Render Graph

Introduction
Render graph is a directed acyclic graph that can be used to specify the dependency of render passes. It is a convenient way to manage the rendering especially when using low level API such as D3D12. There are many great resources talked about it such as this and this. In this post I will talk about how the render graph is set up, render pass reordering as well as resources barrier management.

Render Graph set up
To have a simplified view of render graph, we can treat each node inside a graph as single render pass. For example we can have a graph for a simple deferred renderer like this:
Render passes dependency within a render graph
By having such graph, we can derive the dependency of the render passes, remove unused render pass, as well as reorder them. In my toy graphics engine, I use a simple scheme to reorder render passes. Taking the below render graph as an example, the render pass are added as following order:
A render graph example
We can group it into several dependency levels like this:
split render passes into several dependency levels
Within each level, the passes are independent and can be reordered freely, so the render passes are enqueued into command list as the following order:
Reordered render passes
Between each dependency level, we batch resources barrier to transit the resources to the correct state.

Transient Resources
The above view is just a simplified view of the graph. In fact, each render pass consist of a number of inputs and outputs. Every input/output is a graphics resource (e.g. texture). And render passes are connected through such resources within a render graph.
Render graph connecting render passes and resources
As you can see in the above example, there are many transient resources used (e.g. depth buffer, shadow map, etc). We handle such transient resources by using a texture pool. Texture will be reused after it is no longer need by previous pass (placed resources is not used for simplicity). When building a render graph, we compute the life time of every transient resources (i.e. the dependency level that the resource start/end). So we can free the transient resources when the execution go beyond a dependency level and reuse them for later render pass. So to specify a render pass input/output in my engine, I only need to specify their size/format and don't need to worry about the resources creation and the transient resources pool will create the textures as well as the required resources view (e.g. SRV/DSV/RTV).

Conclusion
In this post, I have described how render passes are reordered inside render graph, when barrier are inserted and transient resources handling. But I have not implemented parallel recording of command lists and async compute. It really takes much more effort to use D3D12 than D3D11. I think the current state of my hobby graphics engine is good enough to use. Looks like I can start learning DXR after spending lots of effort on basic D3D12 set up code. =]
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