Add offscreen render targets and rename AssetRegistry to ResourceRegistry

Add a RenderTarget resource so the scene can render offscreen instead of
straight to the swapchain. The editor uses it to show the scene inside the
Viewport panel.

- Rename AssetRegistry to ResourceRegistry (AssetTraits -> ResourceTraits,
  assets() -> resources()) and move the pools from engine/src/assets to
  engine/src/resources. It now covers non-asset GPU resources as well.
- Replace loadMesh() with create<T>(...), and add create<RenderTarget>(),
  requestResize(), getExtent() and isAlive().
- Add RenderTarget and RenderTargetPool with RenderTargetHandle.
  Resizes are deferred and applied at the start of the next frame, when the
  GPU is idle.
- Add Application::setSceneTarget(). With no target set, the scene still
  renders to the swapchain. With a target set, the scene renders offscreen
  and the swapchain receives only the UI. Frame extent and camera projection
  follow the target's size.
- Add sophia::UI (Application::ui()) with image() and getTextureId() to draw
  a render target in an ImGui window, sizing the target to the available
  space. ImGuiRenderer now registers the target textures for this.
- ViewportPanel takes the UI and a target, and draws the scene image in place
  of the flat background fill.
- Update the testbed, the boids example, and the glossary for the rename.

engine/src/application.cpp

@@ -20,6 +20,7 @@
 #include "mesh.hpp"
 #include "pipelines/graphics_pipeline.hpp"
 #include "render_context.hpp"
+#include "render_target.hpp"
 #include "shader.hpp"
 #include "systems/model_render_system.hpp"
 #include "window.hpp"
@@ -41,6 +42,38 @@ struct PushConstants {
 
 std::vector<std::string> commandLineArgs;
 
+void beginColorPass(vk::CommandBuffer commandBuffer, vk::ImageView view,
+                    vk::Extent2D extent) {
+  vk::ClearValue clearValue{};
+  clearValue.color =
+      vk::ClearColorValue{std::array<float, 4>{0.01f, 0.01f, 0.02f, 1.0f}};
+
+  vk::RenderingAttachmentInfo colorAttachment{};
+  colorAttachment.imageView = view;
+  colorAttachment.imageLayout = vk::ImageLayout::eColorAttachmentOptimal;
+  colorAttachment.loadOp = vk::AttachmentLoadOp::eClear;
+  colorAttachment.storeOp = vk::AttachmentStoreOp::eStore;
+  colorAttachment.clearValue = clearValue;
+
+  vk::RenderingInfo renderingInfo{};
+  renderingInfo.renderArea = vk::Rect2D{{0, 0}, extent};
+  renderingInfo.layerCount = 1;
+  renderingInfo.colorAttachmentCount = 1;
+  renderingInfo.pColorAttachments = &colorAttachment;
+
+  commandBuffer.beginRendering(renderingInfo);
+
+  vk::Viewport viewport{0.0f,
+                        0.0f,
+                        static_cast<float>(extent.width),
+                        static_cast<float>(extent.height),
+                        0.0f,
+                        1.0f};
+  vk::Rect2D scissor{{0, 0}, extent};
+  commandBuffer.setViewport(0, viewport);
+  commandBuffer.setScissor(0, scissor);
+}
+
 }  // namespace
 
 class Application::Impl {
@@ -55,7 +88,9 @@ class Application::Impl {
   void run() {
     ModelRenderSystem modelRenderSystem{device, renderContext};
     ImGuiRenderer imguiRenderer{window, device, renderContext.getImageFormat(),
-                                renderContext.getImageCount()};
+                                renderContext.getImageCount(),
+                                this->application.resourceRegistry};
+    this->application._ui.renderer = &imguiRenderer;
 
     this->application.consoleCommandRegistry.registerCommand(
         "reset_layout", "Restore the default panel layout",
@@ -83,10 +118,22 @@ class Application::Impl {
       double elapsedTime =
           std::chrono::duration<double>(currentTime - startTime).count();
 
