QuaternionEngine/docs/RenderGraph.md

Render Graph: Per‑Frame Scheduling, Barriers, and Dynamic Rendering

Lightweight render graph that builds a per‑frame DAG from pass declarations, computes the necessary resource barriers/layout transitions, and records passes with dynamic rendering when attachments are declared.

Why

• Centralize synchronization and image layout transitions across passes.
• Make passes declarative: author declares reads/writes; the graph inserts barriers and begins/ends rendering.
• Keep existing pass classes (IRenderPass) while migrating execution to the graph.

High‑Level Flow

• Engine creates the graph each frame and imports swapchain/G‑Buffer images: src/core/vk_engine.cpp:303.
• Each pass registers its work by calling register_graph(graph, ...) and declaring resources via a builder.
• The graph appends a present chain (copy HDR drawImage → swapchain, then transition to PRESENT), optionally inserting ImGui before present.
• compile() topologically sorts passes by data dependencies (read/write) and computes per‑pass barriers.
• execute(cmd) emits barriers, begins dynamic rendering if attachments were declared, calls the pass record lambda, and ends rendering.

Core API

• RenderGraph::add_pass(name, RGPassType type, BuildCallback build, RecordCallback record)

  • Declare image/buffer accesses and attachments inside build using RGPassBuilder.
  • Do your actual rendering/copies in record using resolved Vulkan objects from RGPassResources.
  • See: src/render/rg_graph.h:36, src/render/rg_graph.cpp:51.

• RenderGraph::compile() → builds ordering and per‑pass Vk*MemoryBarrier2 lists. See src/render/rg_graph.cpp:83.

• RenderGraph::execute(cmd) → emits barriers and dynamic rendering begin/end. See src/render/rg_graph.cpp:592.

• Import helpers for engine images: import_draw_image(), import_depth_image(), import_gbuffer_*(), import_swapchain_image(index). See src/render/rg_graph.cpp:740.

• Present chain: add_present_chain(draw, swapchain, appendExtra) inserts Copy→Present passes and lets you inject extra passes (e.g., ImGui) in between. See src/render/rg_graph.cpp:705.

Declaring a Pass

Use register_graph(...) on your pass to declare resources and record work. The graph handles transitions and dynamic rendering.

void MyPass::register_graph(RenderGraph* graph,
                            RGImageHandle draw,
                            RGImageHandle depth) {
  graph->add_pass(
    "MyPass",
    RGPassType::Graphics,
    // Build: declare resources + attachments
    [draw, depth](RGPassBuilder& b, EngineContext*) {
      b.read(draw,  RGImageUsage::SampledFragment); // example read
      b.write_color(draw);                          // render target
      b.write_depth(depth, /*clear*/ false);        // depth test
    },
    // Record: issue Vulkan commands (no begin/end rendering needed)
    [this, draw](VkCommandBuffer cmd, const RGPassResources& res, EngineContext* ctx) {
      VkPipeline p{}; VkPipelineLayout l{};
      ctx->pipelines->getGraphics("my_pass", p, l); // hot‑reload safe
      vkCmdBindPipeline(cmd, VK_PIPELINE_BIND_POINT_GRAPHICS, p);
      VkViewport vp{0,0,(float)ctx->getDrawExtent().width,(float)ctx->getDrawExtent().height,0,1};
      vkCmdSetViewport(cmd, 0, 1, &vp);
      VkRect2D sc{{0,0}, ctx->getDrawExtent()};
      vkCmdSetScissor(cmd, 0, 1, &sc);
      vkCmdDraw(cmd, 3, 1, 0, 0);
    }
  );
}

Builder Reference (RGPassBuilder)

• Images

  • read(RGImageHandle, RGImageUsage) → sample/read usage for this pass.
  • write(RGImageHandle, RGImageUsage) → write usage (compute/storage/transfer).
  • write_color(RGImageHandle, bool clearOnLoad=false, VkClearValue clear={}) → declares a color attachment.
  • write_depth(RGImageHandle, bool clearOnLoad=false, VkClearValue clear={}) → declares a depth attachment.

• Buffers

  • read_buffer(RGBufferHandle, RGBufferUsage) / write_buffer(RGBufferHandle, RGBufferUsage).
  • Convenience import: read_buffer(VkBuffer, RGBufferUsage, size, name) and write_buffer(VkBuffer, ...) dedup by raw handle.

See src/render/rg_builder.h:39 and impl in src/render/rg_builder.cpp:20.

Resource Model (RGResourceRegistry)

• Imported vs transient resources are tracked uniformly with lifetime indices (firstUse/lastUse).
• Imports are deduplicated by VkImage/VkBuffer and keep initial layout/stage/access as the starting state.
• Transients are created via ResourceManager and auto‑destroyed at end of frame using the frame deletion queue.
• See src/render/rg_resources.h:11 and src/render/rg_resources.cpp:1.

Synchronization and Layouts

• For each pass, compile() compares previous state with desired usage and, if needed, adds a pre‑pass barrier:
  • Images: VkImageMemoryBarrier2 with stage/access/layout derived from RGImageUsage.
  • Buffers: VkBufferMemoryBarrier2 with stage/access derived from RGBufferUsage.
• Initial state comes from the imported descriptor; if unknown, buffers default to TOP_OF_PIPE.
• Format/usage checks:
  • Warns if binding a depth format as color (and vice‑versa).
  • Warns if a transient resource is used with flags it wasn’t created with.

Image usage → layout/stage examples (subset):

• SampledFragment → SHADER_READ_ONLY_OPTIMAL, FRAGMENT_SHADER.
• ColorAttachment → COLOR_ATTACHMENT_OPTIMAL, COLOR_ATTACHMENT_OUTPUT (read|write).
• DepthAttachment → DEPTH_ATTACHMENT_OPTIMAL, EARLY|LATE_FRAGMENT_TESTS.
• TransferDst → TRANSFER_DST_OPTIMAL, TRANSFER.
• Present → PRESENT_SRC_KHR, BOTTOM_OF_PIPE.

Buffer usage → stage/access examples:

• IndexRead → INDEX_INPUT, INDEX_READ.
• VertexRead → VERTEX_INPUT, VERTEX_ATTRIBUTE_READ.
• UniformRead → ALL_GRAPHICS|COMPUTE, UNIFORM_READ.
• StorageReadWrite → COMPUTE|FRAGMENT, SHADER_STORAGE_READ|WRITE.

Built‑In Pass Wiring (Current)

• Resource uploads (if any) → Background (compute) → Geometry (G‑Buffer) → Lighting (deferred) → Transparent → CopyToSwapchain → ImGui → PreparePresent.
• See registrations: src/core/vk_engine.cpp:321–src/core/vk_engine.cpp:352.

Notes & Limits

• No aliasing or transient pooling yet; images created via create_* are released end‑of‑frame.
• Graph scheduling uses a topological order by data dependency; it does not parallelize across queues.
• Load/store control for attachments is minimal (clearOnLoad, store on RGAttachmentInfo).
• Render area is the min of all declared attachment extents and EngineContext::drawExtent.

Debugging

• Each pass is wrapped with a debug label (RG: <name>).
• Compile prints warnings for suspicious usages or format mismatches.