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<main>
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<h1 id="framegraph"><a class="header" href="#framegraph">FrameGraph</a></h1>
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<p>FrameGraph is a framework within Filament for computing resources needed to
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render a frame. The framework enables declaring dependencies between resources.</p>
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<p>For example, when rendering shadows, we would need to first compute and store the
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shadow map into a texture resource, and then the later color pass would then
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sample that texture to attenuate the final output color. That creates a
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dependency on the shadow map from the color pass. Filament uses FrameGraph to
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declare that dependency.</p>
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<h2 id="details"><a class="header" href="#details">Details</a></h2>
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<h3 id="dependency-graph"><a class="header" href="#dependency-graph">Dependency Graph</a></h3>
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<p>The core of this framework is
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<a href="https://github.com/google/filament/blob/main/libs/gltfio/src/DependencyGraph.h">a class that defines a dependency graph</a> — that is, the class
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defines nodes and connections between nodes. This class makes assumptions about
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the types of its nodes. Like many other classes within Filament, this class is
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without virtual function declaration to avoid paying the cost of virtual calls.</p>
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<p>This class has additional functions to detect whether there is a cycle in the
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graph, and it is able to cull unreachable nodes.</p>
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<h3 id="framegraph-1"><a class="header" href="#framegraph-1">FrameGraph</a></h3>
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<p>A frame graph consists of two types of nodes</p>
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<ul>
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<li>Resource
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<ul>
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<li>This represents a generic resource such as a texture</li>
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<li>90% of the time, this is a texture.</li>
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</ul>
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</li>
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<li>Pass
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<ul>
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<li>This represents a "computation/rendering process"</li>
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<li>It takes a set of resources</li>
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<li>It outputs a set of resources</li>
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</ul>
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</li>
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</ul>
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<p>Edges can be created in the following three directions:</p>
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<ul>
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<li>Resource → Pass = A read</li>
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<li>Pass → Resource = A write</li>
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<li>Resource → Resource = A resource/subresource relationship.</li>
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</ul>
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<h3 id="an-example"><a class="header" href="#an-example">An example</a></h3>
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<p>To better understand FrameGraph, we consider the following graphical
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representation of a real graph. In this graph, blue nodes denote "Resources" and
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orange nodes denote "Passes."</p>
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<p><img src="../images/framegraph.png" alt="Sample frame graph" /></p>
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<p>In this graph, we see that the "Color Pass" takes as input the "Shadowmap",
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which has edges going into it, meaning that it's a texture array. The output of
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the "Color Pass" are "viewRenderTarget" and "Depth Buffer."</p>
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<p>Note that there is an outgoing edge from "viewRenderTarget", where the color
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buffer will be used as input in the post-processing passes. But since "Depth
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Buffer" is not relevant to the rest of the rendering, it does not have an
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outgoing edge.</p>
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<p>Since the graph is guaranteed to be acyclic, we can produce a
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dependency-respecting ordering of the nodes by traversal of the graph (e.g.
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topological sort).</p>
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<h3 id="example-code"><a class="header" href="#example-code">Example code</a></h3>
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<p>We take a snippet of in production code to look through the details of building
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a graph.</p>
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<pre><code>struct StructurePassData {
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FrameGraphId<FrameGraphTexture> depth;
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FrameGraphId<FrameGraphTexture> picking;
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};
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...
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// generate depth pass at the requested resolution
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auto& structurePass = fg.addPass<StructurePassData>("Structure Pass",
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[&](FrameGraph::Builder& builder, auto& data) {
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bool const isES2 = mEngine.getDriverApi().getFeatureLevel() == FeatureLevel::FEATURE_LEVEL_0;
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data.depth = builder.createTexture("Structure Buffer", {
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.width = width, .height = height,
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.levels = uint8_t(levelCount),
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.format = isES2 ? TextureFormat::DEPTH24 : TextureFormat::DEPTH32F });
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// workaround: since we have levels, this implies SAMPLEABLE (because of the gl
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// backend, which implements non-sampleables with renderbuffers, which don't have levels).
