src/gallium/docs/source/tgsi.rst

TGSI
====

TGSI, Tungsten Graphics Shader Infrastructure, is an intermediate language
for describing shaders. Since Gallium is inherently shaderful, shaders are
an important part of the API. TGSI is the only intermediate representation
used by all drivers.

Basics
------

All TGSI instructions, known as *opcodes*, operate on arbitrary-precision
floating-point four-component vectors. An opcode may have up to one
destination register, known as *dst*, and between zero and three source
registers, called *src0* through *src2*, or simply *src* if there is only
one.

Some instructions, like :opcode:`I2F`, permit re-interpretation of vector
components as integers. Other instructions permit using registers as
two-component vectors with double precision; see :ref:`Double Opcodes`.

When an instruction has a scalar result, the result is usually copied into
each of the components of *dst*. When this happens, the result is said to be
*replicated* to *dst*. :opcode:`RCP` is one such instruction.

Instruction Set
---------------

Core ISA
^^^^^^^^^^^^^^^^^^^^^^^^^

These opcodes are guaranteed to be available regardless of the driver being
used.

.. opcode:: ARL - Address Register Load

.. math::

  dst.x = \lfloor src.x\rfloor

  dst.y = \lfloor src.y\rfloor

  dst.z = \lfloor src.z\rfloor

  dst.w = \lfloor src.w\rfloor


.. opcode:: MOV - Move

.. math::

  dst.x = src.x

  dst.y = src.y

  dst.z = src.z

  dst.w = src.w


.. opcode:: LIT - Light Coefficients

.. math::

  dst.x = 1

  dst.y = max(src.x, 0)

  dst.z = (src.x > 0) ? max(src.y, 0)^{clamp(src.w, -128, 128))} : 0

  dst.w = 1


.. opcode:: RCP - Reciprocal

This instruction replicates its result.

.. math::

  dst = \frac{1}{src.x}


.. opcode:: RSQ - Reciprocal Square Root

This instruction replicates its result.

.. math::

  dst = \frac{1}{\sqrt{|src.x|}}


.. opcode:: EXP - Approximate Exponential Base 2

.. math::

  dst.x = 2^{\lfloor src.x\rfloor}

  dst.y = src.x - \lfloor src.x\rfloor

  dst.z = 2^{src.x}

  dst.w = 1


.. opcode:: LOG - Approximate Logarithm Base 2

.. math::

  dst.x = \lfloor\log_2{|src.x|}\rfloor

  dst.y = \frac{|src.x|}{2^{\lfloor\log_2{|src.x|}\rfloor}}

  dst.z = \log_2{|src.x|}

  dst.w = 1


.. opcode:: MUL - Multiply

.. math::

  dst.x = src0.x \times src1.x

  dst.y = src0.y \times src1.y

  dst.z = src0.z \times src1.z

  dst.w = src0.w \times src1.w


.. opcode:: ADD - Add

.. math::

  dst.x = src0.x + src1.x

  dst.y = src0.y + src1.y

  dst.z = src0.z + src1.z

  dst.w = src0.w + src1.w


.. opcode:: DP3 - 3-component Dot Product

This instruction replicates its result.

.. math::

  dst = src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z


.. opcode:: DP4 - 4-component Dot Product

This instruction replicates its result.

.. math::

  dst = src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z + src0.w \times src1.w


.. opcode:: DST - Distance Vector

.. math::

  dst.x = 1

  dst.y = src0.y \times src1.y

  dst.z = src0.z

  dst.w = src1.w


.. opcode:: MIN - Minimum

.. math::

  dst.x = min(src0.x, src1.x)

  dst.y = min(src0.y, src1.y)

  dst.z = min(src0.z, src1.z)

  dst.w = min(src0.w, src1.w)


.. opcode:: MAX - Maximum

.. math::

  dst.x = max(src0.x, src1.x)

  dst.y = max(src0.y, src1.y)

  dst.z = max(src0.z, src1.z)

  dst.w = max(src0.w, src1.w)


