skia / external / github.com / KhronosGroup / OpenGL-Registry / 6bd92218f5ab5ee6c9fd02c0993dcae3ebb5c063 / . / extensions / SGIX / SGIX_fragment_lighting_space.txt

XXX - not done yet | |

Name | |

SGIX_fragment_lighting_space | |

Name Strings | |

GL_SGIX_fragment_lighting_space | |

Version | |

$Date: 1998/05/12 23:52:07 $ $Revision: 1.14 $ | |

Number | |

118 | |

Dependencies | |

EXT_fragment_lighting is required. | |

EXT_coordinate_frame is required. | |

Overview | |

This extension specifies that the fragment lighting vectors including | |

the view vector, light vectors, halfangle vectors and | |

spotlight direction vectors, be transformed into either eye-space, | |

object-space or tangent-space on a per-vertex basis. | |

The default is eye-space. | |

Combined with the ability to take a normal vector from texture memory, | |

as specified by the SGIX_light_texture extension, | |

the ability to transform other shading vectors to the same space | |

as the normal vector allows advanced shading effects such as | |

bump mapping to be realized. | |

The tangent-space transform uses the per-vertex tangent and binormal | |

vector introduced by the EXT_coordinate_frame extension. | |

Issues | |

* A suggestion for a different way to attack this is to introduce the | |

concept of a per-vertex matrix which is applied to lighting vectors. | |

Coupled with a new matrix-gen function, we could generate the necessary | |

per-vertex matrices. | |

* Are we really dependent upon fragment light? | |

Theoretically, no, but doing lighting in a space other than | |

eye-space doesn't help one much unless we can do something | |

like get normal vectors from texture. | |

For that reason it doesn't seem to make sense to burden per-vertex | |

lighting with this generality. | |

* If we are dependent upon fragment light are we not also dependent | |

upon SGIX_light_texture? | |

It would seem to make little sense to implement light_space and not | |

light texture because the combination of the two of them is what | |

makes bump mapping possible. But since there is nothing gained by | |

making light_space dependent upon light_texture we won't make it | |

dependent upon light_texture | |

New Procedures and Functions | |

FragmentLightSpaceSGIX(enum space); | |

New Tokens | |

Accepted by the <space> parameter of FragmentLightSpaceSGIX: | |

EYE_SPACE_SGIX 0x8436 | |

TANGENT_SPACE_SGIX 0x8437 | |

OBJECT_SPACE_SGIX 0x8438 | |

Accepted by the <pname> parameter of GetBooleanv, GetIntegerv, | |

GetFloatv, and GetDoublev: | |

FRAGMENT_LIGHT_SPACE_SGIX 0x843D | |

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

The fragment_light.spec section 3.9.2 Lighting Operation is amended: | |

If FragmentLightSpaceSGIX specifies | |

EYE_SPACE_SGIX <default> | |

then calculations are carried out in eye space. | |

If FragmentLightSpaceSGIX specifies | |

TANGENT_SPACE_SGIX | |

then calculations are carried out in tangent space by | |

constructing an orthonormal basis from the per-vertex tangent, | |

binormal and normal vectors and using that matrix to transform | |

eye-space shading vectors into tangent space on a | |

per-vertex basis. The vectors are normalized before being passed | |

to the rasterization stage where the fragment lighting calculations | |

are performed. | |

The matrix is constructed as follows: | |

if NORMALIZE is enabled, | |

b = cross(normal,tangent); | |

t = cross(b,n); | |

t = normalize(t); | |

b = normalize(b); | |

n = normalize(normal); | |

else | |

t = tangent | |

b = binormal | |

n = normal | |

the matrix | |

M = ( t - - ) | |

( b - - ) | |

( n - - ) | |

maps (1,0,0) to t, | |

(0,1,0) to b, | |

(0,0,1) to n. | |

Because it is an orthogonal matrix, its inverse is its transpose: | |

Mi = ( t b n ) | |

( | | | ) | |

( | | | ) | |

and maps t to (1,0,0), | |

b to (0,1,0), | |

n to (0,0,1), ie, maps vectors to tangent space. | |

Any shading vectors required by the fragment lighting rasterization | |

stages, which could include the view vector V, light vector VPli, | |

halfangle vector hi and spotlight direction vector, sdli, | |

are first computed in eye space on a per-vertex basis and then | |

transformed by Mi to the tangent space shading vectors, Vts, Ltsi, Htsi, sdltsi. | |

if FragmentLightSpaceSGIX specifies | |

OBJECT_SPACE_SGIX | |

then fragment lighting calculations can be carried out in object space | |

by transforming the eye-space shading vectors by the | |

inverse of the current modelview matrix and subsequently normalizing | |

them. | |

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) | |

tables are extended with state values from New State section | |

Additions to the GLX Specification | |

None | |

GLX Protocol | |

XXX - not yet complete | |

Errors | |

INVALID_ENUM is generated if FragmentLightSpaceSGIX parameter | |

<space> is not EYE_SPACE_SGIX, OBJECT_SPACE_SGIX, or TANGENT_SPACE_SGIX. | |

INVALID_OPERATION is generated if FragmentLightSpaceSGIX is called | |

between the execution of Begin and the corresponding execution of End. | |

New State | |

Get Value Get Command Type Initial Value Attribute | |

--------- ----------- ---- ------------- --------- | |

FRAGMENT_LIGHT_SPACE_SGIX GetIntegerv Z3 EYE_SPACE_SGIX lighting/enable | |

New Implementation Dependent State | |

None | |