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+Intel Surface Layout
+
+Introduction
+============
+isl is a small library that calculates the layout of Intel GPU surfaces, queries
+those layouts, and queries the properties of surface formats.
+
+
+Independence from User APIs
+===========================
+isl's API is independent of any user-facing graphics API, such as OpenGL and
+Vulkan. This independence allows isl to be used a shared component by multiple
+Intel drivers.
+
+Rather than mimic the user-facing APIs, the isl API attempts to reflect Intel
+hardware: the actual memory layout of Intel GPU surfaces and how one programs
+the GPU to use those surfaces. For example:
+
+  - The tokens of `enum isl_format` (such as `ISL_FORMAT_R8G8B8A8_UNORM`)
+    match those of the hardware enum `SURFACE_FORMAT` rather than the OpenGL
+    or Vulkan format tokens.  And the values of `isl_format` and
+    `SURFACE_FORMAT` are identical.
+
+  - The OpenGL and Vulkan APIs contain depth and stencil formats. However the
+    hardware enum `SURFACE_FORMAT` does not, and therefore neither does `enum
+    isl_format`. Rather than define new pixel formats that have no hardware
+    counterpart, isl records the intent to use a surface as a depth or stencil
+    buffer with the usage flags `ISL_SURF_USAGE_DEPTH_BIT` and
+    `ISL_SURF_USAGE_STENCIL_BIT`.
+
+  - `struct isl_surf` distinguishes between the surface's logical dimension
+    from the user API's perspective (`enum isl_surf_dim`, which may be 1D, 2D,
+    or 3D) and the layout of those dimensions in memory (`enum isl_dim_layout`).
+
+
+Surface Units
+=============
+
+Intro
+-----
+ISL takes care in its equations to correctly handle conversion among surface
+units (such as pixels and compression blocks) and to carefully distinguish
+between a surface's logical layout in the client API and its physical layout
+in memory.
+
+Symbol names often explicitly declare their unit with a suffix:
+
+   - px: logical pixels
+   - sa: physical surface samples
+   - el: physical surface elements
+   - sa_rows: rows of physical surface samples
+   - el_rows: rows of physical surface elements
+
+Logical units are independent of hardware generation and are closely related
+to the user-facing API (OpenGL and Vulkan). Physical units are dependent on
+hardware generation and reflect the surface's layout in memory.
+
+Definitions
+-----------
+- Logical Pixels (px):
+
+  The surface's layout from the perspective of the client API (OpenGL and
+  Vulkan) is in units of logical pixels. Logical pixels are independent of the
+  surface's layout in memory.
+
+  A surface's width and height, in units of logical pixels, is not affected by
+  the surface's sample count. For example, consider a VkImage created with
+  VkImageCreateInfo{width=w0, height=h0, samples=s0}. The surface's width and
+  height at level 0 is, in units of logical pixels, w0 and h0 regardless of
+  the value of s0.
+
+  For example, the logical array length of a 3D surface is always 1, even on
+  Gen9 where the surface's memory layout is that of an array surface
+  (ISL_DIM_LAYOUT_GEN4_2D).
+
+- Physical Surface Samples (sa):
+
+  For a multisampled surface, this unit has the obvious meaning.
+  A singlesampled surface, from ISL's perspective, is simply a multisampled
+  surface whose sample count is 1.
+
+  For example, consider a 2D single-level non-array surface with samples=4,
+  width_px=64, and height_px=64 (note that the suffix 'px' indicates logical
+  pixels). If the surface's multisample layout is ISL_MSAA_LAYOUT_INTERLEAVED,
+  then the extent of level 0 is, in units of physical surface samples,
+  width_sa=128, height_sa=128, depth_sa=1, array_length_sa=1. If
+  ISL_MSAA_LAYOUT_ARRAY, then width_sa=64, height_sa=64, depth_sa=1,
+  array_length_sa=4.
+
+- Physical Surface Elements (el):
+
+  This unit allows ISL to treat compressed and uncompressed formats
+  identically in many calculations.
+
+  If the surface's pixel format is compressed, such as ETC2, then a surface
+  element is equivalent to a compression block. If uncompressed, then
+  a surface element is equivalent to a surface sample. As a corollary, for
+  a given surface a surface element is at least as large as a surface sample.
+
+Errata
+------
+ISL acquired the term 'surface element' from the Broadwell PRM [1], which
+defines it as follows:
+
+   An element is defined as a pixel in uncompresed surface formats, and as
+   a compression block in compressed surface formats. For MSFMT_DEPTH_STENCIL
+   type multisampled surfaces, an element is a sample.
+
+
+References
+==========
+[1]: Broadwell PRM >> Volume 2d: Command Reference: Structures >>
+     RENDER_SURFACE_STATE Surface Vertical Alignment (p325)