.. opcode:: SLT - Set On Less Than

.. math::

  dst.x = (src0.x < src1.x) ? 1 : 0

  dst.y = (src0.y < src1.y) ? 1 : 0

  dst.z = (src0.z < src1.z) ? 1 : 0

  dst.w = (src0.w < src1.w) ? 1 : 0


.. opcode:: SGE - Set On Greater Equal Than

.. math::

  dst.x = (src0.x >= src1.x) ? 1 : 0

  dst.y = (src0.y >= src1.y) ? 1 : 0

  dst.z = (src0.z >= src1.z) ? 1 : 0

  dst.w = (src0.w >= src1.w) ? 1 : 0


.. opcode:: MAD - Multiply And Add

.. math::

  dst.x = src0.x \times src1.x + src2.x

  dst.y = src0.y \times src1.y + src2.y

  dst.z = src0.z \times src1.z + src2.z

  dst.w = src0.w \times src1.w + src2.w


.. opcode:: SUB - Subtract

.. math::

  dst.x = src0.x - src1.x

  dst.y = src0.y - src1.y

  dst.z = src0.z - src1.z

  dst.w = src0.w - src1.w


.. opcode:: LRP - Linear Interpolate

.. math::

  dst.x = src0.x \times src1.x + (1 - src0.x) \times src2.x

  dst.y = src0.y \times src1.y + (1 - src0.y) \times src2.y

  dst.z = src0.z \times src1.z + (1 - src0.z) \times src2.z

  dst.w = src0.w \times src1.w + (1 - src0.w) \times src2.w


.. opcode:: CND - Condition

.. math::

  dst.x = (src2.x > 0.5) ? src0.x : src1.x

  dst.y = (src2.y > 0.5) ? src0.y : src1.y

  dst.z = (src2.z > 0.5) ? src0.z : src1.z

  dst.w = (src2.w > 0.5) ? src0.w : src1.w


.. opcode:: DP2A - 2-component Dot Product And Add

.. math::

  dst.x = src0.x \times src1.x + src0.y \times src1.y + src2.x

  dst.y = src0.x \times src1.x + src0.y \times src1.y + src2.x

  dst.z = src0.x \times src1.x + src0.y \times src1.y + src2.x

  dst.w = src0.x \times src1.x + src0.y \times src1.y + src2.x


.. opcode:: FRC - Fraction

.. math::

  dst.x = src.x - \lfloor src.x\rfloor

  dst.y = src.y - \lfloor src.y\rfloor

  dst.z = src.z - \lfloor src.z\rfloor

  dst.w = src.w - \lfloor src.w\rfloor


.. opcode:: CLAMP - Clamp

.. math::

  dst.x = clamp(src0.x, src1.x, src2.x)

  dst.y = clamp(src0.y, src1.y, src2.y)

  dst.z = clamp(src0.z, src1.z, src2.z)

  dst.w = clamp(src0.w, src1.w, src2.w)


.. opcode:: FLR - Floor

This is identical to :opcode:`ARL`.

.. math::

  dst.x = \lfloor src.x\rfloor

  dst.y = \lfloor src.y\rfloor

  dst.z = \lfloor src.z\rfloor

  dst.w = \lfloor src.w\rfloor


.. opcode:: ROUND - Round

.. math::

  dst.x = round(src.x)

  dst.y = round(src.y)

  dst.z = round(src.z)

  dst.w = round(src.w)


.. opcode:: EX2 - Exponential Base 2

This instruction replicates its result.

.. math::

  dst = 2^{src.x}


.. opcode:: LG2 - Logarithm Base 2

This instruction replicates its result.

.. math::

  dst = \log_2{src.x}


.. opcode:: POW - Power

This instruction replicates its result.

.. math::

  dst = src0.x^{src1.x}

.. opcode:: XPD - Cross Product

.. math::

  dst.x = src0.y \times src1.z - src1.y \times src0.z

  dst.y = src0.z \times src1.x - src1.z \times src0.x

  dst.z = src0.x \times src1.y - src1.x \times src0.y

  dst.w = 1


.. opcode:: ABS - Absolute

.. math::

  dst.x = |src.x|

  dst.y = |src.y|

  dst.z = |src.z|

  dst.w = |src.w|


.. opcode:: RCC - Reciprocal Clamped

This instruction replicates its result.

XXX cleanup on aisle three

.. math::

  dst = (1 / src.x) > 0 ? clamp(1 / src.x, 5.42101e-020, 1.884467e+019) : clamp(1 / src.x, -1.884467e+019, -5.42101e-020)


.. opcode:: DPH - Homogeneous Dot Product

This instruction replicates its result.