+      // Resizing waits for the GPU, so it has to happen before any of this
+      // frame's work (or ImGui's references to the targets) is recorded.
+      this->application.resourceRegistry.applyPendingResizes();
+
+      RenderTarget* sceneTarget =
+          this->sceneTargetHandle.isValid()
+              ? &this->application.resourceRegistry.getRenderTarget(
+                    this->sceneTargetHandle)
+              : nullptr;
+
       vk::Extent2D extent = renderContext.getExtent();
+      vk::Extent2D sceneExtent =
+          sceneTarget ? sceneTarget->getExtent() : extent;
       FrameInfo frameInfo{frameIndex, elapsedTime, deltaTime,
-                          glm::vec2(static_cast<float>(extent.width),
-                                    static_cast<float>(extent.height)),
+                          glm::vec2(static_cast<float>(sceneExtent.width),
+                                    static_cast<float>(sceneExtent.height)),
                           this->application.camera};
       this->application.onUpdate(frameInfo);
       this->application.camera.setProjection(
@@ -105,38 +152,6 @@ class Application::Impl {
       this->application.panelRegistry.update(frameInfo);
       this->application.onRender();
 
-      renderContext.transitionCurrentImage(
-          commandBuffer, vk::ImageLayout::eColorAttachmentOptimal);
-
-      vk::ClearValue clearValue{};
-      clearValue.color =
-          vk::ClearColorValue{std::array<float, 4>{0.01f, 0.01f, 0.02f, 1.0f}};
-
-      vk::RenderingAttachmentInfo colorAttachment{};
-      colorAttachment.imageView = renderContext.getCurrentImageView();
-      colorAttachment.imageLayout = vk::ImageLayout::eColorAttachmentOptimal;
-      colorAttachment.loadOp = vk::AttachmentLoadOp::eClear;
-      colorAttachment.storeOp = vk::AttachmentStoreOp::eStore;
-      colorAttachment.clearValue = clearValue;
-
-      vk::RenderingInfo renderingInfo{};
-      renderingInfo.renderArea = vk::Rect2D{{0, 0}, extent};
-      renderingInfo.layerCount = 1;
-      renderingInfo.colorAttachmentCount = 1;
-      renderingInfo.pColorAttachments = &colorAttachment;
-
-      commandBuffer.beginRendering(renderingInfo);
-
-      vk::Viewport viewport{0.0f,
-                            0.0f,
-                            static_cast<float>(extent.width),
-                            static_cast<float>(extent.height),
-                            0.0f,
-                            1.0f};
-      vk::Rect2D scissor{{0, 0}, extent};
-      commandBuffer.setViewport(0, viewport);
-      commandBuffer.setScissor(0, scissor);
-
       std::vector<ModelRenderSystem::MeshInstance> instances;
       instances.reserve(this->meshes.size() + this->drawCalls.size());
       for (const std::unique_ptr<Mesh>& mesh : this->meshes) {
@@ -144,10 +159,28 @@ class Application::Impl {
       }
       instances.insert(instances.end(), this->drawCalls.begin(),
                        this->drawCalls.end());
-
-      modelRenderSystem.render(commandBuffer, instances, frameInfo);
       this->drawCalls.clear();
 
+      if (sceneTarget) {
+        Image& targetImage = sceneTarget->getImage();
+        targetImage.transitionLayout(commandBuffer,
+                                     vk::ImageLayout::eColorAttachmentOptimal);
+        beginColorPass(commandBuffer, targetImage.getView(), sceneExtent);
+        modelRenderSystem.render(commandBuffer, instances, frameInfo);
+        commandBuffer.endRendering();
+        targetImage.transitionLayout(commandBuffer,
+                                     vk::ImageLayout::eShaderReadOnlyOptimal);
+      }
+
+      renderContext.transitionCurrentImage(
+          commandBuffer, vk::ImageLayout::eColorAttachmentOptimal);
+      beginColorPass(commandBuffer, renderContext.getCurrentImageView(),
+                     extent);
+
+      if (!sceneTarget) {
+        modelRenderSystem.render(commandBuffer, instances, frameInfo);
+      }
+
       imguiRenderer.endFrame(commandBuffer);
 
       commandBuffer.endRendering();
@@ -164,6 +197,8 @@ class Application::Impl {
 
     device.waitIdle();
 
