Rendering without a vertex buffer

[streifmi]

Does screen-13 support rendering without binding a vertex buffer (i.e. only using an index buffer)? I tried the most obvious thing and simply didn't set one during record_subpass, but that causes the following validation error:

[2022-10-28T20:47:07Z ERROR screen_13::driver::instance] 🆘 vkCmdDrawIndexed: VkPipeline 0x4fac1c0000000032[] expects that this Command Buffer's vertex binding Index 0 should be set via vkCmdBindVertexBuffers. This is because pVertexBindingDescriptions[0].binding value is 0. The Vulkan spec states: All vertex input bindings accessed via vertex input variables declared in the vertex shader entry point's interface must have either valid or VK_NULL_HANDLE buffers bound (https://www.khronos.org/registry/vulkan/specs/1.3-extensions/html/vkspec.html#VUID-vkCmdDrawIndexed-None-04007)

[attackgoat]

Thanks for the detailed report!

This is not currently supported, but probably should be. Adding it looks straight forward - I've created #42 to cover this use case. If you feel inclined, please check out the test that change adds to fuzzer.rs and see if that works for you. I'd like to improve the API and use the Into<AnyBufferBinding> pattern if possible so expect some changes over the next few days.

[streifmi]

Thanks for the fast response! I'm a bit surprised that this even requires VK_NULL_HANDLE. The validation error might be a bit misleading here. Looking at this it should be possible to just use an empty vertex input state (and for my use case still bind an index buffer). Though I am by no means a Vulkan expert, so I might be totally wrong of course.

EDIT: I still had vertex attribute bindings in the vertex shader - that's the cause of the validation error. But even if I remove that, it does not work: the screen remains black, but there's no validation errors anymore. I'll investigate further.

[attackgoat]

This code demonstrates drawing without a vertex buffer while having no vertex input variables in the shader.

I suppose it's kind of an odd thing to declare input in the shader without providing data while drawing. Apparently it's an actual thing people wanted? The PR makes this scenario possible but I'm wondering if it provides any value.