CODEWITHSUNDEEP
vulkan
Alan_Greenspan
Alan_Greenspan

Reputation: 1

How do I render to multiple 3d targets in Vulkan?

I have some legacy DX11 code that renders to multiple 3d render targets. Destination target is passed via SV_TARGETxx and the slice is set via SV_RenderTargetArrayIndex in GS. Is there any way to do the same in Vulkan?

My plan is to create individual view for each slice of each 3d target and pass them all together as attachments to a single frame buffer, then in GS I can have something like gl_Layer = sliceNo + targetOffsets[xx]. Is there any better solution?

Upvotes: 0

Views: 2396

Answers (2)

Jesse Hall
Jesse Hall

Reputation: 6787

In Vulkan, the GS SV_RenderTargetArrayIndex is called Layer in SPIR-V or gl_Layer in GLSL. It behaves the same as in D3D. You create one view per 3D target, and attach that to the framebuffer. The Layer output from the GS will say which layer (of all the targets) the output primitive is drawn to.

In Vulkan there's no "true" 3D framebuffer attachments, in the sense that after projection to screen space coordinates everything exists in a 2D plane. So attachment image views can have 2D_ARRAY dimensionality, but not 3D. The Image and image view parameter compatibility requirements table says that given a 3D image, you can create a 2D_ARRAY image view with layerCount >= 1. Note that you have to create the image with the VK_IMAGE_CREATE_2D_ARRAY_COMPATIBLE_BIT flag.

So if you want to have N 3D render target images:

  • Create your N 3D images, with the VK_IMAGE_CREATE_2D_ARRAY_COMPATIBLE_BIT flag.
  • Create one image view for each image, with VK_IMAGE_VIEW_TYPE_2D_ARRAY and layerCount equal to the number of slices you want to be able to render to.
  • Create a VkRenderPass with one VkAttachmentDescription per 3D render target, plus whatever others you need for depth/stencil, resolve target, etc.
  • Create a VkFrameBuffer based on that VkRenderPass, and pass your image views in the VkFrameBufferCreateInfo::pAttachments array. Set VkFramebufferCreateInfo::layerCount to the number of layers/slices you want to be able to render to.

[Edit: Below paragraph can be ignored based on first comment. Leaving it for transparency.]

I'm confused what you're trying to do with SV_Target[n]. In both D3D and Vulkan, if you've got multiple render targets / color attachments, the fragment shader will write to all of them -- if your fragment shader doesn't provide a value for a bound target, the value written is undefined. So SV_Target[n] is used to tell which shader output variables go to which target, but they don't let you write to some without writing to others. Vulkan works similarly, using output variables gl_FragData[n] in GLSL.

Upvotes: 1

Jherico
Jherico

Reputation: 29240

If you're talking about having 1 draw call rendered from multiple points of view (but otherwise using the same pipeline) then you want VK_KHR_multiview. This is an extension in Vulkan 1.0, but core in 1.1.

There's an example of it's usage here and the corresponding shader functionality is here. It functions similar to what you seem to describe. You attach multiple images from a texture array to a single framebuffer ("rendertarget" in D3D) and then in the vertex shader you can determine which layer you're rendering to via the gl_ViewIndex variable. There's no need for a geometry shader with this approach.

Upvotes: 0

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