confused about render pass in Vulkan API

Question

Recently i started learning Vulkan API,there are some topics that confuses me, my question is what is a render pass, why it is used concurrently with command buffer recording? and finally what are sub pass, sub pass dependencies and attachments? that are commonly related to render pass.

Answer 1

It's the only way to get something drawn (draw commands can only be inside render pass). So don't overthink it. As a begginer you only need to create one render pass with one (mandatory) subpass and that's it. You can understand the depths of it later.

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Also you should give some chance to all those videos and tutorials, which are written at length and with more care than whatever will someone write here in short SO format.

Give the spec a chance (it's not so bad — but avoids redundant semantic and conceptual information). Try to read up some intro by AMD, vulkan-tutorial.com, Vulkan in 30 minutes (this one helped me started anyway — well there was not much more available at the time), API without secrets and watch e.g. Vulkan GDC session Part1, Part2.

Now you heard some people behind it and seen some of the commands. You should get back to us with more specific aspects you do not understand about it.

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OK, I am just gonna add some conceptual description of it here to formally answer the question.

Render pass is sort of a description or a map or a scheme of a graphics job (which revolves around particular organization/use of Image resources). But it does not describe the actual commands nor the actual resources (that is done in command buffer recording for render pass instance between vkCmdBeginRenderPass() and vkCmdEndRenderPass())

Maybe a "black box" or "C++ like declaration" for which you provide implementation later is a good analogy.

Render pass has some set of attachments. Let's think of them as descriptions of needed frame image outputs and temporaries (but not the specific frame images themselves).

Render pass has some set of subpasses. Subpass describes how an attachment will be treated during its execution (e.g. as a color buffer in a color image layout).

Render pass has some set of subpass dependencies. Dependencies describe the execution order between subpasses (it forms a dependency DAG). Dependency also describes an equivalent of a pipeline barrier between two subpasses, or between a subpass and outside of the whole render pass (VK_SUBPASS_EXTERNAL dependency). Subpasses are executed in any order and can overlap (at the leisure of the driver), except for what you describe in the dependencies (or otherwise synchronize).

In command buffer using vkCmdBeginRenderPass() you create render pass instance (you provide actual Images for the attachments with VkFramebuffer, and actual commands which write to them).

The things that are part of the render pass description are executed automagically (the image layout transitions, barriers, and MSAA resolutions).

For the rest you record the commands for subpasses of the renderpass instance for the current CB. You do it sequentially for subpass 0, 1, 2, 3, 4, ... — that is not what the actual execution order will be though — you have described that with the subpass dependencies (and otherwise is at the leisure of the driver).

Then the command buffer with such render pass instanc(es) is submitted to queue and actually being executed.
It is perhaps these indirections that make it harder to grasp. Commands are recorded before they are even executed. And render pass is created before it is even recorded. :)