## 14.4. Fragment Input Attachment Interface

When a fragment stage is present in a pipeline, the fragment shader subpass inputs form an interface with the input attachments of the current subpass. The fragment shader subpass input variables are matched by `InputAttachmentIndex` decorations to the input attachments specified in the `pInputAttachments` array of the `VkSubpassDescription` structure that describes the subpass that the fragment shader is executed in.

The fragment shader subpass input variables with the `UniformConstant` storage class and a decoration of `InputAttachmentIndex` that are statically used by `OpEntryPoint` form the fragment input attachment interface. These variables must be declared with a type of `OpTypeImage`, a `Dim` operand of `SubpassData`, and a `Sampled` operand of 2.

A subpass input variable identified with an `InputAttachmentIndex` decoration of i reads from the input attachment indicated by `pInputAttachments`[i] member of `VkSubpassDescription`. If the subpass input variable is declared as an array of size N, it consumes N consecutive input attachments, starting with the index specified. There must not be more than one input variable with the same `InputAttachmentIndex` whether explicitly declared or implied by an array declaration. The number of available input attachment indices is given by the `maxPerStageDescriptorInputAttachments` member of the `VkPhysicalDeviceLimits` structure.

Variables identified with the `InputAttachmentIndex` must only be used by a fragment stage. The basic data type (floating-point, integer, unsigned integer) of the subpass input must match the basic format of the corresponding input attachment, or the values of subpass loads from these variables are undefined.

See Section 13.1.11, “Input Attachment” for more details.