+    this->application._ui.renderer = nullptr;
+
     auto endTime = std::chrono::high_resolution_clock::now();
     double totalRuntime =
         std::chrono::duration<double>(endTime - startTime).count();
@@ -193,6 +228,14 @@ class Application::Impl {
     this->drawCalls.push_back({&mesh, transform});
   }
 
+  void setSceneTarget(RenderTargetHandle handle) {
+    this->sceneTargetHandle = handle;
+  }
+
+  u32 getColorFormat() const {
+    return static_cast<u32>(this->renderContext.getImageFormat());
+  }
+
   Device& getDevice() { return device; }
 
  private:
@@ -204,6 +247,8 @@ class Application::Impl {
 
   std::vector<std::unique_ptr<Mesh>> meshes;
   std::vector<ModelRenderSystem::MeshInstance> drawCalls;
+
+  RenderTargetHandle sceneTargetHandle = RenderTargetHandle::invalid();
 };
 
 void Application::setCommandLineArgs(int argc, const char* const* argv) {
@@ -214,7 +259,8 @@ Application::Application(const Config& config)
     : impl{std::make_unique<Impl>(*this, config)},
       canvas{this},
       args{std::move(commandLineArgs)},
-      assetRegistry{impl->getDevice()},
+      resourceRegistry{impl->getDevice(), impl->getColorFormat()},
+      _ui{resourceRegistry},
       _debugUI{this->panelRegistry, this->consoleCommandRegistry} {}
 
 Application::~Application() = default;
@@ -226,10 +272,14 @@ void Application::pushMesh(std::vector<CanvasVertex> vertices,
   impl->pushMesh(std::move(vertices), std::move(indices));
 }
 
+void Application::setSceneTarget(RenderTargetHandle handle) {
+  impl->setSceneTarget(handle);
+}
+
 void Application::clearMeshes() { impl->clearMeshes(); }
 
 void Application::drawMesh(MeshHandle handle, const glm::mat4& transform) {
-  impl->drawMesh(assetRegistry.get<Mesh>(handle), transform);
+  impl->drawMesh(resourceRegistry.get<Mesh>(handle), transform);
 }
 
 }  // namespace sophia

engine/src/assets/asset_registry.cpp

@@ -1,60 +0,0 @@
-#include <sophia/asset_registry.hpp>
-#include <sophia/util/assert.hpp>
-
-#include "mesh.hpp"
-#include "pools/mesh_pool.hpp"
-
-namespace sophia {
-
-class AssetRegistry::Impl {
- public:
-  explicit Impl(Device& device) : meshPool{device} {}
-
-  MeshPool meshPool;
-  // TexturePool texturePool;
-  // MaterialPool materialPool;
-};
-
-AssetRegistry::AssetRegistry(Device& device)
-    : impl{std::make_unique<Impl>(device)} {}
-
-AssetRegistry::~AssetRegistry() = default;
-
-template <>
-MeshHandle AssetRegistry::load<Mesh>(const std::string& path) {
-  (void)path;
-  SOPHIA_ASSERT(false,
-                "Mesh::load(path) is not implemented yet - no model file "
-                "format is supported; build one via Mesh::Builder for now");
-  return MeshHandle{};
-}
-
-template <typename T>
-typename AssetTraits<T>::Handle load(const std::string&) {
-  static_assert(sizeof(T) == 0,
-                "load<T> is not implemented for this asset type");
-}
-
-template <>
-void AssetRegistry::release<Mesh>(MeshHandle handle) {
-  impl->meshPool.release(handle);
-}
-
-template <>
-const Mesh& AssetRegistry::get<Mesh>(MeshHandle handle) const {
-  return impl->meshPool.get(handle);
-}
-
-MeshHandle AssetRegistry::loadMesh(std::vector<CanvasVertex> vertices,
-                                   std::vector<u32> indices) {
-  Mesh::Builder builder{};
-  builder.vertices.reserve(vertices.size());
-  for (const CanvasVertex& vertex : vertices) {
-    builder.vertices.push_back({vertex.position, vertex.color});
-  }
-  builder.indices = std::move(indices);
-
-  return impl->meshPool.create(builder);
-}
-
-}  // namespace sophia

