Vulkan descriptor binding

Question

In my vulkan application i used to draw meshes like this when all the meshes used the same texture

Updatedescriptorsets(texture)

Command buffer record
{
 For each mesh
        Bind transformubo
        Draw mesh
}

But now I want each mesh to have a unique texture so i tried this

Command buffer record
{
 For each mesh
        Bind transformubo
        Updatedescriptorsets (textures[meshindex])
        Draw mesh
}

But it gives an error saying descriptorset is destroyed or updated. I looked in vulkan documentation and found out that I can't update descriptorset during command buffer records. So how can I have a unique texture to each mesh?

Answer 1

If you update a descriptor set then all command buffers that this descriptor set is bound to will become invalid. Invalid command buffers cannot be submitted or be executed by the GPU.

What you basically need to do is to update descriptor sets before you bind them.

This odd behavior is there because in vkCmdBindDescriptorSets some implementations take the vulkan descriptor set, translate it to native descriptor tables and then store it in the command buffer. So if you update the descriptor set after vkCmdBindDescriptorSets the command buffer will be seeing stale data. VK_EXT_descriptor_indexing extension relaxed this behavior under some circumstances.

Answer 2

vkUpdateDescriptorSets is not synchonrized with anything. Therefore, you cannot update a descriptor set while it is in use. You must ensure that all rendering operations that use the descriptor set in question have finished, and that no commands have been placed in command buffers that use the set in question.

It's basically like a global variable; you can't have people accessing a global variable from numerous threads without some kind of synchronization. And Vulkan doesn't synchronize access to descriptor sets.

There are several ways to deal with this. You can give each object its own descriptor set. This is usually done by having the frequently changing descriptor set data be of a higher index than the less frequently changing data. That way, you're not changing every descriptor for each object, only the ones that change on a per-object basis.

You can use push constant data to index into large tables/array textures. So the descriptor set would have an array texture or an array of textures (if you have dynamic indexing for arrays of textures). A push constant would provide an index, which is used by the shader to fetch that particular object's texture from the array texture/array of textures. This makes frequent changes fairly cheap, and the same index can also be used to give each object its own transformation matrices (by fetching into an array of matrices).

If you have the extension VK_KHR_push_descriptor available, then you can integrate changes to descriptors directly into the command buffer. How much better this is than the push constant mechanism is of course implementation-dependent.