Vulkan texture rendering on multiple meshes

Question

I am in the middle of rendering different textures on multiple meshes of a model, but I do not have much clues about the procedures. Someone suggested for each mesh, create its own descriptor sets and call vkCmdBindDescriptorSets() and vkCmdDrawIndexed() for rendering like this:

    // Pipeline with descriptor set layout that matches the shared descriptor sets
vkCmdBindPipeline(...pipelines.mesh...);
...
// Mesh A
vkCmdBindDescriptorSets(...&meshA.descriptorSet... );
vkCmdDrawIndexed(...);
// Mesh B
vkCmdBindDescriptorSets(...&meshB.descriptorSet... );
vkCmdDrawIndexed(...);

However, the above approach is quite different from the chopper sample and vulkan's samples that makes me have no idea where to start the change. I really appreciate any help to guide me to a correct direction.

Cheers

Answer 1

You have a conceptual object which is made of multiple meshes which have different texturing needs. The general ways to deal with this are:

1. Change descriptor sets between parts of the object. Painful, but it works on all Vulkan-capable hardware.

2. Employ array textures. Each individual mesh fetches its data from a particular layer in the array texture. Of course, this restricts you to having each sub-mesh use textures of the same size. But it works on all Vulkan-capable hardware (up to 128 array elements, minimum). The array layer for a particular mesh can be provided as a push-constant, or a base instance if that's available.

   Note that if you manage to be able to do it by base instance, then you can render the entire object with a multi-draw indirect command. Though it's not clear that a short multi-draw indirect would be faster than just baking a short sequence of drawing commands into a command buffer.

3. Employ sampler arrays, as Sascha Willems suggests. Presumably, the array index for the sub-mesh is provided as a push-constant or base instance. The problem is that, regardless of how that array index is provided, it will have to be a dynamically uniform expression. And Vulkan implementations are not required to allow you to index a sampler array with a dynamically uniform expression. The base requirement is just a constant expression.

   This limits you to hardware that supports the shaderSampledImageArrayDynamicIndexing feature. So you have to ask for that, and if it's not available, then you've got to work around that with #1 or #2. Or just don't run on that hardware. But the last one means that you can't run on any mobile hardware, since most of them don't support this feature as of yet.

   Note that I am not saying you shouldn't use this method. I just want you to be aware that there are costs. There's a lot of hardware out there that can't do this. So you need to plan for that.

Answer 2

The person that suggested the above code fragment was me I guess ;)

This is only one way of doing it. You don't necessarily have to create one descriptor set per mesh or per texture. If your mesh e.g. uses 4 different textures, you could bind all of them at once to different binding points and select them in the shader.

And if you a take a look at NVIDIA's chopper sample, they do it pretty much the same way only with some more abstraction.

The example also sets up descriptor sets for the textures used :

VkDescriptorSet *textureDescriptors = m_renderer->getTextureDescriptorSets();

binds them a few lines later :

VkDescriptorSet sets[3] = { sceneDescriptor, textureDescriptors[0], m_transform_descriptor_set };
vkCmdBindDescriptorSets(m_draw_command[inCommandIndex], VK_PIPELINE_BIND_POINT_GRAPHICS, layout, 0, 3, sets, 0, NULL);

and then renders the mesh with the bound descriptor sets :

vkCmdDrawIndexedIndirect(m_draw_command[inCommandIndex], sceneIndirectBuffer, 0, inCount, sizeof(VkDrawIndexedIndirectCommand));
vkCmdDraw(m_draw_command[inCommandIndex], 1, 1, 0, 0);

If you take a look at initDescriptorSets you can see that they also create separate descriptor sets for the cubemap, the terrain, etc.

The LunarG examples should work similar, though if I'm not mistaken they never use more than one texture?