engine/src/image.cpp

@@ -63,6 +63,18 @@ void Image::transitionLayout(vk::CommandBuffer commandBuffer,
     barrier.dstAccessMask = {};
     srcStage = vk::PipelineStageFlagBits::eColorAttachmentOutput;
     dstStage = vk::PipelineStageFlagBits::eBottomOfPipe;
+  } else if (this->layout == vk::ImageLayout::eColorAttachmentOptimal &&
+             newLayout == vk::ImageLayout::eShaderReadOnlyOptimal) {
+    barrier.srcAccessMask = vk::AccessFlagBits::eColorAttachmentWrite;
+    barrier.dstAccessMask = vk::AccessFlagBits::eShaderRead;
+    srcStage = vk::PipelineStageFlagBits::eColorAttachmentOutput;
+    dstStage = vk::PipelineStageFlagBits::eFragmentShader;
+  } else if (this->layout == vk::ImageLayout::eShaderReadOnlyOptimal &&
+             newLayout == vk::ImageLayout::eColorAttachmentOptimal) {
+    barrier.srcAccessMask = vk::AccessFlagBits::eShaderRead;
+    barrier.dstAccessMask = vk::AccessFlagBits::eColorAttachmentWrite;
+    srcStage = vk::PipelineStageFlagBits::eFragmentShader;
+    dstStage = vk::PipelineStageFlagBits::eColorAttachmentOutput;
   } else if (this->layout == vk::ImageLayout::ePresentSrcKHR &&
              newLayout == vk::ImageLayout::eColorAttachmentOptimal) {
     barrier.srcAccessMask = {};

engine/src/imgui_renderer.cpp

@@ -5,6 +5,10 @@
 #include <imgui_impl_vulkan.h>
 #include <imgui_internal.h>
 
+#include <sophia/resource_registry.hpp>
+
+#include "render_target.hpp"
+
 namespace sophia {
 
 namespace {
@@ -20,7 +24,8 @@ void dockAll(const std::vector<std::string>& windows, ImGuiID nodeId) {
 
 ImGuiRenderer::ImGuiRenderer(Window& window, Device& device,
                              vk::Format colorAttachmentFormat,
-                             u32 minImageCount) {
+                             u32 minImageCount, ResourceRegistry& resources)
+    : resources{resources} {
   // 1. Construct ImGui context
   IMGUI_CHECKVERSION();
   ImGui::CreateContext();
@@ -67,12 +72,17 @@ ImGuiRenderer::ImGuiRenderer(Window& window, Device& device,
 }
 
 ImGuiRenderer::~ImGuiRenderer() {
+  for (const auto& [index, texture] : textures) {
+    ImGui_ImplVulkan_RemoveTexture(texture.descriptorSet);
+  }
   ImGui_ImplVulkan_Shutdown();
   ImGui_ImplGlfw_Shutdown();
   ImGui::DestroyContext();
 }
 
 void ImGuiRenderer::beginFrame() {
+  releaseStaleTextures();
+
   ImGui_ImplVulkan_NewFrame();
   ImGui_ImplGlfw_NewFrame();
   ImGui::NewFrame();
@@ -82,6 +92,45 @@ void ImGuiRenderer::beginFrame() {
   applyLayoutIfNeeded(dockspaceId, DOCKSPACE_FLAGS);
 }
 
