0;
+
+    // New texture id from the render engine
     public: GLuint newTextureId = 0;
+
+    // Intermediate FBO for copying texture data
+    public: GLuint rhiFbo  = 0;
+
+    // Texture id for internal texture that holds texture data from
+    // the render engine texture. The texture data is in GL_RGB format
+    public: GLuint rhiTextureId = 0;
+
     public: QSize size {0, 0};
     public: QSize newSize {0, 0};
     public: QMutex mutex;
@@ -195,6 +206,13 @@ void RenderThreadRhiOpenGL::ShutDown()
 /////////////////////////////////////////////////
 TextureNodeRhiOpenGL::~TextureNodeRhiOpenGL()
 {
+  // clean up opengl resources owned by this rhi
+  QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
+  if (this->dataPtr->rhiFbo)
+    f->glDeleteFramebuffers(1, &this->dataPtr->rhiFbo);
+  if (this->dataPtr->rhiTextureId)
+    f->glDeleteTextures(1, &this->dataPtr->rhiTextureId);
+
   delete this->dataPtr->texture;
   this->dataPtr->texture = nullptr;
 }
@@ -265,13 +283,63 @@ void TextureNodeRhiOpenGL::PrepareNode()
     delete this->dataPtr->texture;
     this->dataPtr->texture = nullptr;
 
+    GLuint texForQt = this->dataPtr->newTextureId;
+
+    // Get the internal texture format of the GL texture from the
+    // rendering engine
+    QOpenGLFunctions *f = QOpenGLContext::currentContext()->functions();
+    f->glBindTexture(GL_TEXTURE_2D, this->dataPtr->newTextureId);
+    GLint texFormat = 0;
+    glGetTexLevelParameteriv(
+        GL_TEXTURE_2D,
+        0,
+        GL_TEXTURE_INTERNAL_FORMAT,
+        &texFormat
+    );
+    f->glBindTexture(GL_TEXTURE_2D, 0);
+
+    // if render engine's texture is in GL_SRGB format, convert it to GL_RGB
+    // as that is the format of QSG textures, otherwise the scene will appear
+    // dark.
+    if (texFormat == GL_SRGB || texFormat == GL_SRGB8 ||
+        texFormat == GL_SRGB_ALPHA || texFormat == GL_SRGB8_ALPHA8)
+    {
+      // Get the currently bound fbo so we can restore it later.
+      GLint currentFbo;
+      f->glGetIntegerv(GL_FRAMEBUFFER_BINDING, &currentFbo);
+
+      // Bind the texture from the rendering engine to our own RHI fbo
+      if (!this->dataPtr->rhiFbo)
+        f->glGenFramebuffers(1, &this->dataPtr->rhiFbo);
+      f->glBindFramebuffer(GL_FRAMEBUFFER, this->dataPtr->rhiFbo);
+      f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+          GL_TEXTURE_2D, this->dataPtr->newTextureId, 0);
+
+      // Copy the source render engine texture data to our own internal texture
+      // The copied texture is the one we will pass to Qt
+      if (!this->dataPtr->rhiTextureId)
+        f->glGenTextures(1, &this->dataPtr->rhiTextureId);
+      f->glBindTexture(GL_TEXTURE_2D, this->dataPtr->rhiTextureId);
+      f->glCopyTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, 0, 0,
+          this->dataPtr->newSize.width(), this->dataPtr->newSize.height(), 0);
+
+      // Unbind the render engine texture from RHI fbo and rebind the
+      // previously bound FBO.
+      f->glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0,
+          GL_TEXTURE_2D, 0, 0);
+      f->glBindTexture(GL_TEXTURE_2D, 0);
+      f->glBindFramebuffer(GL_FRAMEBUFFER, (GLuint) currentFbo);
+
+      texForQt = this->dataPtr->rhiTextureId;
+    }
+
 #if QT_VERSION < QT_VERSION_CHECK(5, 14, 0)
 # ifndef _WIN32
 #   pragma GCC diagnostic push
 #   pragma GCC diagnostic ignored "-Wdeprecated-declarations"
 # endif
     this->dataPtr->texture = this->dataPtr->window->createTextureFromId(
-        this->dataPtr->newTextureId,
+        texForQt,
         this->dataPtr->newSize,
         QQuickWindow::TextureIsOpaque);
 # ifndef _WIN32
@@ -281,7 +349,7 @@ void TextureNodeRhiOpenGL::PrepareNode()
     this->dataPtr->texture =
         this->dataPtr->window->createTextureFromNativeObject(
             QQuickWindow::NativeObjectTexture,
-            static_cast<void*>(&this->dataPtr->newTextureId),
+            static_cast<void*>(&texForQt),
             0,
             this->dataPtr->newSize);
 #endif