OpenVR-AdvancedSettings-frame/src/render/RenderBackend.h

// Adapted from the vrdashboard template (FrameAssistant commit 4a5b090),
// verified on the Steam Frame. See docs/steamframe/DESIGN.md, section 3.

#pragma once

#include <openvr.h>

#include <QByteArrayList>
#include <QImage>
#include <QQuickRenderTarget>
#include <QSGRendererInterface>
#include <QSize>
#include <QString>
#include <functional>
#include <memory>

class QQuickWindow;

// Requirements the VR runtime imposes on the graphics device. All optional:
// without them (snapshot mode, tests) backends pick defaults.
struct RenderInterop
{
    std::function<QByteArrayList()> vulkanInstanceExtensions;
    std::function<QByteArrayList( VkPhysicalDevice_T* )> vulkanDeviceExtensions;
    // The physical device driving the HMD, for a given VkInstance.
    std::function<VkPhysicalDevice_T*( VkInstance_T* )> vulkanOutputDevice;
};

// Abstraction over the graphics API used to render the Qt Quick scene into a
// texture that SteamVR can consume. Everything API-specific (context/device
// setup, texture allocation, layout transitions, handing the texture to OpenVR,
// CPU readback) lives behind this interface; OverlayController and the
// VR/input code stay API-agnostic.
//
// Call order (driven by OverlayController):
//   initialize → configureWindow → [QQuickRenderControl::initialize] →
//   attachWindow → createRenderTarget → per frame: beginFrame … [Qt renders] …
//   finishFrame → vrTexture → releaseRenderTarget → detachWindow → [Qt destroys
//   its device/context]
class RenderBackend
{
public:
    virtual ~RenderBackend() = default;

    virtual QString name() const = 0;

    // Must be applied via QQuickWindow::setGraphicsApi() before any
    // QQuickWindow exists.
    virtual QSGRendererInterface::GraphicsApi graphicsApi() const = 0;

    // Creates API-level objects (context / instance). Requires a
    // QGuiApplication.
    virtual bool initialize( const RenderInterop& interop, QString* error ) = 0;

    // Tells Qt Quick which device/context/instance to use. Before
    // QQuickRenderControl::initialize().
    virtual void configureWindow( QQuickWindow* window ) = 0;

    // Picks up objects Qt created during QQuickRenderControl::initialize()
    // (e.g. VkDevice).
    virtual bool attachWindow( QQuickWindow* window, QString* error ) = 0;
    // Releases everything created on Qt's device; called before Qt destroys
    // it.
    virtual void detachWindow() = 0;

    // Human-readable GPU/driver description (valid after attachWindow).
    virtual QString deviceName() const = 0;

    // Bind the API context to the current thread (no-op for APIs without
    // one).
    virtual bool makeCurrent() = 0;
    virtual void doneCurrent() = 0;

    // (Re)allocates the shared color texture; returns the target for
    // QQuickWindow::setRenderTarget.
    virtual QQuickRenderTarget createRenderTarget( QSize size ) = 0;
    // Frees the texture. Called before Qt's device/context goes away.
    virtual void releaseRenderTarget() = 0;

    // Before Qt renders a frame / after QQuickRenderControl::endFrame().
    // Backends put the texture into the state Qt resp. SteamVR expects
    // (layouts, flushes).
    virtual void beginFrame() = 0;
    virtual void finishFrame() = 0;

    // Blocks until the GPU has finished all submitted work (benchmarks:
    // makes the asynchronous OpenGL path comparable to the synchronous
    // Vulkan one).
    virtual void waitForGpu() = 0;

    // Texture description for IVROverlay::SetOverlayTexture. Only valid
    // while the render target exists; must be submitted with the context
    // current (see makeCurrent()).
    virtual vr::Texture_t vrTexture() = 0;

    // UV bounds for IVROverlay::SetOverlayTextureBounds (compensates
    // API-specific origin).
    virtual vr::VRTextureBounds_t vrTextureBounds() const = 0;

    // Reads the last finished frame back to the CPU (snapshots, tests).
    // Slow.
    virtual QImage readback() = 0;
};

enum class GraphicsApi
{
    OpenGL,
    Vulkan
};

// Returns nullptr if the API was not compiled in (Vulkan is optional, see
// OVRAS_HAS_VULKAN).
std::unique_ptr<RenderBackend> createRenderBackend( GraphicsApi api );

// "opengl" / "vulkan" (case-insensitive). Returns false for unknown names.
bool parseGraphicsApi( const QString& name, GraphicsApi* api );