Name

     EXT_texenv_op

Name Strings

     GL_EXT_texenv_op

Contact

     Tom Frisinger (tfrisinger 'at' atitech.com)
     Michael Gold (gold 'at' nvidia.com)

Status

    XXX - Not complete yet!!!

Version

    $Date: 1999/12/28 01:40:34 $ $Revision: 1.4 $
    ATI Date: 1999/03/02 12:00:00  Revision: 1.1

Number

     XXX

Dependencies

    OpenGL 1.1 is required.
    ARB_multitexture is required.
    This specification is written against the OpenGL 1.2 specification.

Overview

     The EXT_texenv_op extension defines a more expressive means for
     programming the operations performed by the texture environment. It
     defines new texture environment equations and allows explicit control
     over the operands to the new equations. This explicit control can be
     independently applied to the alpha and RGB color fragments.

Issues

   * Should we include f, iv, and fv versions of the TexEnvOperand call.

        No, there seems to be no compelling reason to do so.

   * Should we try to wrap this entire interface into TexEnv or at least
     the specification of the operation?

        Move operation specification into TexEnv, but keep TexEnvOperandiEXT
        for operand specification.

   * Should a stage argument be added to TexEnvOperandiEXT to decouple from
     the current unit selector?

        No. This goes against the ARB_multitexture implementation. We
        have however allowed TEXTUREn_ARB as a value to the source
        parameter of TexEnvOperandiEXT. This allows the texture fragment
        to come from the texture bound to any legal texture unit stage.

   * Should a generic scale and bias term be added?

        Allow scales of 1.0, 2.0, and 4.0 to be applied to the output of
        each stage. The scales can be independently applied to RGB and
        Alpha. Generic biasing isn't allowed. However the ADD_SIGNED_EXT
        operation effectively allows a bias of -0.5.

   * Should alpha and RGB operands be completely orthogonal?

     Yes.

   * This spec becomes much more powerful when ARB_multitexture is
     available. However, it seemed like having the new texture environment
     functions and control over operands available to implementations that
     don't support ARB_multitexture could be useful.

        Make ARB_multitexture required. We define PREVIOUS_EXT to be
        INCOMING_EXT for the texture unit 0 case so we could loosen up the
        spec if need be.

   * IDENTITY_EXT and ONE_MINUS_EXT are the legal values to the mapping
     parameter of TexEnvOperandiEXT. Other names such as COPY, INVERT, or
     COMPLEMENT_EXT may or may not be more appropriate.

        Keep IDENTITY_EXT and ONE_MINUS_EXT they seem to be as good as any.

   * Should a third operand be introduced such that the blending operations
     could be generalized to the following equation?

       Cv = CopA*(1- CopC) + CopB* CopC

        Yes, this is the INTERPOLATE_EXT operation. Interpolation in the
        alpha channel is also permitted.

  *  Should we make the OpenGL 1.1 modes programmable? For example should
     setting the env. mode to MODULATE have the effect of changing the
     TEXTURE_ENV_MODE_ALPHA_EXT, TEXTURE_ENV_MODE_RGB_EXT, OPERANDS, and
     possibly the OPERATIONS to a given state that would correspond with
     tables 3.18 and 3.19?

        Instead of this approach, we decided to go with the
        non-programmable approach. For example MODULATE works as it
        always did according to table 3.18 and 3.19 and does not change
        TEXTURE_ENV_MODE_ALPHA_EXT, TEXTURE_ENV_MODE_RGB_EXT, OPERAND,
        or OPERATION state. This adds a small amount of burden to the
        driver but seems more consistent.

   * ALPHA_SCALE and RGB_SCALE_EXT accept only 1.0, 2.0 and 4.0. This look
     like we are really doing a shift. Should the value be restricted to an
     integer and indicate a shift instead of a scale?

        There is no reason to limit the domain of SCALE to an integer,
        even if we only support integral values today. Lets keep it a
        floating point value to allow future generalization.