.. math::

  dst = src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z + src1.w


.. opcode:: COS - Cosine

This instruction replicates its result.

.. math::

  dst = \cos{src.x}


.. opcode:: DDX - Derivative Relative To X

.. math::

  dst.x = partialx(src.x)

  dst.y = partialx(src.y)

  dst.z = partialx(src.z)

  dst.w = partialx(src.w)


.. opcode:: DDY - Derivative Relative To Y

.. math::

  dst.x = partialy(src.x)

  dst.y = partialy(src.y)

  dst.z = partialy(src.z)

  dst.w = partialy(src.w)


.. opcode:: KILP - Predicated Discard

  discard


.. opcode:: PK2H - Pack Two 16-bit Floats

  TBD


.. opcode:: PK2US - Pack Two Unsigned 16-bit Scalars

  TBD


.. opcode:: PK4B - Pack Four Signed 8-bit Scalars

  TBD


.. opcode:: PK4UB - Pack Four Unsigned 8-bit Scalars

  TBD


.. opcode:: RFL - Reflection Vector

.. math::

  dst.x = 2 \times (src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z) / (src0.x \times src0.x + src0.y \times src0.y + src0.z \times src0.z) \times src0.x - src1.x

  dst.y = 2 \times (src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z) / (src0.x \times src0.x + src0.y \times src0.y + src0.z \times src0.z) \times src0.y - src1.y

  dst.z = 2 \times (src0.x \times src1.x + src0.y \times src1.y + src0.z \times src1.z) / (src0.x \times src0.x + src0.y \times src0.y + src0.z \times src0.z) \times src0.z - src1.z

  dst.w = 1

.. note::

   Considered for removal.


.. opcode:: SEQ - Set On Equal

.. math::

  dst.x = (src0.x == src1.x) ? 1 : 0

  dst.y = (src0.y == src1.y) ? 1 : 0

  dst.z = (src0.z == src1.z) ? 1 : 0

  dst.w = (src0.w == src1.w) ? 1 : 0


.. opcode:: SFL - Set On False

This instruction replicates its result.

.. math::

  dst = 0

.. note::

   Considered for removal.


.. opcode:: SGT - Set On Greater Than

.. math::

  dst.x = (src0.x > src1.x) ? 1 : 0

  dst.y = (src0.y > src1.y) ? 1 : 0

  dst.z = (src0.z > src1.z) ? 1 : 0

  dst.w = (src0.w > src1.w) ? 1 : 0


.. opcode:: SIN - Sine

This instruction replicates its result.

.. math::

  dst = \sin{src.x}


.. opcode:: SLE - Set On Less Equal Than

.. math::

  dst.x = (src0.x <= src1.x) ? 1 : 0

  dst.y = (src0.y <= src1.y) ? 1 : 0

  dst.z = (src0.z <= src1.z) ? 1 : 0

  dst.w = (src0.w <= src1.w) ? 1 : 0


.. opcode:: SNE - Set On Not Equal

.. math::

  dst.x = (src0.x != src1.x) ? 1 : 0

  dst.y = (src0.y != src1.y) ? 1 : 0

  dst.z = (src0.z != src1.z) ? 1 : 0

  dst.w = (src0.w != src1.w) ? 1 : 0


.. opcode:: STR - Set On True

This instruction replicates its result.

.. math::

  dst = 1


.. opcode:: TEX - Texture Lookup

  TBD


.. opcode:: TXD - Texture Lookup with Derivatives

  TBD


.. opcode:: TXP - Projective Texture Lookup

  TBD


.. opcode:: UP2H - Unpack Two 16-Bit Floats

  TBD

.. note::

   Considered for removal.

.. opcode:: UP2US - Unpack Two Unsigned 16-Bit Scalars

  TBD

.. note::

   Considered for removal.

.. opcode:: UP4B - Unpack Four Signed 8-Bit Values

  TBD

.. note::

   Considered for removal.

.. opcode:: UP4UB - Unpack Four Unsigned 8-Bit Scalars

  TBD

.. note::

   Considered for removal.

