xref: /third_party/openGLES/extensions/SGIS/SGIS_clip_band_hint.txt

XXX - Not complete yet!!!

Name

    SGIS_clip_band_hint

Name Strings

    GL_SGIS_clip_band_hint

Version

    $Date: 1998/08/07 02:43:19 $ $Revision: 1.1 $

Number

    XXX

Dependencies

    None

Overview

    For graphics systems that decouple geometry processing from the graphics
    hardware to the host system CPU, it is often beneficial to allow the
    host processing code not to have to perform clip checking if geometric
    data is known not to require clipping.  Since many applications already
    perform some amount of coarse-grained view volume clipping, this
    information can be reused efficiently to turn clip-checking on or off.  
    In addition, certain graphics subsystems are capable of performing fast 
    2D viewport or, in some cases, 3D volume "scissoring" operations within
    some coordinate range much faster that the host CPU could re-tesselate
    clipped primitives.

    This extension introduces the notion of an extended rasterizable view
    volume that is an expansion of the clip-space view volume.  This volume
    is the space within which a particular graphics system is much more
    efficient at rejecting fragments that lie outside the view volume than
    it is at performing strict view volume clipping.  This extended volume 
    is characterized by queriable implementation dependent expansion factors
    to the normal view volume.  Clip-checking can be turned on or off 
    through the glHint mechanism.

Issues

    * This extension is essentially a superset of the 
      clip_volume_hint extension.  Do we want to have this extension just
      introduce the notion of clip band factors instead and use the
      clip_volume_hint extension for the hint?

    * It is possible that for certain implementation the X, Y and Z factors 
      are dependent the size of the viewport and are thus not necessarily 
      constant throughout the execution of an application.  Is there 
      a better way of describing this and suggesting when to query these 
      factors?

    * Better name?  Should it have a different branding that SGIS?

    * No mention has been made in the description on the effect user clip
      planes have on the whether the clip-checking hint is honored or not.  
      Clip-checking should probably always be performed for user defined 
      clip planes independent of the state of the hint.


New Procedures and Functions

    None

New Tokens

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

        CLIP_CHECK_HINT_SGIS	XXXXX

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

        CLIP_BAND_FACTOR_X_SGIS	XXXXX
        CLIP_BAND_FACTOR_Y_SGIS	XXXXX
        CLIP_BAND_FACTOR_Z_SGIS	XXXXX

Additions to Chapter 2 of the 1.1 Specification (OpenGL Operation)

    Before the last paragraph of section 2.11 (Clipping), the following text
    is added: (Should it go in the section 5.6 (Hints) instead?)

    If it is known that primitives lie within the view volume,
    the clip checking operation may be disabled through the use of the
    CLIP_CHECK_HINT_SGIS hint.  In addition, clip-checking may be disabled
    for graphics subsystems that are able to rasterize correctly primitives
    that extend past the view volume but are still within some rasterizable
    volume.  This extended clip volume is defined in clip coordinates by,

		-(w_c * bf_x) <= x_c <= (w_c * bf_x)
		-(w_c * bf_y) <= y_c <= (w_c * bf_y)
		-(w_c * bf_z) <= z_c <= (w_c * bf_z)

    where bf_x, bf_y and bf_z are implementation dependent values (greater
    than or equal to 1.0) and can be queried by calling glGet with a 
    <value> of CLIP_BAND_FACTOR_X_SGIS, CLIP_BAND_FACTOR_Y_SGIS and 
    CLIP_BAND_FACTOR_X_SGIS respectively.  Clip-checking is disabled when 
    the <hint> parameter to glHint is FASTEST, it is enabled for all other
    values.

    Since implementations may choose to base the clip band factors on a
    function of the viewport size, it is recommended that these factors be
    queried after the viewport which will be used for rendering has been 
    set.

    Results of rasterization for geometric primitives that extend past the 
    rasterizable volume when clip checking has been disabled are 
    undefined.  Finally, the state of the current raster position is not 
    affected by the value of clip_band_hint and is always clipped against 
    the actual clip volume.

Additions to Chapter 3 of the 1.1 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.1 Specification (Per-Fragment Operations
and the Frame Buffer)

    None

Additions to Chapter 5 of the 1.1 Specification (Special Functions)

    To the list of hints in section 5.6 (Hints) add:

    CLIP_CHECK_HINT, indicating whether to turn clip-checking on or off.

Additions to Chapter 6 of the 1.1 Specification (State and State Requests)

    None

Additions to the GLX Specification

    XXX - Not complete yet!!!

GLX Protocol

    XXX - Not complete yet!!!

Errors


New State

								Initial
    Get Value				Get Command	Type	Value		Attrib
    ---------				-----------	----	-------		------

    CLIP_CHECK_HINT_SGIS		GetIntegerv	Z3	DONT_CARE	Hint


New Implementation Dependent State

									Minimum
    Get Value				Get Command	Type		Value
    ---------				-----------	----		-------
    
    CLIP_BAND_FACTOR_X_SGIS		GetFloatv	R		1.0
    CLIP_BAND_FACTOR_Y_SGIS		GetFloatv	R		1.0
    CLIP_BAND_FACTOR_Z_SGIS		GetFloatv	R		1.0