CODEWITHSUNDEEP
vulkan
May
May

Reputation: 121

Do you need synchronization between 2 consecutive renderpasses if the 2nd render pass is depending on the 1st one's output

say the first RenderPass generates an output of a rendered texture, then the second RenderPass sample the texture in the shader and render it to the swapchain .

i don't know if i can do this kind of rendering inside a single RenderPass using subpasses, can subpass attachments have different sizes? in other words , can subpass behave like render textures?

Upvotes: 0

Views: 884

Answers (1)

Varrak
Varrak

Reputation: 768

It seems like there's a few separate questions wrapped up here, so I'll have a stab at answering them in order!

First, there's no need to implicitly synchronize two renderpasses where the second relies on the output from the first, provided they are either recorded on the same command buffer in the correct order, or (if recorded on separate command buffers) submitted in the correct order. The GPU will consume commands in the order submitted, so there's an implicit synchronization there.

If you are consuming the output from a renderpass (or subpass) by sampling inside a shader (which you will need to if the sizes are different, see below), rather than setting up a subpass output as an input attachment in a later subpass then you will need to do so in a separate render pass.

If you are consuming the output from a previous subpass as an input attachment, that implies you are using pixel local load operations inside your shader (where framebuffer attachments written in a previous subpass are read from at the exact same location in a subsequent subpass). This requires attachments be the same size, since all operations occur at the same pixel location.

From the Vulkan specification:

The subpasses in a render pass all render to the same dimensions, and fragments for pixel (x,y,layer) in one subpass can only read attachment contents written by previous subpasses at that same (x,y,layer) location. For multi-pixel fragments, the pixel read from an input attachment is selected from the pixels covered by that fragment in an implementation-dependent manner. However, this selection must be made consistently for any fragment with the same shading rate for the lifetime of the VkDevice.

So if your attachments vary in size, this would imply you need to consume your initial output in a separate renderpass.

Upvotes: 2

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