CODEWITHSUNDEEP
vulkan
Zebrafish
Zebrafish

Reputation: 13876

Why isn't there a 3D array image in Vulkan?

In the Vulkan API it was seen as valuable to include a VK_IMAGE_VIEW_TYPE_CUBE_ARRAY, but not a 3D array:

typedef enum VkImageViewType {
    VK_IMAGE_VIEW_TYPE_1D = 0,
    VK_IMAGE_VIEW_TYPE_2D = 1,
    VK_IMAGE_VIEW_TYPE_3D = 2,
    VK_IMAGE_VIEW_TYPE_CUBE = 3,
    VK_IMAGE_VIEW_TYPE_1D_ARRAY = 4,
    VK_IMAGE_VIEW_TYPE_2D_ARRAY = 5,
    VK_IMAGE_VIEW_TYPE_CUBE_ARRAY = 6,
} VkImageViewType;

Each 6 layers of view for a cube array is another cube. I'm actually struggling to think of a use case for a cube array, and I don't really think it would be useful for 3D array, but why does the cube get an array type and not the 3D image. How is this cube array even supposed to be used? Is there even a cube array sampler?

Upvotes: 1

Views: 1138

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

Cube maps, cube map arrays, and 2D array textures are, in terms of the bits and bytes of storage, ultimately the same thing. All of these views are created from the same kind of image. You have to specify if you need a layered 2D image to be usable as an array or a cubemap (or both), but conceptually, they're all just the same thing.

Each mipmap level consists of L images of a size WxH, where W and H shrink based on the original size and the current mipmap level. L is the number of layers specified at image creation time, and it does not change with the mipmap level. Put simply, there are a constant number of 2D images per mipmap level. Cubemaps and cubemap arrays require L to be either 6 or a multiple of 6 respectively, but it's still constant.

A 3D image is not that. Each mipmap level consists of a single image of size WxHxD, where W, H, and D shrink based on the original size and current mipmap level. Even if you think of a mipmap level of a 3D image as being D number of WxH images, the number D is not constant between mipmap levels.

These are not the same things.

To have a 3D array image, you would need to have each mipmap level contain L 3D images of size WxHxD, where L is the same for each mipmap level.

As for the utility of a cubemap array, it's the same utility you would get out of a 2D array compared to a single 2D image. You use array textures when you need to specify one of a number of images to sample. It's just that in one case, each image is a single 2D image, while in another case, each image is a cubemap.

For a more specific example, many advanced forms of shadow mapping require the use of multiple shadow maps, selected at runtime. That's a good use case for an array texture. You can apply these techniques to point lights through the use of cube maps, but now you need to have the individual images in the array be cube maps, not just single 2D images.

Upvotes: 5

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