CODEWITHSUNDEEP
glslvulkan
AzP
AzP

Reputation: 1101

Dynamic size arrays in Uniform buffer in Vulkan

I recently re-wrote some code to use Shader Storage Buffer Object in OpenGL, to send in dynamic sized arrays into GLSL, vastly increasing performance when drawing many procedurally generated objects.

What I have is a couple of thousand points, and for each point I render a procedurally generated circular billboard. Each one can in turn have different colors and radius, as well as a few other characteristics (represented as bools or enums)

I fill a vector with these positions, packed together with the radius and color. Then I upload it as a Shader Storage Buffer Object with dynamic size. I create a dummy VAO, containing 0 vbos, but call the draw command with the same amount of points that I have. Inside the shader, I then iterate through this array, using the gl_VertexID, and generate a quad (two triangles) with texture coordinates, for each point.

I'm looking for a way of doing the same in Vulkan. Is there some way in Vulkan, to pass a dynamic sized array into a shader? Reading about Shader Storage Buffer objects in Graham Seller's Vulkan book, it only mentions them being read-write, but not capable of dynamically sized arrays.

Edit: It seems that storage buffers are in fact capable of dynamic sized arrays, based on Sasha Willems particle example. Is there a way of doing the same thing via uniforms?

Upvotes: 1

Views: 3884

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473242

I may be misunderstanding your question. SSBOs have identical behavior and functionality between the two APIs. That's why they're named the same. An unbounded array at the end of a storage block will have its length defined at runtime, based on what data you provide.

The size of a buffer descriptor is not hard coded into the descriptor set layout; it's something you set with VkWriteDescriptorSet. Now unlike the offset, you cannot change a descriptor set's size without changing the descriptor itself. That is, you don't have an equivalent to vkCmdBindDescriptorSets's pDynamicOffsets field. So you have to actually update the descriptor in-situ to change the length.

But that just requires double-buffering your descriptor set; it shouldn't be a problem.

Is there a way of doing the same thing via uniforms?

Again, the answer is the same for Vulkan as for OpenGL: no.

Upvotes: 3

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