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// (should the gl driver revert to textures, in that case?)
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data.depth = builder.write(data.depth,
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FrameGraphTexture::Usage::DEPTH_ATTACHMENT | FrameGraphTexture::Usage::SAMPLEABLE);
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if (config.picking) {
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data.picking = builder.createTexture("Picking Buffer", {
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.width = width, .height = height,
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.format = isES2 ? TextureFormat::RGBA8 : TextureFormat::RG32F });
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data.picking = builder.write(data.picking,
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FrameGraphTexture::Usage::COLOR_ATTACHMENT);
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}
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builder.declareRenderPass("Structure Target", {
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.attachments = { .color = { data.picking }, .depth = data.depth },
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.clearFlags = TargetBufferFlags::COLOR0 | TargetBufferFlags::DEPTH
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});
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},
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[=, renderPass = pass](FrameGraphResources const& resources,
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auto const&, DriverApi&) mutable {
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Variant structureVariant(Variant::DEPTH_VARIANT);
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structureVariant.setPicking(config.picking);
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auto out = resources.getRenderPassInfo();
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renderPass.setRenderFlags(structureRenderFlags);
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renderPass.setVariant(structureVariant);
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renderPass.appendCommands(mEngine, RenderPass::CommandTypeFlags::SSAO);
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renderPass.sortCommands(mEngine);
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renderPass.execute(mEngine, resources.getPassName(), out.target, out.params);
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}
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);
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</code></pre>
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<p>The <code>addPass</code> method creates a node and it take in two lambda functions as its
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parameter. The first lambda sets up the resources that will be used in the
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execution of the Pass. This lambda is executed immediately and synchronously when
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<code>addPass</code> is called. The second lambda is the actual execution of the pass; it is
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executed when the graph has been completed and is traversed.</p>
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<h3 id="what-does-it-do"><a class="header" href="#what-does-it-do">What does it do?</a></h3>
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<p>In the above, we see through a graph and code what a frame graph looks like and
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how to build it. We provide here a more detailed description of what it does:</p>
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<ul>
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<li>Manages the lifetime of the resources
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<ul>
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<li>Know how the resources are allocated, when it is used, and when it can
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be freed</li>
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</ul>
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</li>
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<li>Calculates the usage bit of the texture resource
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<ul>
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<li>The usage bit is used to indicate what the resources are used for: for
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example, will it be blitted to or sampled from?</li>
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</ul>
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</li>
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<li>Calculates the load/store bits of the rendertargets within a renderpass.
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<ul>
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<li>For example, if we are rendering into a texture, we would want to mark
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it with the bit "keep" as oppose to "discard".</li>
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</ul>
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</li>
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</ul>
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<h2 id="additional-details"><a class="header" href="#additional-details">Additional details</a></h2>
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<ul>
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<li>In a previous version of FrameGraph, there were only edges between Resource
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and Pass. For example, a Pass and Pass edge would not make logical sense.
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The following iteration, allowed for edges between two Resource nodes to
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indicate that one is a subresource of another (i.e. a layer in a mip-mapped
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texture).</li>
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<li>There are two extra features of FrameGraph that are important but has a lot
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subtlety, and, incidentally, their inclusion added great complexity to the
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implementation
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<ul>
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<li>Importing/exporting resources outside of the graph
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<ul>
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<li>In most cases, the graph and its resources are "alive" for only for
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a frame.</li>
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<li>For techniques like TAA (Temporal Anti-aliasing), we need to be able
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to import past output into the current FrameGraph</li>
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</ul>
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</li>
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</ul>
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</li>
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<li>Future Work
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<ul>
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<li>For CPU only passes, explore multi-threading and re-ordering of the Pass
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nodes</li>
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<li>A graphical debugger for online debugging session in the spirit of
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<code>matdbg</code>.</li>
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</ul>
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</li>
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<li>"RenderGraph" might be a more fitting name for this framework.</li>
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</ul>
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