New Procedures and Functions

     void TexEnvOperandiEXT(enum pname, int source, int mapping,
                            int component);

     void GetTexEnvOperandivEXT(enum pname, enum query, int *param);

New Tokens

     Accepted by <pname> parameter of TexEnvf, TexEnvi, TexEnvfv, TexEnviv,
     GetTexEnviv, and GetTexEnvfv:

        TEXTURE_ENV_MODE_ALPHA_EXT                    0x8570
        TEXTURE_ENV_MODE_RGB_EXT                      0x8571
        RGB_SCALE_EXT                                 0x8572
        ALPHA_OPERATION_EXT                           0x8590
        RGB_OPERATION_EXT                             0x8598

     Accepted by the <param> parameter of TexEnvf, TexEnvi, TexEnvfv, and
     TexEnviv when the <pname> parameter is TEXTURE_ENV_MODE,
     TEXTURE_ENV_MODE_ALPHA_EXT, or TEXTURE_ENV_MODE_RGB_EXT:

        A_TIMES_B_OP_C_EXT                            0x8573
        INTERPOLATE_EXT                               0x8574

     Accepted by the <param> parameter of TexEnvf, TexEnvi, TexEnvfv, and
     TexEnviv when the <pname> parameter is ALPHA_OPERATION_EXT or
     RGB_OPERATION_EXT:

        ADD_SIGNED_EXT                                0x8575
        SUTRACT_EXT                                   0x8576

     Accepted by the <pname> parameter of TexEnvOperandiEXT and
     GetTexEnvOperandivEXT:

        ALPHA_OPERAND_A_EXT                           0x8580
        ALPHA_OPERAND_B_EXT                           0x8581
        ALPHA_OPERAND_C_EXT                           0x8582
        RGB_OPERAND_A_EXT                             0x8588
        RGB_OPERAND_B_EXT                             0x8589
        RGB_OPERAND_C_EXT                             0x858A

     Accepted by the source parameter to TexEnvOperandiExt:

        CONSTANT_EXT                                  0x8577
        INCOMING_EXT                                  0x8578
        PREVIOUS_EXT                                  0x8579

     Accepted by the mapping parameter to TexEnvOperandiExt:

        IDENTITY_EXT                                  0x857A
        ONE_MINUS_EXT                                 0x857B

     Accepted by the query parameter to GetTexEnvOperandivEXT:

        SOURCE_EXT                                    0x857C
        MAPPING_EXT                                   0x857D
        COMPONENT_EXT                                 0x857E

Additions to Chapter 1 of the GL Specification (Introduction)

     None

Additions to Chapter 2 of the GL Specification (OpenGL Operation)

     None

Additions to Chapter 3 of the GL Specification (Rasterization)

     Function             Operation         Equation
     --------             ---------         --------
     INTERPOLATE_EXT                        Av = AopA * (1-AopC) + AopB * AopC
     A_TIMES_B_OP_C_EXT   ADD               Av = AopA * AopB + AopC
                          ADD_SIGNED_EXT    Av = AopA * AopB + AopC - 0.5
                          SUBTRACT_EXT      Av = AopA * AopB - AopC

     Table 3.20: Alpha texture environment functions, operations and equations


     Function             Operation         Equation
     --------             ---------         --------
     INTERPOLATE_EXT                        Cv = CopA * (1-CopC) + CopB * CopC
     A_TIMES_B_OP_C_EXT   ADD               Cv = CopA * CopB + CopC
                          ADD_SIGNED_EXT    Cv = CopA * CopB + CopC - 0.5
                          SUBTRACT_EXT      Cv = CopA * CopB - CopC

     Table 3.21: RGB texture environment functions, operations and equations


     Operand                     Source             Mapping          Component
     -------                     ------             -------          ---------
     ALPHA_OPERAND_A_EXT         TEXTURE            IDENTITY_EXT     ALPHA
     ALPHA_OPERAND_B_EXT         TEXTUREn_ARB       ONE_MINUS_EXT
     ALPHA_OPERAND_C_EXT         CONSTANT_EXT
                                 INCOMING_EXT
                                 PREVIOUS_EXT
                                 ZERO
     RGB_OPERAND_A_EXT           TEXTURE            IDENTITY_EXT     ALPHA
     RGB_OPERAND_B_EXT           TEXTUREn_ARB       ONE_MINUS_EXT    RGB
     RGB_OPERAND_C_EXT           CONSTANT_EXT
                                 INCOMING_EXT
                                 PREVIOUS_EXT
                                 ZERO

