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 I2F, permit re-interpretation of vector components as integers. Other instructions permit using registers as two-component vectors with double precision; see Double ISA.

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. RCP is one such instruction.

Instruction Set

Core ISA

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

ARL (Address Register Load)

dst.x = \lfloor src.x\rfloor

dst.y = \lfloor src.y\rfloor

dst.z = \lfloor src.z\rfloor

dst.w = \lfloor src.w\rfloor

MOV (Move)

dst.x = src.x

dst.y = src.y

dst.z = src.z

dst.w = src.w

LIT (Light Coefficients)

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

RCP (Reciprocal)

This instruction replicates its result.

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

RSQ (Reciprocal Square Root)

This instruction replicates its result.

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

EXP (Approximate Exponential Base 2)

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

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

dst.z = 2^{src.x}

dst.w = 1

LOG (Approximate Logarithm Base 2)

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

MUL (Multiply)

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

ADD (Add)

dst.x = src0.x + src1.x

dst.y = src0.y + src1.y

dst.z = src0.z + src1.z

dst.w = src0.w + src1.w

DP3 (3-component Dot Product)

This instruction replicates its result.

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

DP4 (4-component Dot Product)

This instruction replicates its result.

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

DST (Distance Vector)

dst.x = 1

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

dst.z = src0.z

dst.w = src1.w

MIN (Minimum)

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)

MAX (Maximum)

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)

SLT (Set On Less Than)

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

SGE (Set On Greater Equal Than)

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

MAD (Multiply And Add)

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

SUB (Subtract)

dst.x = src0.x - src1.x

dst.y = src0.y - src1.y

dst.z = src0.z - src1.z

dst.w = src0.w - src1.w

LRP (Linear Interpolate)

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

CND (Condition)

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

DP2A (2-component Dot Product And Add)

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

FRC (Fraction)

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

CLAMP (Clamp)

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)

FLR (Floor)

This is identical to ARL.

dst.x = \lfloor src.x\rfloor

dst.y = \lfloor src.y\rfloor

dst.z = \lfloor src.z\rfloor

dst.w = \lfloor src.w\rfloor

ROUND (Round)

dst.x = round(src.x)

dst.y = round(src.y)

dst.z = round(src.z)

dst.w = round(src.w)

EX2 (Exponential Base 2)

This instruction replicates its result.

dst = 2^{src.x}

LG2 (Logarithm Base 2)

This instruction replicates its result.

dst = \log_2{src.x}

POW (Power)

This instruction replicates its result.

dst = src0.x^{src1.x}

XPD (Cross Product)

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

ABS (Absolute)

dst.x = |src.x|

dst.y = |src.y|

dst.z = |src.z|

dst.w = |src.w|

RCC (Reciprocal Clamped)

This instruction replicates its result.

XXX cleanup on aisle three

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)

DPH (Homogeneous Dot Product)

This instruction replicates its result.

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

COS (Cosine)

This instruction replicates its result.

dst = \cos{src.x}

DDX (Derivative Relative To X)

dst.x = partialx(src.x)

dst.y = partialx(src.y)

dst.z = partialx(src.z)

dst.w = partialx(src.w)

DDY (Derivative Relative To Y)

dst.x = partialy(src.x)

dst.y = partialy(src.y)

dst.z = partialy(src.z)

dst.w = partialy(src.w)

KILP (Predicated Discard)
discard
PK2H (Pack Two 16-bit Floats)
TBD
PK2US (Pack Two Unsigned 16-bit Scalars)
TBD
PK4B (Pack Four Signed 8-bit Scalars)
TBD
PK4UB (Pack Four Unsigned 8-bit Scalars)
TBD
RFL (Reflection Vector)

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.

SEQ (Set On Equal)

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

SFL (Set On False)

This instruction replicates its result.

dst = 0

Note

Considered for removal.

SGT (Set On Greater Than)

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

SIN (Sine)

This instruction replicates its result.

dst = \sin{src.x}

SLE (Set On Less Equal Than)

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

SNE (Set On Not Equal)

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

STR (Set On True)

This instruction replicates its result.

dst = 1

TEX (Texture Lookup)
TBD
TXD (Texture Lookup with Derivatives)
TBD
TXP (Projective Texture Lookup)
TBD
UP2H (Unpack Two 16-Bit Floats)
TBD

Note

Considered for removal.

UP2US (Unpack Two Unsigned 16-Bit Scalars)
TBD

Note

Considered for removal.

UP4B (Unpack Four Signed 8-Bit Values)
TBD

Note

Considered for removal.

UP4UB (Unpack Four Unsigned 8-Bit Scalars)
TBD

Note

Considered for removal.