.. opcode:: X2D - 2D Coordinate Transformation

.. math::

  dst.x = src0.x + src1.x \times src2.x + src1.y \times src2.y

  dst.y = src0.y + src1.x \times src2.z + src1.y \times src2.w

  dst.z = src0.x + src1.x \times src2.x + src1.y \times src2.y

  dst.w = src0.y + src1.x \times src2.z + src1.y \times src2.w

.. note::

   Considered for removal.


.. opcode:: ARA - Address Register Add

  TBD

.. note::

   Considered for removal.

.. opcode:: ARR - Address Register Load With Round

.. math::

  dst.x = round(src.x)

  dst.y = round(src.y)

  dst.z = round(src.z)

  dst.w = round(src.w)


.. opcode:: BRA - Branch

  pc = target

.. note::

   Considered for removal.

.. opcode:: CAL - Subroutine Call

  push(pc)
  pc = target


.. opcode:: RET - Subroutine Call Return

  pc = pop()


.. opcode:: SSG - Set Sign

.. math::

  dst.x = (src.x > 0) ? 1 : (src.x < 0) ? -1 : 0

  dst.y = (src.y > 0) ? 1 : (src.y < 0) ? -1 : 0

  dst.z = (src.z > 0) ? 1 : (src.z < 0) ? -1 : 0

  dst.w = (src.w > 0) ? 1 : (src.w < 0) ? -1 : 0


.. opcode:: CMP - Compare

.. math::

  dst.x = (src0.x < 0) ? src1.x : src2.x

  dst.y = (src0.y < 0) ? src1.y : src2.y

  dst.z = (src0.z < 0) ? src1.z : src2.z

  dst.w = (src0.w < 0) ? src1.w : src2.w


.. opcode:: KIL - Conditional Discard

.. math::

  if (src.x < 0 || src.y < 0 || src.z < 0 || src.w < 0)
    discard
  endif


.. opcode:: SCS - Sine Cosine

.. math::

  dst.x = \cos{src.x}

  dst.y = \sin{src.x}

  dst.z = 0

  dst.w = 1


.. opcode:: TXB - Texture Lookup With Bias

  TBD


.. opcode:: NRM - 3-component Vector Normalise

.. math::

  dst.x = src.x / (src.x \times src.x + src.y \times src.y + src.z \times src.z)

  dst.y = src.y / (src.x \times src.x + src.y \times src.y + src.z \times src.z)

  dst.z = src.z / (src.x \times src.x + src.y \times src.y + src.z \times src.z)

  dst.w = 1


.. opcode:: DIV - Divide

.. math::

  dst.x = \frac{src0.x}{src1.x}

  dst.y = \frac{src0.y}{src1.y}

  dst.z = \frac{src0.z}{src1.z}

  dst.w = \frac{src0.w}{src1.w}


.. opcode:: DP2 - 2-component Dot Product

This instruction replicates its result.

.. math::

  dst = src0.x \times src1.x + src0.y \times src1.y


.. opcode:: TXL - Texture Lookup With LOD

  TBD


.. opcode:: BRK - Break

  TBD


.. opcode:: IF - If

  TBD


.. opcode:: ELSE - Else

  TBD


.. opcode:: ENDIF - End If

  TBD


.. opcode:: PUSHA - Push Address Register On Stack

  push(src.x)
  push(src.y)
  push(src.z)
  push(src.w)

.. note::

   Considered for cleanup.

.. note::

   Considered for removal.

.. opcode:: POPA - Pop Address Register From Stack

  dst.w = pop()
  dst.z = pop()
  dst.y = pop()
  dst.x = pop()

.. note::

   Considered for cleanup.

.. note::

   Considered for removal.


Compute ISA
^^^^^^^^^^^^^^^^^^^^^^^^

These opcodes are primarily provided for special-use computational shaders.
Support for these opcodes indicated by a special pipe capability bit (TBD).

XXX so let's discuss it, yeah?

