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.