+ImTextureID ImGuiRenderer::getTextureId(RenderTargetHandle handle) {
+  vk::ImageView view =
+      std::as_const(resources).getRenderTarget(handle).getImage().getView();
+
+  auto it = textures.find(handle.index);
+  if (it != textures.end()) {
+    if (it->second.generation == handle.generation && it->second.view == view) {
+      return (ImTextureID)it->second.descriptorSet;
+    }
+    ImGui_ImplVulkan_RemoveTexture(it->second.descriptorSet);
+    textures.erase(it);
+  }
+
+  // ImGui owns the sampler; it only needs the view and the layout the image is
+  // left in once the scene pass has finished.
+  VkDescriptorSet descriptorSet = ImGui_ImplVulkan_AddTexture(
+      static_cast<VkImageView>(view), VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL);
+  textures[handle.index] = {handle.generation, view, descriptorSet};
+  return (ImTextureID)descriptorSet;
+}
+
+// Resizing and releasing a target both wait for the GPU before destroying its
+// image, so nothing in flight can still be using a stale texture.
+void ImGuiRenderer::releaseStaleTextures() {
+  for (auto it = textures.begin(); it != textures.end();) {
+    RenderTargetHandle handle{it->first, it->second.generation};
+    bool stale =
+        !resources.isAlive(handle) ||
+        std::as_const(resources).getRenderTarget(handle).getImage().getView() !=
+            it->second.view;
+    if (stale) {
+      ImGui_ImplVulkan_RemoveTexture(it->second.descriptorSet);
+      it = textures.erase(it);
+    } else {
+      ++it;
+    }
+  }
+}
+
 void ImGuiRenderer::setDefaultLayout(DockLayout layout) {
   defaultLayout = std::move(layout);
   seedIfUnsaved = true;

engine/src/imgui_renderer.hpp

@@ -2,31 +2,50 @@
 
 #include <imgui.h>
 
+#include <sophia/handles.hpp>
 #include <sophia/types.hpp>
 #include <sophia/ui/dock_layout.hpp>
+#include <unordered_map>
 
 #include "device.hpp"
 #include "window.hpp"
 
 namespace sophia {
 
+class ResourceRegistry;
+
 class ImGuiRenderer {
  public:
   ImGuiRenderer(const ImGuiRenderer&) = delete;
   ImGuiRenderer& operator=(const ImGuiRenderer&) = delete;
 
   ImGuiRenderer(Window& window, Device& device,
-                vk::Format colorAttachmentFormat, u32 minImageCount);
+                vk::Format colorAttachmentFormat, u32 minImageCount,
+                ResourceRegistry& resources);
   ~ImGuiRenderer();
 
   void beginFrame();
   void endFrame(vk::CommandBuffer commandBuffer);
 
+  ImTextureID getTextureId(RenderTargetHandle handle);
+
   void setDefaultLayout(DockLayout layout);
   void resetLayout();
 
  private:
   void applyLayoutIfNeeded(ImGuiID dockspaceId, ImGuiDockNodeFlags flags);
+  void releaseStaleTextures();
+
+  struct Texture {
+    u32 generation;
+    vk::ImageView view;
+    VkDescriptorSet descriptorSet;
+  };
+
+  ResourceRegistry& resources;
+
+  // Keyed by RenderTargetHandle::index.
+  std::unordered_map<u32, Texture> textures;
 
   DockLayout defaultLayout;
   bool seedIfUnsaved = false;

engine/src/mesh.hpp

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <memory>
-#include <sophia/asset_registry.hpp>
+#include <sophia/resource_registry.hpp>
 #include <sophia/types.hpp>
 #include <vector>
 

