GL_SGIX_sprite

Name

    SGIX_sprite

Name Strings

    GL_SGIX_sprite

Version

    $Date: 1996/04/09 22:54:47 $ $Revision: 1.11 $

Number

    52

Dependencies

    None

Overview

    This extension provides support for viewpoint dependent alignment
    of geometry, in particular geometry that rotates about a point or
    a specified axis to face the eye point.  The primary use is for
    quickly rendering roughly cylindrically or spherically symmetric
    objects, e.g. trees, smoke, clouds, etc. using geometry textured
    with a partially transparent texture map.

    Rendering sprite geometry requires applying a transformation to
    primitives before the current model view. This matrix includes a
    rotation which is computed based on the current model view matrix
    and a translation which is specified explicitly
    (SPRITE_TRANSLATION_SGIX). The current model view matrix itself
    is not modified.

    Primitives are first transformed by a rotation, depending on the
    sprite mode:

	SPRITE_AXIAL_SGIX: The front of the object is rotated about
	an axis so that it faces the eye as much as the axis
	constraint allows.  This is used for roughly rendering cylindrical
	objects such as trees in visual simulation. 

	SPRITE_OBJECT_ALIGNED_SGIX: The front of the object is
	rotated about a point to face the eye with the remaining
	rotational degree of freedom specified by aligning the top
	of the object with a specified axis in object coordinates.
	This is used for spherical objects and special effects such
	as smoke which must maintain an alignment in object
	coordinates for realism.

	SPRITE_EYE_ALIGNED_SGIX: The front of the object is rotated
	about a point to face the eye with the remaining rotational
	degree of freedom specified by aligning the top of the object
	with a specified axis in eye coordinates. This is used for
	rendering sprites which must maintain an alignment on the
	screen, such as 3D annotations.

    The axis of rotation or alignment, SPRITE_AXIS_SGIX, can be 
    an arbitrary direction to support geocentric coordinate frames
    in which "up" is not along X, Y or Z.

    Sprite geometry is modeled in a canonical frame: +Z is the up
    vector. -Y is the front vector which is rotated to point towards
    the eye. In the discussion below, the eye vector is the vector to
    the eye from the origin of the model view frame translated by the
    sprite position.

New Procedures and Functions

	void SpriteParameteriSGIX(enum pname, int param);
	void SpriteParameterfSGIX(enum pname, float param);
	void SpriteParameterivSGIX(enum pname, int* params);
	void SpriteParameterfvSGIX(enum pname, float* params);
			

Issues

    * Should the canonical modeling frame of the geometry be specifiable?
      - requires API for "up" and "front" vectors; additional math

    * Should sprites for annotation be clip or eye aligned?  Clip
      alignment is a nearly trivial extension of object aligned. 
      However, once the projection matrix assumes rotation things
      are unpredictable and we should avoid this complexity.
      

New Tokens

    Accepted by the <cap> parameter of Enable, Disable, and IsEnabled,
    and by the <pname> parameter of GetBooleanv, GetIntegerv,
    GetFloatv, and GetDoublev:

	SPRITE_SGIX

    Accepted by the <pname> parameter of SpriteParameteriSGIX, and 
    SpriteParameterfSGIX:

	SPRITE_MODE_SGIX

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

	SPRITE_MODE_SGIX
	SPRITE_AXIS_SGIX
	SPRITE_TRANSLATION_SGIX

    Accepted by the <param> parameter of SpriteParameteriSGIX and
    and SpriteParameterfSGIX, and by the <params> parameter of
    SpriteParameterivSGIX and SpriteParameterfvSGIX, when the
    <pname> parameter is SPRITE_MODE_SGIX:

        SPRITE_AXIAL_SGIX
	SPRITE_OBJECT_ALIGNED_SGIX
	SPRITE_EYE_ALIGNED_SGIX
    

Additions to Chapter 2 of the 1.0 Specification (OpenGL Operation)

    SPRITE_SGIX is enabled and disabled using Enable and Disable with
    target equal to SPRITE_SGIX.  When SPRITE_SGIX is enabled, the
    current sprite mode, axis, translation and the ModelView matrix
    are used to generate a sprite transformation which is applied to 
    subsequent primitives before applying the current model view matrix.

    When the sprite mode is SPRITE_AXIAL_SGIX, the sprite transformation
    is arranged such that, after sprite and ModelView transformation,
    the front vector is pointing as nearly towards the origin of the
    eye coordinate system as possible.

    Each primitive is first rotated so that the sprite up vector aligns
    with the sprite axis which is specified in the current model view
    frame. Then the primitive is rotated about the axis so its
    front vector is parallel with the projection of the eye vector
    into the plane perpendicular to the rotation axis. Then the
    primitive is translated by the specified translation. 

    There are four unique matrices involved in computing the transformation
    required for sprite geometry:

	M - model-view matrix
	T - sprite translation matrix
	A - sprite axis rotation matrix
	R - sprite up vector/axis alignment rotation matrix

    Note that in SPRITE_AXIAL_SGIX mode, the R matrix is a function of M,
    T and A since the eye vector is derived from the inverse transpose matrix 
    of the compound (M * T * A). In both the SPRITE_OBJECT_ALIGNED_SGIX and 
    the SPRITE_EYE_ALIGNED_SGIX modes, the eye vector is extracted from the 
    inverse transpose of the compound (M * T).

