NoesisGUI

Rendering Architecture

NoesisGUI rendering architecture is designed to have two threads working in parallel: the ui thread and the render thread. Although, as explained later, this is not strictly mandatory. You can have more threads or even have no extra threads at all. Under no circumstances our core library will create threads under the hood. This responsibility is delegated to the client who is in charge of invoking Noesis from the appropriate thread.

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UI Thread

This is the thread where all logical interactions between the user and the application happen. Things like event dispatching and layout processing occurs in this thread. Note that depending on the complexity of the application this can be the Main thread or a separate UI thread.

The object in charge of providing this interaction is the View. It can be created by calling GUI::CreateView().

// Loads XAML and creates a view with it
Ptr<FrameworkElement> xaml = Noesis::GUI::LoadXaml<FrameworkElement>("Button.xaml");
Ptr<IView> view = Noesis::GUI::CreateView(xaml);

// Sets logical size
view->SetSize(width, height);

View instances are not thread-safe. All invocations must happen in the same thread where view was created.

// Send input events to view
view->MouseButtonDown(x, y, button);
view->MouseButtonUp(x, y, button);
view->MouseDoubleClick(x, y, button);
view->MouseMove(x, y);

NOTE

Any violation of this thread-safety is reported as an error in 'checked' builds.

Once per frame the view instance must be ticked to update its internal representation. At this step the current state is locked and stored to be consumed by the render thread described in the next section. From a high-level perspective this can be seen as taking a frame snapshot of the current UI state.

// Updates view
view->Update(time);

Note that although you can only interact with the view from the owner thread, you are allowed to create several views in different threads. This way you can update each view in parallel. Although the overhead of each view is low, it is recommended to only have one view per surface. In a normal scenario, you create a view for the main camera and a separate view for each render texture that is needed, reusing them as much as possible.

NOTE

'Update' will block if it is invoked many times without the corresponding 'UpdateRenderTree' call in the render thread

Render Thread

This is the thread that directly interacts with the GPU through the RenderDevice abstract class. Being Noesis rendering agnostic, it is client code responsibility to provide one RenderDevice implementation. The Application Framework provides many reference implementations that can be used as a starting point to create a custom one. RenderDevice exposes a few functions to control the internal Vector Graphics context created. Default values should work well for most scenarios.

Ptr<GLRenderDevice> device = *new GLRenderDevice();
device->SetGlyphCacheWidth(2048);
device->SetGlyphCacheHeight(2048);
device->SetOffscreenSampleCount(1);

Each view contains a renderer that must be initialize with a RenderDevice implementation. All interactions with the view in the render thread are isolated through the interface IRenderer. This interface is obtained by calling GetRenderer().

view->GetRenderer()->Init(device);

Each time you need to render a new frame you must call UpdateRenderTree() to collect pending update commands from the UI thread. This is like grabbing a frame from the UI thread. The rendering is decomposed into two stages:

  • Offscreen (RenderOffscreen()). Sometimes, for example when using effects like shadows or when using opacity groups, the view needs to render content to intermediate textures. Those drawing commands happen at this stage. It is very important that this stage takes place before binding the main surface. This is especially important in tiled architectures.
  • Onscreen (Render()). At this point the view renders primitives to the currently active framebuffer. For example, when rendering HUD interfaces this stage happens after rendering 3D content and before swapping or resolving the back buffer.

NOTE

IntegrationGLUT is a minimalist integration sample showcasing these concepts.

// Applies last changes happened in view
view->GetRenderer()->UpdateRenderTree();

// Generates offscreen textures
view->GetRenderer()->RenderOffscreen();

// ------->
// HERE: Insert code to render your 3D scene
// <-------

// Render UI in the active render target and viewport dimensions
view->GetRenderer()->Render();

Note that several views can be managed in the same render thread just by initializing all of them with the same render device. That way internal resources like ramps and glyphs are shared across all views. RenderDevice is a heavyweight class. Extra instances should be avoided whenever possible.

NOTE

Extra parallelization may happen at RenderDevice implementation level. For example, using deferred contexts in D3D11 or similar concepts in other APIs. Doing this, generating GPU commands for the UI can be done in parallel with the rest of frame commands. This is useful for architectures already using Jobs or Tasks.

The common scenario is having just one render thread. But sometimes interacting with different render devices in different threads is necessary. In this case, each thread must have its own instance of RenderDevice. Instances of RenderDevice are not thread-safe and must not be shared. Following this pattern, each render thread is in in charge of collecting updates from all the views that it initialized.

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