     Table 3.22: Inputs to TexEnvOperandiEXT


     Texture                          LUMINANCE
     Format      ALPHA     LUMINANCE    ALPHA     INTENSITY   RGB    RGBA
     -------     -----     ---------  ----------  ---------   ---    ----
     RGB          1.0       Lt         Lt          It          Ct     Ct
     ALPHA        At        1.0        At          It          1.0    At

     Table 3.23: Texture fragment sources

3.8.9 Texture Environments and Texture Functions

     The command

        void TexEnv{if}( enum target, enum pname, T param ) ;
        void TexEnv{if}v( enum target, enum pname, T params ) ;

     sets parameters of the texture environment that specifies how texture
     values are interpreted when texturing a fragment. target must
     currently be the symbolic constant TEXTURE_ENV. pname is a symbolic
     constant indicating the parameter to be set. In the first form of the
     command, param is a value to which to set a single-valued parameter;
     in the second form, params is a pointer to an array of parameters:
     either a single symbolic constant or a value or group of values to
     which the parameter should be set. The possible environment parameters
     are TEXTURE_ENV_MODE, TEXTURE_ENV_MODE_ALPHA_EXT,
     TEXTURE_ENV_MODE_RGB_EXT, ALPHA_OPERATION_EXT, RGB_OPERATION_EXT,
     ALPHA_SCALE_EXT, RGB_SCALE_EXT, and TEXTURE_ENV_COLOR.
     TEXTURE_ENV_MODE, TEXTURE_ENV_MODE_ALPHA_EXT, and
     TEXTURE_ENV_MODE_RGB_EXT may be set to one of REPLACE, MODULATE,
     DECAL, BLEND, A_TIMES_B_OP_C_EXT, or INTERPOLATE_EXT;
     ALPHA_OPERATION_EXT and RGB_OPERATION_EXT may be set to one of ADD,
     ADD_SIGNED_EXT, SUBTRACT_EXT; ALPHA_SCALE and RGB_SCALE_EXT are
     single-precision floating point values that may have a value of 1.0,
     2.0, or 4.0. TEXTURE_ENV_COLOR is set to an RGBA color providing four
     single-precision floating-point values in the range [0, 1] (values
     outside this range are clamped to it). If integers are provided for
     TEXTURE_ENV_COLOR, then they are converted to floating-point as
     specified in table 2.6 for signed integers.

     The value of TEXTURE_ENV_MODE specifies an implicit texture function
     if it is REPLACE, MODULATE, DECAL, or BLEND. The result of this
     function depends on the fragment and the texture array value. The
     precise form of the function depends on the base internal formats of
     the texture arrays that were last specified. In the following two
     tables, Rf, Gf, Bf, and Af are the primary color components of the
     incoming fragment; Rt, Gt, Bt, At, Lt, and It are the filtered texture
     values; Rc, Gc, Bc, and Ac are the texture environment color values;
     and Rv, Gv, Bv, and Av are the primary color components computed by
     the texture function. All of these color values are in the range [0,
     1]. The REPLACE and MODULATE texture functions are specified in table
     3.18, and the DECAL and BLEND texture functions are specified in table
     3.19.

     The texture function used to generate the Alpha component (Av) and the
     RGB component (Rv, Gv, Bv) can be independently specified using
     TEXTURE_ENV_MODE_ALPHA_EXT and TEXTURE_ENV_MODE_RGB_EXT respectively.
     The alpha component will be computed in accordance to the alpha
     equation for the given alpha texture function as specified in tables
     3.18 and 3.19. The RGB components will be computed in accordance to
     the RGB equations for the given RGB texture function as specified in
     tables 3.18 and 3.19. Queries to TEXTURE_ENV_MODE are supported for
     compatibility and are equivalent to querying TEXTURE_ENV_MODE_RGB_EXT.

