27000 Dragons and 10'000 Lights: GPU-Driven Clustered Forward Renderer

Summary

During the course Advanced Computer Graphics and Applications I had lots of time (and freedom!) to develop something interesting. While this is an article about computer graphics, it's also about high-performance parallelization strategies.I have written a GPU-Driven forward rendering using clustered shading. It can render 27'000 stanford dragons with 10'000 lights at 1080p in over 60fps on a GTX 1070 GPU. In this post, I will present exactly how my renderer achieves this performance.dragons.GPU-Driven What?In a conventional renderer, there is a certain kind of separation in memory and ownership between the GPU and CPU. Usually, the GPU owns texture data, meshes and other assets—while the CPU owns entity data (position, velocity, etc). This is usually a good design, as the entity data is modified many times on the CPU and uploaded once (through a uniform or similar) for each frame to the GPU.This however, means that certain barriers need to be put up between writing the per-object data and rendering it. Importantly, using a single uniform for all objects requires that each object is rendered sequentially in it's own draw call.Our goal is to reduce the number of draw calls to as few as possible. In both GL and Vulkan, there is exists an API for indirect multi draws. These differ from normal draw calls in that the CPU dispatches the instruction with the draw details (index count, start vertex index) already present on the GPU in a buffer. This way, a single API call can execute multiple draw calls. It results in a few limitations —for example, re-binding resources or changing shaders between draw calls is not possible—but is a tradeoff for performance.In my renderer, the entity (object) data is kept in a contigous buffer, and any modifications to it are marked. Before rendering a frame, all modified parts are uploaded to the GPU.MemoryVk Indexed Draw IndirectCommandGPU Buffers used for driving rendering.In the above figure, the GPU buffers chosen are shown. As some ot...