Why do we need multiple render passes and subpasse

Question

I had some experience with DirectX12 in the past and I don't remember something similar to render passes in Vulkan so I can't make an analogy. If I'm understanding correctly command buffers inside the same subpass doesn't need to be synchronized. So why to complicate and make multiple of them? Why can't I just take one command buffer and put all my frame related info there?

Answer 1

Imagine that the GPU cannot render to images directly. Imagine that it can only render to special framebuffer memory storage, which is completely separate from regular image memory. You cannot talk to this framebuffer memory directly, and you cannot allocate from it. However, during a rendering operation, you can copy data from images into it, read data out of it into images, and of course render to this internal memory.

Now imagine that your special framebuffer memory is fixed in size, a size which is smaller than the size of the overall framebuffer you want to render to (perhaps much smaller). To be able to render to images that are bigger than your framebuffer memory, you basically have to execute all rendering commands for those targets multiple times. To avoid running vertex processing multiple times, you need a way to store the output of vertex processing stages.

Furthermore, when generating rendering commands, you need to have some idea of how to apportion your framebuffer memory. You may have to divide up your framebuffer memory differently if you're rendering to one 32-bpp image than if you're rendering to two. And how you assign your framebuffer memory can affect how your fragment shader code works. After all, this framebuffer rendering memory may be directly accessible by the fragment shader during a rendering operation.

That is the basic idea of the render pass model: you are rendering to special framebuffer memory, of an indeterminate size. Every aspect of the render pass system's complexity is based on this conceptual model.

Subpasses are the part where you determine exactly which things you're rendering to at the moment. Because this affects framebuffer memory arrangement, graphics pipelines are always built by referring to a subpass of a render pass. Similarly, secondary command buffers that are to be executed within a subpass must provide the subpass it will be used within.

When a render pass instance begins execution on a queue, it (conceptually) copies the attachment images we intend to render to into framebuffer rendering memory. At the end of the render pass, the data we render is copied back out to the attachment images.

During the execution of a render pass instance, the data for attachment images is considered "indeterminate". While the model says that we're copying into framebuffer rendering memory, Vulkan doesn't want to force implementations to actually copy stuff if they directly render to images.

As such, Vulkan merely states that no operation can access images that are being used as attachments, except for those which access the images as attachments. For example, you cannot read an attachment image as a texture. But you can read from it as an input attachment.

This is a conceptual description of the way tile-based renderers work. And this is the conceptual model that is the foundation of the Vulkan render pass architecture. Render targets are not accessible memory; they're special things that can only be accessed in special ways.

You can't "just" read from a G-buffer because, while you're rendering to that G-buffer, it exists in special framebuffer memory that isn't in the image yet.

Answer 2

Both features primarily exist for tile-based GPUs, which are common in mobile but, historically, uncommon on desktop computers. That's why DX12 doesn't have an equivalent, and Metal (iOS) does. Though both Nvidia's and AMD's recent architectures do a variant of tile-based rendering now also, and with the recent Windows-on-ARM PCs using Qualcomm chips (tile-based GPU), it will be interesting to see how DX12 evolves.

The benefit of render passes is that during pixel shading, you can keep the framebuffer data in on-chip memory instead of constantly reading and writing external memory. Caches help some, but without reordering pixel shading, the cache tends to thrash quite a bit since it's not large enough to store the entire framebuffer. A related benefit is you can avoid reading in previous framebuffer contents if you're just going to completely overwrite them anyway, and avoid writing out framebuffer contents at the end of the render pass if they're not needed after it's over. In many applications, tile-based GPUs never have to read and write depth buffer data or multisample data to or from external memory, which saves a lot of bandwidth and power.

Subpasses are an advanced feature that, in some cases, allow the driver to effectively merge multiple render passes into one. The goal and underlying mechanism is similar to the OpenGL ES Pixel Local Storage extension, but the API is a bit different in order to allow more GPU architectures to support it and to make it more extensible / future-proof. The classic example where this helps is with basic deferred shading: the first subpass writes out gbuffer data for each pixel, and later subpasses use that to light and shade pixels. Gbuffers can be huge, so keeping all of that on-chip and never having to read or write it to main memory is a big deal, especially on mobile GPUs which tend to be more bandwidth- and power-constrained.