     The value of TEXTURE_ENV_MODE, TEXTURE_ENV_MODE_ALPHA_EXT, or
     TEXTURE_ENV_MODE_RGB_EXT specifies an explicit texture function if it
     is INTERPOLATE_EXT or A_TIMES_B_OP_C_EXT. Specifying TEXTURE_ENV_MODE
     with a value of INTERPOLATE_EXT or A_TIMES_B_OP_C_EXT will result in
     the setting of both TEXTURE_ENV_MODE_ALPHA_EXT and
     TEXTURE_ENV_MODE_RGB_EXT to the given value. The result of this
     function depends on the currently selected operands and in the case of
     A_TIMES_B_OP_C_EXT the operation. In tables 3.20, 3.21, 3.22, and 3.23
     the following variables are used:

     AopA = ALPHA_OPERAND_A_EXT
     AopB = ALPHA_OPERAND_B_EXT
     AopC = ALPHA_OPERAND_C_EXT
     CopA = RGB_OPERAND_A_EXT
     CopB = RGB_OPERAND_B_EXT
     CopC = RGB_OPERAND_C_EXT

     All of these color and alpha values are clamped to the range [0,1].

     and:

     Ac, Af, At, Av, Ct, It, and Lt have the same meaning as in tables 3.18
     and 3.19.

     See tables 3.20 and 3.21 for the specifics of the texture functions.

     ALPHA_OPERATION_EXT specifies the texture environment operation to be
     applied to the alpha component of the color fragment.
     RGB_OPERATION_EXT specifies the texture environment operation to be
     applied to the RGB component of the color fragment. This operation is
     only applicable when the texture environment mode is A_TIMES_B_OP_C
     and can have a value of ADD, ADD_SIGNED_EXT, or SUBTRACT_EXT. See
     tables 3.20 and 3.21 for texture operation equations.

     The values for ALPHA_SCALE and RGB_SCALE_EXT specify the amount of
     scaling to be applied to the Alpha and RGB output of each texture unit
     stage. Valid values are 1.0, 2.0, and 4.0. Scaling happens prior to
     clamping of Rv, Gv, Bv, and Av to the range [0,1].

     The command

        void TexEnvOperandiEXT(enum pname, int source, int mapping, int
                               component);

     is used to set the operands to the texture function operation. <pname>
     is a symbolic constant indicating the parameter to be set; the
     possible constants and corresponding parameters are summarized in
     table 3.22.

     When <pname> is set to ALPHA_OPERAND_l_EXT, the alpha component of
     operand l is modified. When <pname> is set to RGB_OPERAND_l_EXT, the RGB
     component of operand l is modified.

     <source> specifies the source of the operand data. TEXTURE specifies to
     use the filtered texture fragment from the current texture unit as an
     operand. TEXTUREn_ARB specifies to use the filtered texture fragment
     from the nth texture unit. When <source> is set to TEXTURE or
     TEXTUREn_ARB the value of the texture fragment depends on the base
     internal formats of the texture arrays that were last specified (see
     Table 3.23). CONSTANT_EXT specifies to use the constant texture
     environment color as the fragment. INCOMING_EXT specifies to use the
     incoming fragment color. PREVIOUS_EXT indicates that the input to the
     operand comes from the specified output of the previous texture unit.
     If the current selected texture unit is texture unit 0, then
     PREVIOUS_EXT has the same meaning as INCOMING_EXT (i.e. the input to
     operand is the incoming fragment).

     <mapping> specifies what operation will be done to the source fragment
     prior to its use as an operand. IDENTITY_EXT specifies that no
     operation will be done on the fragment. ONE_MINUS specifies that 1.0
     minus the fragment color will be used as an operand.

     <component> specifies which part of the source fragment color to use.
     <component> must be ALPHA when pname is ALPHA_OPERAND_l_EXT. Valid
     values for <component> are ALPHA and RGB when <pname> is
     RGB_OPERAND_l_EXT.

     The state required for the current texture environment consists of two
     six-valued integers indicating the texture rgb and alpha functions;
     the four floating-point TEXTURE_ENV_COLOR values; two three-valued
     integers representing the RGB and ALPHA operations; two floating point
     RGB_SCALE_EXT and ALPHA_SCALE values; six n-valued integers
     representing the RGB and ALPHA OPERAND A,B and C values, where n
     equals five plus the number of texture units; six two-valued integers
     representing the operand mappings; and three two-valued integers
     representing the component selection of the RGB_OPERANDs. In the
     initial state, the texture RGB and ALPHA functions are MODULATE; the
     TEXTURE_ENV_COLOR is (0,0,0,0); the RGB and ALPHA operations are ADD;
     the RGB and ALPHA scales are 1.0; the RGB_OPERAND_A_EXT and
     ALPHA_OPERAND_A_EXT sources are both PREVIOUS_EXT; the
     RGB_OPERAND_B_EXT and ALPHA_OPERAND_B_EXT sources are both TEXTURE;
     the RGB_OPERAND_C_EXT and ALPHA_OPERAND_C_EXT sources are both ZERO;
     all operand mappings are IDENTITY_EXT; all RGB operand components are
     RGB; and all ALPHA operand mappings are ALPHA.