.. opcode:: CEIL - Ceiling

.. math::

  dst.x = \lceil src.x\rceil

  dst.y = \lceil src.y\rceil

  dst.z = \lceil src.z\rceil

  dst.w = \lceil src.w\rceil


.. opcode:: I2F - Integer To Float

.. math::

  dst.x = (float) src.x

  dst.y = (float) src.y

  dst.z = (float) src.z

  dst.w = (float) src.w


.. opcode:: NOT - Bitwise Not

.. math::

  dst.x = ~src.x

  dst.y = ~src.y

  dst.z = ~src.z

  dst.w = ~src.w


.. opcode:: TRUNC - Truncate

.. math::

  dst.x = trunc(src.x)

  dst.y = trunc(src.y)

  dst.z = trunc(src.z)

  dst.w = trunc(src.w)


.. opcode:: SHL - Shift Left

.. math::

  dst.x = src0.x << src1.x

  dst.y = src0.y << src1.x

  dst.z = src0.z << src1.x

  dst.w = src0.w << src1.x


.. opcode:: SHR - Shift Right

.. math::

  dst.x = src0.x >> src1.x

  dst.y = src0.y >> src1.x

  dst.z = src0.z >> src1.x

  dst.w = src0.w >> src1.x


.. opcode:: AND - Bitwise And

.. math::

  dst.x = src0.x & src1.x

  dst.y = src0.y & src1.y

  dst.z = src0.z & src1.z

  dst.w = src0.w & src1.w


.. opcode:: OR - Bitwise Or

.. math::

  dst.x = src0.x | src1.x

  dst.y = src0.y | src1.y

  dst.z = src0.z | src1.z

  dst.w = src0.w | src1.w


.. opcode:: MOD - Modulus

.. math::

  dst.x = src0.x \bmod src1.x

  dst.y = src0.y \bmod src1.y

  dst.z = src0.z \bmod src1.z

  dst.w = src0.w \bmod src1.w


.. opcode:: XOR - Bitwise Xor

.. math::

  dst.x = src0.x \oplus src1.x

  dst.y = src0.y \oplus src1.y

  dst.z = src0.z \oplus src1.z

  dst.w = src0.w \oplus src1.w


.. opcode:: SAD - Sum Of Absolute Differences

.. math::

  dst.x = |src0.x - src1.x| + src2.x

  dst.y = |src0.y - src1.y| + src2.y

  dst.z = |src0.z - src1.z| + src2.z

  dst.w = |src0.w - src1.w| + src2.w


.. opcode:: TXF - Texel Fetch

  TBD


.. opcode:: TXQ - Texture Size Query

  TBD


.. opcode:: CONT - Continue

  TBD

.. note::

   Support for CONT is determined by a special capability bit,
   ``TGSI_CONT_SUPPORTED``. See :ref:`Screen` for more information.


Geometry ISA
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

These opcodes are only supported in geometry shaders; they have no meaning
in any other type of shader.

.. opcode:: EMIT - Emit

  TBD


.. opcode:: ENDPRIM - End Primitive

  TBD


GLSL ISA
^^^^^^^^^^

These opcodes are part of :term:`GLSL`'s opcode set. Support for these
opcodes is determined by a special capability bit, ``GLSL``.

.. opcode:: BGNLOOP - Begin a Loop

  TBD


.. opcode:: BGNSUB - Begin Subroutine

  TBD


.. opcode:: ENDLOOP - End a Loop

  TBD


.. opcode:: ENDSUB - End Subroutine

  TBD


.. opcode:: NOP - No Operation

  Do nothing.


.. opcode:: NRM4 - 4-component Vector Normalise

This instruction replicates its result.

.. math::

  dst = \frac{src.x}{src.x \times src.x + src.y \times src.y + src.z \times src.z + src.w \times src.w}


ps_2_x
^^^^^^^^^^^^

XXX wait what

.. opcode:: CALLNZ - Subroutine Call If Not Zero

  TBD


.. opcode:: IFC - If

  TBD


.. opcode:: BREAKC - Break Conditional

  TBD

.. _doubleopcodes:

Double ISA
^^^^^^^^^^^^^^^

The double-precision opcodes reinterpret four-component vectors into
two-component vectors with doubled precision in each component.

