CODEWITHSUNDEEP
vulkan
alagris
alagris

Reputation: 2206

Vulkan vkCmdDraw with variable instance count

When I define command buffer I need to specify vertex count and instance count beforehand. Does it mean that if I want to update the number of instances dynamically, I need to recompile the entire command buffer all over again? Just changing this single number seems like a small and innocent tweak. There should be a more efficient way of doing that.

Upvotes: 1

Views: 740

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473302

vkCmdDrawIndirect allows for dispatch operations whose parameters are fetched from a VkBuffer. This allows you to change the storage in that buffer object, and that will be reflected in the indirect draw call that the CB uses...

Assuming you did proper synchronzation, at any rate.

After all, you cannot modify the values in storage associated with a VkBuffer while a command that could be using that storage is executing. So if you want to change the data in that memory, you will need some kind of synchronization between the final indirect draw command that reads from the buffer and whatever process writes the data. If it is an on-GPU process (a copy from mapped memory, for example), then it's fairly easy.

However, event setting is not something you can do within a render pass, so the set will have to wait until the entire render pass is over.

The most efficient way to handle this is to double-buffer your draw indirect buffers. On one frame, you write to one piece of memory and execute commands that read from it. On the next frame, you write to a different piece of memory while the GPU is executing commands that write to the previous one. On the third frame, you go back to the first piece of memory (using the synchronization you set up to ensure that the GPU is finished).

Of course, if you're insisting on static command buffers, this means that the command buffers themselves must also be double-buffered. One CB reads the indirect data from one buffer, and the other CB reads from the other.

Upvotes: 1

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