Additions to Chapter 4 of the GL Specification (Per-Fragment
Operations and the Framebuffer)

     None

Additions to Chapter 5 of the GL Specification (Special Functions)

     None

Additions to Chapter 6 of the GL Specification (State and State Requests)

     3. Enumerated Queries

        void GetTexEnvOperandivEXT(enum pname, enum query, int *param);

     GetTexEnvOperandivEXT places information about <query> (a symbolic
     constant) for <pname> (also symbolic constant) in <param>.

Additions to the GLX Specification

     None

GLX Protocol

     TBD

Errors

     INVALID_ENUM is generated when target or pname parameters of TexEnvi
     or TexEnvf are not one of the accepted defined values, or when params
     should have a defined constant value (based on the value of pname) and
     does not.

     INVALID_ENUM is generated when pname, source, mapping, or component
     parameters of TexEnvOperandiEXT are not one of the accepted defined
     values.

     INVALID_ENUM is generated when pname parameter of
     GetTexEnvOperandEXTiv is not one of the accepted defined values.

     INVALID_OPERATION is generated if TexEnvOperandiEXT, or
     GetTexEnvOperandivEXT are executed between execution of Begin and the
     corresponding execution of End.

New State

     All new state belongs to Section 3.8.9. All new state has an attribute
     of 'texture'. These are additions/modifications to table 6.14.


  Get Value                   Type   Get Command            Initial Value
  ---------                   ----   -----------            ------------ 
  TEXTURE_ENV_MODE_ALPHA_EXT  Z6     GetTexEnviv            MODULATE     
  TEXTURE_ENV_MODE_RGB_EXT    Z6     GetTexEnviv            MODULATE 
  ALPHA_SCALE                 R      GetTexEnvfv            1.0      
  RGB_SCALE_EXT               R      GetTexEnvfv            1.0      
  ALPHA_OPERATION_EXT         Z3     GetTexEnviv            ADD      
  RGB_OPERATION_EXT           Z3     GetTexEnviv            ADD      
  SOURCE_EXT                  6 X    GetTexEnvOperandivEXT  PREVIOUS_EXT for
                              Z6                            ALPHA_OPERAND_A_EXT

                                                            PREVIOUS_EXT for
                                                            RGB_OPERAND_A_EXT

                                                            TEXTURE for
                                                            ALPHA_OPERAND_B_EXT

                                                            TEXTURE for
                                                            RGB_OPERAND_B_EXT

                                                            ZERO for
                                                            ALPHA_OPERAND_C_EXT

                                                            ZERO for
                                                            RGB_OPERAND_C_EXT

  MAPPING_EXT                 6 X    GetTexEnvOperandivEXT  IDENTITY_EXT for
                              Z2                            ALPHA_OPERAND_A_EXT

                                                            IDENTITY_EXT for
                                                            RGB_OPERAND_A_EXT

                                                            IDENTITY_EXT for
                                                            ALPHA_OPERAND_B_EXT

                                                            IDENTITY_EXT for
                                                            RGB_OPERAND_B_EXT

                                                            IDENTITY_EXT for
                                                            ALPHA_OPERAND_C_EXT

                                                            IDENTITY_EXT for
                                                            RGB_OPERAND_C_EXT

  COMPONENT_EXT               6 X    GetTexEnvOperandivEXT  ALPHA for
                              Z2                            ALPHA_OPERAND_A_EXT

                                                            RGB for
                                                            RGB_OPERAND_A_EXT

                                                            ALPHA for
                                                            ALPHA_OPERAND_B_EXT

                                                            RGB for
                                                            RGB_OPERAND_B_EXT

                                                            ALPHA for
                                                            ALPHA_OPERAND_C_EXT

                                                            RGB for
                                                            RGB_OPERAND_C_EXT

     Table 6.14: Texture Environment and Generation

New Implementation Dependent State

     None