Support for these opcodes is XXX undecided. :T

.. opcode:: DADD - Add

.. math::

  dst.xy = src0.xy + src1.xy

  dst.zw = src0.zw + src1.zw


.. opcode:: DDIV - Divide

.. math::

  dst.xy = src0.xy / src1.xy

  dst.zw = src0.zw / src1.zw

.. opcode:: DSEQ - Set on Equal

.. math::

  dst.xy = src0.xy == src1.xy ? 1.0F : 0.0F

  dst.zw = src0.zw == src1.zw ? 1.0F : 0.0F

.. opcode:: DSLT - Set on Less than

.. math::

  dst.xy = src0.xy < src1.xy ? 1.0F : 0.0F

  dst.zw = src0.zw < src1.zw ? 1.0F : 0.0F

.. opcode:: DFRAC - Fraction

.. math::

  dst.xy = src.xy - \lfloor src.xy\rfloor

  dst.zw = src.zw - \lfloor src.zw\rfloor


.. opcode:: DFRACEXP - Convert Number to Fractional and Integral Components

Like the ``frexp()`` routine in many math libraries, this opcode stores the
exponent of its source to ``dst0``, and the significand to ``dst1``, such that
:math:`dst1 \times 2^{dst0} = src` .

.. math::

  dst0.xy = exp(src.xy)

  dst1.xy = frac(src.xy)

  dst0.zw = exp(src.zw)

  dst1.zw = frac(src.zw)

.. opcode:: DLDEXP - Multiply Number by Integral Power of 2

This opcode is the inverse of :opcode:`DFRACEXP`.

.. math::

  dst.xy = src0.xy \times 2^{src1.xy}

  dst.zw = src0.zw \times 2^{src1.zw}

.. opcode:: DMIN - Minimum

.. math::

  dst.xy = min(src0.xy, src1.xy)

  dst.zw = min(src0.zw, src1.zw)

.. opcode:: DMAX - Maximum

.. math::

  dst.xy = max(src0.xy, src1.xy)

  dst.zw = max(src0.zw, src1.zw)

.. opcode:: DMUL - Multiply

.. math::

  dst.xy = src0.xy \times src1.xy

  dst.zw = src0.zw \times src1.zw


.. opcode:: DMAD - Multiply And Add

.. math::

  dst.xy = src0.xy \times src1.xy + src2.xy

  dst.zw = src0.zw \times src1.zw + src2.zw


.. opcode:: DRCP - Reciprocal

.. math::

   dst.xy = \frac{1}{src.xy}

   dst.zw = \frac{1}{src.zw}

.. opcode:: DSQRT - Square Root

.. math::

   dst.xy = \sqrt{src.xy}

   dst.zw = \sqrt{src.zw}


Explanation of symbols used
------------------------------


Functions
^^^^^^^^^^^^^^


  :math:`|x|`       Absolute value of `x`.

  :math:`\lceil x \rceil` Ceiling of `x`.

  clamp(x,y,z)      Clamp x between y and z.
                    (x < y) ? y : (x > z) ? z : x

  :math:`\lfloor x\rfloor` Floor of `x`.

  :math:`\log_2{x}` Logarithm of `x`, base 2.

  max(x,y)          Maximum of x and y.
                    (x > y) ? x : y

  min(x,y)          Minimum of x and y.
                    (x < y) ? x : y

  partialx(x)       Derivative of x relative to fragment's X.

  partialy(x)       Derivative of x relative to fragment's Y.

  pop()             Pop from stack.

  :math:`x^y`       `x` to the power `y`.

  push(x)           Push x on stack.

  round(x)          Round x.

  trunc(x)          Truncate x, i.e. drop the fraction bits.


Keywords
^^^^^^^^^^^^^


  discard           Discard fragment.

  pc                Program counter.

  target            Label of target instruction.


Other tokens
---------------


Declaration
^^^^^^^^^^^


Declares a register that is will be referenced as an operand in Instruction
tokens.

File field contains register file that is being declared and is one
of TGSI_FILE.

UsageMask field specifies which of the register components can be accessed
and is one of TGSI_WRITEMASK.

Interpolate field is only valid for fragment shader INPUT register files.
It specifes the way input is being interpolated by the rasteriser and is one
of TGSI_INTERPOLATE.

If Dimension flag is set to 1, a Declaration Dimension token follows.

If Semantic flag is set to 1, a Declaration Semantic token follows.

CylindricalWrap bitfield is only valid for fragment shader INPUT register
files. It specifies which register components should be subject to cylindrical
wrapping when interpolating by the rasteriser. If TGSI_CYLINDRICAL_WRAP_X
is set to 1, the X component should be interpolated according to cylindrical
wrapping rules.


Declaration Semantic
^^^^^^^^^^^^^^^^^^^^^^^^

  Vertex and fragment shader input and output registers may be labeled
  with semantic information consisting of a name and index.