engine/src/render_target.cpp

@@ -0,0 +1,52 @@
+#include "render_target.hpp"
+
+#include <algorithm>
+
+namespace sophia {
+
+RenderTarget::RenderTarget(Device& device, vk::Format format, u32 width,
+                           u32 height)
+    : device{device}, format{format} {
+  create(std::max(width, 1u), std::max(height, 1u));
+}
+
+RenderTarget::~RenderTarget() { destroy(); }
+
+vk::Extent2D RenderTarget::getExtent() const {
+  vk::Extent3D extent = this->image->getExtent();
+  return {extent.width, extent.height};
+}
+
+void RenderTarget::requestResize(u32 width, u32 height) {
+  this->pendingExtent = vk::Extent2D{std::max(width, 1u), std::max(height, 1u)};
+}
+
+void RenderTarget::applyPendingResize() {
+  if (!this->pendingExtent) {
+    return;
+  }
+
+  vk::Extent2D extent = *this->pendingExtent;
+  this->pendingExtent.reset();
+  if (extent == getExtent()) {
+    return;
+  }
+
+  // Earlier frames may still be sampling the old image (e.g. through the UI).
+  this->device.waitIdle();
+  destroy();
+  create(extent.width, extent.height);
+}
+
+void RenderTarget::create(u32 width, u32 height) {
+  this->image = Image::Builder(this->device)
+                    .setExtent(width, height)
+                    .setFormat(this->format)
+                    .asColorAttachment()
+                    .withView()
+                    .build();
+}
+
+void RenderTarget::destroy() { this->image.reset(); }
+
+}  // namespace sophia

engine/src/render_target.hpp

@@ -0,0 +1,51 @@
+#pragma once
+
+#include <memory>
+#include <optional>
+#include <sophia/types.hpp>
+#include <vulkan/vulkan.hpp>
+
+#include "device.hpp"
+#include "image.hpp"
+
+namespace sophia {
+
+/**
+ * A place rendering can go. Currently always backed by an offscreen color
+ * image.
+ *
+ * Owned by the ResourceRegistry and addressed through a RenderTargetHandle.
+ */
+class RenderTarget {
+ public:
+  RenderTarget(Device& device, vk::Format format, u32 width, u32 height);
+  ~RenderTarget();
+
+  RenderTarget(const RenderTarget&) = delete;
+  RenderTarget& operator=(const RenderTarget&) = delete;
+
+  Image& getImage() const { return *this->image; }
+
+  vk::Extent2D getExtent() const;
+
+  void requestResize(u32 width, u32 height);
+
+  /**
+   * Applies a pending resize. Waits for the GPU, so it must run before any of
+   * the frame's work (or any UI reference to the image) is recorded.
+   */
+  void applyPendingResize();
+
+ private:
+  void create(u32 width, u32 height);
+  void destroy();
+
+  Device& device;
+  vk::Format format;
+
+  std::unique_ptr<Image> image;
+
+  std::optional<vk::Extent2D> pendingExtent;
+};
+
+}  // namespace sophia

engine/src/resources/pools/mesh_pool.cpp

@@ -1,4 +1,4 @@
-#include "assets/pools/mesh_pool.hpp"
+#include "resources/pools/mesh_pool.hpp"
 