    Given the coordinates of SPRITE_TRANSLATION_SGIX the sprite translation 
    matrix (T) is constructed. Both A and R are rotation matrices generated 
    by computing an angle of rotation and the sprite axis (computing sine and 
    cosine of the rotation angle suffices for computing a rotation matrix). 
    The computation of the matrices (T, A, R) are given in Chapter 2.9 of 
    the 1.0 OpenGL Specification (Matrices). 

    The angle and axis of rotation parameters for generating rotation matrix 
    A are computed as follows:

	    cosTheta = (V0 (dot) V1);
	    rotAxis = (V0 (cross) V1);
	    sinTheta = |rotAxis|;

    V0 is the canonical up vector. V1 is the sprite axis (SPRITE_AXIS_SGIX)
    when in SPRITE_AXIAL_SGIX mode. In either the SPRITE_OBJECT_ALIGNED_SGIX 
    and the SPRITE_EYE_ALIGNED_SGIX modes V1 is the sprite axis aligned by 
    removing its component along the eye vector. The computation of V1 in
    the latter cases is as follows:

	    scalar = Eye (dot) spriteAxis;
	    alignedSpriteAxis = spriteAxis - (scalar * Eye); 

    The rotation axis for computing the R matrix is the canonical up
    vector. The angle of rotation is computed as follows:

	    cosTheta = (Eye (dot) canonicalFront);
	    sinTheta = (Eye (dot) canonicalRight);

    Sprite primitive geometry is transformed from object coordinates
    to eye coordinates by the following compound matrix (MM):

		MM = M * T * A * R

    Sprite geometry is not limited to vertex based primitives and evaluators
    may apply as well.

    Normals associated with sprite geometry are transformed by the inverse 
    transpose of the compound matrix (MM). In general, the user would not 
    want clip planes to be transformed by the compound matrix. In a similar 
    manner, raster position and texture coordinates (for certain texgen 
    modes). The user should be warrant that once glClipPlanes, glTexGen or 
    glRasterPos are called within a glEnable/Disable (SPRITE_SGIX) delimitors, 
    the associated geometry will be transformed by the compound matrix (MM).

    When the sprite mode is SPRITE_OBJECT_ALIGN or SPRITE_EYE_ALIGN,
    the sprite transformation rotates the front vector towards the eye
    while keeping the up vector as aligned as possible with the
    current sprite axis, i.e. the up vector is rotated to be parallel
    to the projection of the current sprite axis into the plane
    perpendicular to the front vector.  Depending on the current
    sprite mode, the sprite axis is taken to be expressed in object
    (SPRITE_OBJECT_ALIGNED_SGIX) or transformed into eye 
    (SPRITE_EYE_ALIGNED_SGIX) coordinates. The transformation into
    eye space uses the compound (M * T).  

    Regardless of the sprite transformation mode, the projection matrix 
    obviously assumes no rotation elements. This is again to warrant the 
    user against overriding the sprite transformation. 

Additions to Chapter 3 of the 1.0 Specification (Rasterization)

    None

Additions to Chapter 4 of the 1.0 Specification (Per-Fragment Operations and the Frame Buffer)

    None 

Additions to Chapter 5 of the 1.0 Specification (Special Functions)

    None

Additions to Chapter 6 of the 1.0 Specification (State and State Requests)

    None

Additions to the GLX Specification

    None

Dependencies on SGI_extension_name

    None

Errors

    INVALID_ENUM is generated if SpriteParameteriSGIX or SpriteParameterfSGIX
    parameter <pname> is not SPRITE_MODE_SGIX.

    INVALID_ENUM is generated if SpriteParameterivSGIX or SpriteParameterfvSGIX
    parameter <pname> is not SPRITE_AXIS_SGIX or SPRITE_TRANSLATION_SGIX.

    INVALID_ENUM is generated if SpriteParameteriSGIX or SpriteParameterfSGIX
    parameter <pname> is SPRITE_MODE_SGIX, and parameter <param> is not
    SPRITE_AXIAL_SGIX, SPRITE_OBJECT_ALIGNED_SGIX, or SPRITE_EYE_ALIGNED_SGIX.

    INVALID_OPERATION is generated if glSpriteParameterSGIX is executed
    between the execution of glBegin and the corresponding execution of
    glEnd.

New State

                                                Initial
    Get Value                Get Command  Type  Value   	  Attrib
    ---------                -----------  ----  -------           ------
    SPRITE_SGIX		     IsEnabled	   B	FALSE	          transform/enable    
    SPRITE_MODE_SGIX	     GetIntegerv   I	SPRITE_AXIAL_SGIX transform
    SPRITE_AXIS_SGIX	     GetFloatv	   V	(0,0,1)           transform
    SPRITE_TRANSLATION_SGIX  GetFloatv	   V	(0,0,0)           transform

New Implementation Dependent State

    None

Implementation Support

   List of OpenGL implementations supporting the GL_SGIX_sprite extension

Original File

   Original text file for the GL_SGIX_sprite extension