  Follows Declaration token if Semantic bit is set.

  Since its purpose is to link a shader with other stages of the pipeline,
  it is valid to follow only those Declaration tokens that declare a register
  either in INPUT or OUTPUT file.

  SemanticName field contains the semantic name of the register being declared.
  There is no default value.

  SemanticIndex is an optional subscript that can be used to distinguish
  different register declarations with the same semantic name. The default value
  is 0.

  The meanings of the individual semantic names are explained in the following
  sections.

TGSI_SEMANTIC_POSITION
""""""""""""""""""""""

For vertex shaders, TGSI_SEMANTIC_POSITION indicates the vertex shader
output register which contains the homogeneous vertex position in the clip
space coordinate system.  After clipping, the X, Y and Z components of the
vertex will be divided by the W value to get normalized device coordinates.

For fragment shaders, TGSI_SEMANTIC_POSITION is used to indicate that
fragment shader input contains the fragment's window position.  The X
component starts at zero and always increases from left to right.
The Y component starts at zero and always increases but Y=0 may either
indicate the top of the window or the bottom depending on the fragment
coordinate origin convention (see TGSI_PROPERTY_FS_COORD_ORIGIN).
The Z coordinate ranges from 0 to 1 to represent depth from the front
to the back of the Z buffer.  The W component contains the reciprocol
of the interpolated vertex position W component.

Fragment shaders may also declare an output register with
TGSI_SEMANTIC_POSITION.  Only the Z component is writable.  This allows
the fragment shader to change the fragment's Z position.



TGSI_SEMANTIC_COLOR
"""""""""""""""""""

For vertex shader outputs or fragment shader inputs/outputs, this
label indicates that the resister contains an R,G,B,A color.

Several shader inputs/outputs may contain colors so the semantic index
is used to distinguish them.  For example, color[0] may be the diffuse
color while color[1] may be the specular color.

This label is needed so that the flat/smooth shading can be applied
to the right interpolants during rasterization.



TGSI_SEMANTIC_BCOLOR
""""""""""""""""""""

Back-facing colors are only used for back-facing polygons, and are only valid
in vertex shader outputs. After rasterization, all polygons are front-facing
and COLOR and BCOLOR end up occupying the same slots in the fragment shader,
so all BCOLORs effectively become regular COLORs in the fragment shader.


TGSI_SEMANTIC_FOG
"""""""""""""""""

Vertex shader inputs and outputs and fragment shader inputs may be
labeled with TGSI_SEMANTIC_FOG to indicate that the register contains
a fog coordinate in the form (F, 0, 0, 1).  Typically, the fragment
shader will use the fog coordinate to compute a fog blend factor which
is used to blend the normal fragment color with a constant fog color.

Only the first component matters when writing from the vertex shader;
the driver will ensure that the coordinate is in this format when used
as a fragment shader input.


TGSI_SEMANTIC_PSIZE
"""""""""""""""""""

Vertex shader input and output registers may be labeled with
TGIS_SEMANTIC_PSIZE to indicate that the register contains a point size
in the form (S, 0, 0, 1).  The point size controls the width or diameter
of points for rasterization.  This label cannot be used in fragment
shaders.

When using this semantic, be sure to set the appropriate state in the
:ref:`rasterizer` first.


TGSI_SEMANTIC_GENERIC
"""""""""""""""""""""

All vertex/fragment shader inputs/outputs not labeled with any other
semantic label can be considered to be generic attributes.  Typical
uses of generic inputs/outputs are texcoords and user-defined values.


TGSI_SEMANTIC_NORMAL
""""""""""""""""""""

Indicates that a vertex shader input is a normal vector.  This is
typically only used for legacy graphics APIs.


TGSI_SEMANTIC_FACE
""""""""""""""""""

This label applies to fragment shader inputs only and indicates that
the register contains front/back-face information of the form (F, 0,
0, 1).  The first component will be positive when the fragment belongs
to a front-facing polygon, and negative when the fragment belongs to a
back-facing polygon.


TGSI_SEMANTIC_EDGEFLAG
""""""""""""""""""""""

For vertex shaders, this sematic label indicates that an input or
output is a boolean edge flag.  The register layout is [F, x, x, x]
where F is 0.0 or 1.0 and x = don't care.  Normally, the vertex shader
simply copies the edge flag input to the edgeflag output.