 #include <sophia/util/assert.hpp>
 

engine/src/resources/pools/render_target_pool.cpp

@@ -0,0 +1,81 @@
+#include "resources/pools/render_target_pool.hpp"
+
+#include <sophia/util/assert.hpp>
+#include <utility>
+
+namespace sophia {
+
+namespace {
+constexpr u32 GENERATION_MASK = (1u << 12) - 1;
+}  // namespace
+
+RenderTargetPool::RenderTargetPool(Device& device, vk::Format format)
+    : device{device}, format{format} {}
+
+RenderTargetPool::~RenderTargetPool() = default;
+
+RenderTargetHandle RenderTargetPool::create(u32 width, u32 height) {
+  auto target = std::make_unique<RenderTarget>(device, format, width, height);
+
+  if (!freeList.empty()) {
+    u32 index = freeList.back();
+    freeList.pop_back();
+    slots[index].target = std::move(target);
+    return RenderTargetHandle{index, slots[index].generation};
+  }
+
+  slots.push_back(Slot{std::move(target), 0});
+  return RenderTargetHandle{static_cast<u32>(slots.size() - 1), 0};
+}
+
+const RenderTargetPool::Slot& RenderTargetPool::validate(
+    RenderTargetHandle handle) const {
+  SOPHIA_ASSERT(handle.isValid(), "invalid RenderTargetHandle");
+  SOPHIA_ASSERT(handle.index < slots.size(),
+                "RenderTargetHandle index out of range:", handle.index);
+
+  const Slot& slot = slots[handle.index];
+  SOPHIA_ASSERT(
+      slot.generation == handle.generation,
+      "stale RenderTargetHandle (generation mismatch):", handle.index);
+  SOPHIA_ASSERT(
+      slot.target != nullptr,
+      "RenderTargetHandle refers to a released target:", handle.index);
+  return slot;
+}
+
+RenderTarget& RenderTargetPool::get(RenderTargetHandle handle) {
+  return const_cast<RenderTarget&>(std::as_const(*this).get(handle));
+}
+
+const RenderTarget& RenderTargetPool::get(RenderTargetHandle handle) const {
+  return *validate(handle).target;
+}
+
+void RenderTargetPool::release(RenderTargetHandle handle) {
+  validate(handle);
+
+  // Earlier frames may still be rendering into or sampling the target.
+  device.waitIdle();
+
+  Slot& slot = slots[handle.index];
+  slot.target.reset();
+  slot.generation = (slot.generation + 1) & GENERATION_MASK;
+  freeList.push_back(handle.index);
+}
+
+bool RenderTargetPool::isAlive(RenderTargetHandle handle) const {
+  return handle.isValid() && handle.index < slots.size() &&
+         slots[handle.index].generation == handle.generation &&
+         slots[handle.index].target != nullptr;
+}
+
+void RenderTargetPool::applyPendingResizes() {
+  for (Slot& slot : slots) {
+    if (slot.target) {
+      slot.target->applyPendingResize();
+    }
+  }
+}
+
+}  // namespace sophia

engine/src/resources/pools/render_target_pool.hpp

@@ -0,0 +1,44 @@
+#pragma once
+
+#include <memory>
+#include <sophia/handles.hpp>
+#include <vector>
+
+#include "device.hpp"
+#include "render_target.hpp"
+
+namespace sophia {
+
+class RenderTargetPool {
+ public:
+  RenderTargetPool(Device& device, vk::Format format);
+  ~RenderTargetPool();
+
+  RenderTargetPool(const RenderTargetPool&) = delete;
+  RenderTargetPool& operator=(const RenderTargetPool&) = delete;
+
+  RenderTargetHandle create(u32 width, u32 height);
+  RenderTarget& get(RenderTargetHandle handle);
+  const RenderTarget& get(RenderTargetHandle handle) const;
+  void release(RenderTargetHandle handle);
+
+  bool isAlive(RenderTargetHandle handle) const;
+
+  void applyPendingResizes();
+
+ private:
+  struct Slot {
+    std::unique_ptr<RenderTarget> target;
+    u32 generation = 0;
+  };
+
+  const Slot& validate(RenderTargetHandle handle) const;
+
+  Device& device;
+  vk::Format format;
+
+  std::vector<Slot> slots;
+  std::vector<u32> freeList;
+};
+
+}  // namespace sophia

