GL_SUN_global_alpha

Name

    SUN_global_alpha

Name Strings

    GL_SUN_global_alpha

Contact

    Jack Middleton, Sun (Jack.Middleton 'at' eng.sun.com)

Status

    Shipping

Version

    $Date: 1999/12/28 01:40:37 $ $Revision: 1.4 $
    SUN Date: 99/06/25 13:05:48  Revision: 1.7

Number

   164

Dependencies

    OpenGL 1.2 is required

Overview

    Transparency is done in OpenGL using alpha blending. An alpha value
    of 0.0 is used for fully transparent objects, while an alpha value
    of 1.0 is used for fully opaque objects.  A value of 0.25 is 75%
    transparent, and so on.

    OpenGL defines alpha as a component of the vertex color state.
    Whenever a color is set, the alpha component is set along with the
    red, green, and blue components.  This means that transparency
    can't be changed for primitives with per-vertex colors without
    modifying the color of each vertex, replacing the old alpha
    component with the new alpha component.  This can be very expensive
    for objects that are drawn using vertex arrays; it all but
    precludes the use of display lists.

    This extension defines a new global alpha attribute that can be
    used to specify an alpha factor that is independent from the alpha
    component of the color value.  The global alpha factor is
    multiplied by the fragment's alpha value after primitive
    rasterization and prior to texture mapping, replacing the
    fragment's alpha value.  The global alpha extension is only
    specified in RGBA mode and must be applied prior to any texture
    mapping operation.  It is enabled by a new GLOBAL_ALPHA flag.

New Procedures and Functions

    void GlobalAlphaFactorbSUN(byte factor);

    void GlobalAlphaFactorsSUN(short factor);

    void GlobalAlphaFactoriSUN(int factor);

    void GlobalAlphaFactorfSUN(float factor);

    void GlobalAlphaFactordSUN(double factor);

    void GlobalAlphaFactorubSUN(ubyte factor);

    void GlobalAlphaFactorusSUN(ushort factor);

    void GlobalAlphaFactoruiSUN(uint factor);

New Tokens

    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled:

	GLOBAL_ALPHA_SUN		0x81D9

    Accepted by the by the <pname> parameter of GetBooleanv,
    GetIntegerv, GetFloatv, and GetDoublev:

	GLOBAL_ALPHA_FACTOR_SUN		0x81DA

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

    None

Additions to Chapter 3 of the GL Specification (Rasterization)

    Global alpha processing is a new stage in the rasterization
    pipeline that occurs after primitive rasterization (including
    pixel rectangles and bitmaps) and before any texture mapping
    operations.  Global alpha processing is only done in RGBA mode.
    It has no effect in color index mode.

    Global alpha processing is enabled or disabled with the Enable and
    Disable commands using the symbolic constant GLOBAL_ALPHA_SUN.
    The global alpha factor is set using one of the GlobalAlphaFactor*SUN
    commands listed above.

    If global alpha processing is enabled, then the global alpha factor
    is multiplied by the incoming fragment's alpha value.  This
    product replaces the fragment's alpha value and is passed to
    subsequent stages in the pipeline.

    Note that for geometric primitives, a functionally equivalent
    method to that of performing a multiplication for each
    fragment is to multiply the alpha values at each vertex of the
    primitive by the global alpha factor prior to the primitive
    rasterization step.

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)

    None

Additions to the GLX / WGL / AGL Specifications

    None

GLX Protocol

    Eight rendering commands are sent to the server as part of
    the glXRender request:

        GlobalAlphaFactorbSUN
	  2		8		rendering command length
	  2		16391		rendering command opcode
	  1		INT8		code
	  1		INT8		pad
	  2		INT16		pad

        GlobalAlphaFactorsSUN
	  2		8		rendering command length
	  2		16392		rendering command opcode
	  2		INT16		code
	  2		INT16		pad

        GlobalAlphaFactoriSUN
	  2		8		rendering command length
	  2		16393		rendering command opcode
	  4		INT32		code

        GlobalAlphaFactorfSUN
	  2		8		rendering command length
	  2		16394		rendering command opcode
	  4		FLOAT32		code

        GlobalAlphaFactordSUN
	  2		12		rendering command length
	  2		16395		rendering command opcode
	  8		FLOAT64		code

        GlobalAlphaFactorubSUN
	  2		8		rendering command length
	  2		16396		rendering command opcode
	  1		CARD8		code
	  1		CARD8		pad
  	  2		CARD16		pad

        GlobalAlphaFactorusSUN
	  2		8		rendering command length
	  2		16397		rendering command opcode
	  2		CARD16		code
	  2		CARD16		pad

        GlobalAlphaFactoruiSUN
	  2		8		rendering command length
	  2		16398		rendering command opcode
	  4		CARD32		code

Errors

    None

New State

    Get Value				Get Command	Type	Initial Value
    ---------				-----------	----	-------------
    GLOBAL_ALPHA_FACTOR_SUN		GetIntegerv,	C	1
					GetFloatv
    GLOBAL_ALPHA_SUN			IsEnabled	B	False

    Get Value				Attribute
    ---------				---------
    GLOBAL_ALPHA_FACTOR_SUN		color-buffer
    GLOBAL_ALPHA_SUN			color-buffer/enable

Revision History

    6/25/99 Added fields from the new extension template.

Implementation Support

   List of OpenGL implementations supporting the GL_SUN_global_alpha extension

Original File

   Original text file for the GL_SUN_global_alpha extension