Edge flags are used to control which lines or points are actually
drawn when the polygon mode converts triangles/quads/polygons into
points or lines.

TGSI_SEMANTIC_STENCIL
""""""""""""""""""""""

For fragment shaders, this semantic label indicates than an output
is a writable stencil reference value. Only the Y component is writable.
This allows the fragment shader to change the fragments stencilref value.


Properties
^^^^^^^^^^^^^^^^^^^^^^^^


  Properties are general directives that apply to the whole TGSI program.

FS_COORD_ORIGIN
"""""""""""""""

Specifies the fragment shader TGSI_SEMANTIC_POSITION coordinate origin.
The default value is UPPER_LEFT.

If UPPER_LEFT, the position will be (0,0) at the upper left corner and
increase downward and rightward.
If LOWER_LEFT, the position will be (0,0) at the lower left corner and
increase upward and rightward.

OpenGL defaults to LOWER_LEFT, and is configurable with the
GL_ARB_fragment_coord_conventions extension.

DirectX 9/10 use UPPER_LEFT.

FS_COORD_PIXEL_CENTER
"""""""""""""""""""""

Specifies the fragment shader TGSI_SEMANTIC_POSITION pixel center convention.
The default value is HALF_INTEGER.

If HALF_INTEGER, the fractionary part of the position will be 0.5
If INTEGER, the fractionary part of the position will be 0.0

Note that this does not affect the set of fragments generated by
rasterization, which is instead controlled by gl_rasterization_rules in the
rasterizer.

OpenGL defaults to HALF_INTEGER, and is configurable with the
GL_ARB_fragment_coord_conventions extension.

DirectX 9 uses INTEGER.
DirectX 10 uses HALF_INTEGER.



Texture Sampling and Texture Formats
------------------------------------

This table shows how texture image components are returned as (x,y,z,w) tuples
by TGSI texture instructions, such as :opcode:`TEX`, :opcode:`TXD`, and
:opcode:`TXP`. For reference, OpenGL and Direct3D conventions are shown as
well.

+--------------------+--------------+--------------------+--------------+
| Texture Components | Gallium      | OpenGL             | Direct3D 9   |
+====================+==============+====================+==============+
| R                  | (r, 0, 0, 1) | (r, 0, 0, 1)       | (r, 1, 1, 1) |
+--------------------+--------------+--------------------+--------------+
| RG                 | (r, g, 0, 1) | (r, g, 0, 1)       | (r, g, 1, 1) |
+--------------------+--------------+--------------------+--------------+
| RGB                | (r, g, b, 1) | (r, g, b, 1)       | (r, g, b, 1) |
+--------------------+--------------+--------------------+--------------+
| RGBA               | (r, g, b, a) | (r, g, b, a)       | (r, g, b, a) |
+--------------------+--------------+--------------------+--------------+
| A                  | (0, 0, 0, a) | (0, 0, 0, a)       | (0, 0, 0, a) |
+--------------------+--------------+--------------------+--------------+
| L                  | (l, l, l, 1) | (l, l, l, 1)       | (l, l, l, 1) |
+--------------------+--------------+--------------------+--------------+
| LA                 | (l, l, l, a) | (l, l, l, a)       | (l, l, l, a) |
+--------------------+--------------+--------------------+--------------+
| I                  | (i, i, i, i) | (i, i, i, i)       | N/A          |
+--------------------+--------------+--------------------+--------------+
| UV                 | XXX TBD      | (0, 0, 0, 1)       | (u, v, 1, 1) |
|                    |              | [#envmap-bumpmap]_ |              |
+--------------------+--------------+--------------------+--------------+
| Z                  | XXX TBD      | (z, z, z, 1)       | (0, z, 0, 1) |
|                    |              | [#depth-tex-mode]_ |              |
+--------------------+--------------+--------------------+--------------+
| S                  | (s, s, s, s) | unknown            | unknown      |
+--------------------+--------------+--------------------+--------------+

.. [#envmap-bumpmap] http://www.opengl.org/registry/specs/ATI/envmap_bumpmap.txt
.. [#depth-tex-mode] the default is (z, z, z, 1) but may also be (0, 0, 0, z)
   or (z, z, z, z) depending on the value of GL_DEPTH_TEXTURE_MODE.