engine/src/resources/resource_registry.cpp

@@ -0,0 +1,104 @@
+#include <sophia/resource_registry.hpp>
+#include <sophia/util/assert.hpp>
+
+#include "mesh.hpp"
+#include "pools/mesh_pool.hpp"
+#include "pools/render_target_pool.hpp"
+#include "render_target.hpp"
+
+namespace sophia {
+
+class ResourceRegistry::Impl {
+ public:
+  Impl(Device& device, vk::Format renderTargetFormat)
+      : meshPool{device}, renderTargetPool{device, renderTargetFormat} {}
+
+  MeshPool meshPool;
+  RenderTargetPool renderTargetPool;
+  // TexturePool texturePool;
+  // MaterialPool materialPool;
+};
+
+ResourceRegistry::ResourceRegistry(Device& device, u32 renderTargetFormat)
+    : impl{std::make_unique<Impl>(
+          device, static_cast<vk::Format>(renderTargetFormat))} {}
+
+ResourceRegistry::~ResourceRegistry() = default;
+
+template <>
+MeshHandle ResourceRegistry::load<Mesh>(const std::string& path) {
+  (void)path;
+  SOPHIA_ASSERT(false,
+                "Mesh::load(path) is not implemented yet - no model file "
+                "format is supported; build one via Mesh::Builder for now");
+  return MeshHandle{};
+}
+
+template <typename T>
+typename ResourceTraits<T>::Handle load(const std::string&) {
+  static_assert(sizeof(T) == 0,
+                "load<T> is not implemented for this resource type");
+}
+
+template <>
+void ResourceRegistry::release<Mesh>(MeshHandle handle) {
+  impl->meshPool.release(handle);
+}
+
+template <>
+void ResourceRegistry::release<RenderTarget>(RenderTargetHandle handle) {
+  impl->renderTargetPool.release(handle);
+}
+
+template <>
+const Mesh& ResourceRegistry::get<Mesh>(MeshHandle handle) const {
+  return impl->meshPool.get(handle);
+}
+
+MeshHandle ResourceRegistry::createResource(std::type_identity<Mesh>,
+                                            std::vector<CanvasVertex> vertices,
+                                            std::vector<u32> indices) {
+  Mesh::Builder builder{};
+  builder.vertices.reserve(vertices.size());
+  for (const CanvasVertex& vertex : vertices) {
+    builder.vertices.push_back({vertex.position, vertex.color});
+  }
+  builder.indices = std::move(indices);
+
+  return impl->meshPool.create(builder);
+}
+
+RenderTargetHandle ResourceRegistry::createResource(
+    std::type_identity<RenderTarget>, u32 width, u32 height) {
+  return impl->renderTargetPool.create(width, height);
+}
+
+void ResourceRegistry::requestResize(RenderTargetHandle handle, u32 width,
+                                     u32 height) {
+  impl->renderTargetPool.get(handle).requestResize(width, height);
+}
+
+glm::vec2 ResourceRegistry::getExtent(RenderTargetHandle handle) const {
+  vk::Extent2D extent =
+      std::as_const(impl->renderTargetPool).get(handle).getExtent();
+  return {static_cast<float>(extent.width), static_cast<float>(extent.height)};
+}
+
+bool ResourceRegistry::isAlive(RenderTargetHandle handle) const {
+  return impl->renderTargetPool.isAlive(handle);
+}
+
+RenderTarget& ResourceRegistry::getRenderTarget(RenderTargetHandle handle) {
+  return impl->renderTargetPool.get(handle);
+}
+
+const RenderTarget& ResourceRegistry::getRenderTarget(
+    RenderTargetHandle handle) const {
+  return std::as_const(impl->renderTargetPool).get(handle);
+}
+
+void ResourceRegistry::applyPendingResizes() {
+  impl->renderTargetPool.applyPendingResizes();
+}
+
+}  // namespace sophia

engine/src/ui/ui.cpp

@@ -0,0 +1,27 @@
+#include <sophia/resource_registry.hpp>
+#include <sophia/ui/ui.hpp>
+#include <sophia/util/assert.hpp>
+
+#include "imgui_renderer.hpp"
+
+namespace sophia {
+
+void UI::image(RenderTargetHandle handle) {
+  const ImVec2 size = ImGui::GetContentRegionAvail();
+  if (size.x < 1.0f || size.y < 1.0f) {
+    return;
+  }
+
+  resources.requestResize(handle, static_cast<u32>(size.x),
+                          static_cast<u32>(size.y));
+  ImTextureID textureId = getTextureId(handle);
+  ImGui::Image(textureId, size);
+}
+
+ImTextureID UI::getTextureId(RenderTargetHandle handle) {
+  SOPHIA_ASSERT(renderer != nullptr,
+                "UI::getTextureId() called before the UI was set up");
+  return renderer->getTextureId(handle);
+}
+
+}  // namespace sophia