X2D (2D Coordinate Transformation)

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.

ARA (Address Register Add)
TBD

Note

Considered for removal.

ARR (Address Register Load With Round)

dst.x = round(src.x)

dst.y = round(src.y)

dst.z = round(src.z)

dst.w = round(src.w)

BRA (Branch)
pc = target

Note

Considered for removal.

CAL (Subroutine Call)
push(pc) pc = target
RET (Subroutine Call Return)

pc = pop()

Potential restrictions: * Only occurs at end of function.

SSG (Set Sign)

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

CMP (Compare)

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

KIL (Conditional Discard)

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

SCS (Sine Cosine)

dst.x = \cos{src.x}

dst.y = \sin{src.x}

dst.z = 0

dst.y = 1

TXB (Texture Lookup With Bias)
TBD
NRM (3-component Vector Normalise)

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

DIV (Divide)

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}

DP2 (2-component Dot Product)

This instruction replicates its result.

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

TXL (Texture Lookup With LOD)
TBD
BRK (Break)
TBD
IF (If)
TBD
ELSE (Else)
TBD
ENDIF (End If)
TBD
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.

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?

CEIL (Ceiling)

dst.x = \lceil src.x\rceil

dst.y = \lceil src.y\rceil

dst.z = \lceil src.z\rceil

dst.w = \lceil src.w\rceil

I2F (Integer To Float)

dst.x = (float) src.x

dst.y = (float) src.y

dst.z = (float) src.z

dst.w = (float) src.w

NOT (Bitwise Not)

dst.x = ~src.x

dst.y = ~src.y

dst.z = ~src.z

dst.w = ~src.w

TRUNC (Truncate)

dst.x = trunc(src.x)

dst.y = trunc(src.y)

dst.z = trunc(src.z)

dst.w = trunc(src.w)

SHL (Shift Left)

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

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

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

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

SHR (Shift Right)

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

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

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

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

AND (Bitwise And)

dst.x = src0.x & src1.x

dst.y = src0.y & src1.y

dst.z = src0.z & src1.z

dst.w = src0.w & src1.w

OR (Bitwise Or)

dst.x = src0.x | src1.x

dst.y = src0.y | src1.y

dst.z = src0.z | src1.z

dst.w = src0.w | src1.w

MOD (Modulus)

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

XOR (Bitwise Xor)

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

SAD (Sum Of Absolute Differences)

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

TXF (Texel Fetch)
TBD
TXQ (Texture Size Query)
TBD
CONT (Continue)
TBD

Note

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

Geometry ISA

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

EMIT (Emit)
TBD
ENDPRIM (End Primitive)
TBD

GLSL ISA

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

BGNLOOP (Begin a Loop)
TBD
BGNSUB (Begin Subroutine)
TBD
ENDLOOP (End a Loop)
TBD
ENDSUB (End Subroutine)
TBD
NOP (No Operation)
Do nothing.
NRM4 (4-component Vector Normalise)

This instruction replicates its result.

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

CALLNZ (Subroutine Call If Not Zero)
TBD
IFC (If)
TBD
BREAKC (Break Conditional)
TBD

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

DADD (Add)

dst.xy = src0.xy + src1.xy

dst.zw = src0.zw + src1.zw

DDIV (Divide)

dst.xy = src0.xy / src1.xy

dst.zw = src0.zw / src1.zw

DSEQ (Set on Equal)

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

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

DSLT (Set on Less than)

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

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

DFRAC (Fraction)

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

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

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 dst1 \times 2^{dst0} = src .

dst0.xy = exp(src.xy)

dst1.xy = frac(src.xy)

dst0.zw = exp(src.zw)

dst1.zw = frac(src.zw)

DLDEXP (Multiply Number by Integral Power of 2)

This opcode is the inverse of DFRACEXP.

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

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

DMIN (Minimum)

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

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

DMAX (Maximum)

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

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

DMUL (Multiply)

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

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

DMAD (Multiply And Add)

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

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

DRCP (Reciprocal)

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

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

DSQRT (Square Root)

dst.xy = \sqrt{src.xy}

dst.zw = \sqrt{src.zw}

Explanation of symbols used

Functions

|x| Absolute value of x.

\lceil x \rceil Ceiling of x.

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

\lfloor x\rfloor Floor of x.

\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.

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 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.

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 TEX, TXD, and 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) [1] (u, v, 1, 1)
Z XXX TBD (z, z, z, 1) [2] (0, z, 0, 1)
[1]http://www.opengl.org/registry/specs/ATI/envmap_bumpmap.txt
[2]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.