skia / external / github.com / KhronosGroup / OpenGL-Registry / d15191e446614fcec3369577495dfb45b131a27a / . / extensions / EXT / EXT_polygon_offset.txt

Name | |

EXT_polygon_offset | |

Name String | |

GL_EXT_polygon_offset | |

Version | |

$Date: 1995/06/17 03:34:49 $ $Revision: 1.12 $ | |

Number | |

3 | |

Dependencies | |

None | |

Overview | |

The depth values of fragments generated by rendering polygons are | |

displaced by an amount that is proportional to the maximum absolute | |

value of the depth slope of the polygon, measured and applied in window | |

coordinates. This displacement allows lines (or points) and polygons | |

in the same plane to be rendered without interaction -- the lines | |

rendered either completely in front of or behind the polygons | |

(depending on the sign of the offset factor). It also allows multiple | |

coplanar polygons to be rendered without interaction, if different | |

offset factors are used for each polygon. Applications include | |

rendering hidden-line images, rendering solids with highlighted edges, | |

and applying `decals' to surfaces. | |

New Procedures and Functions | |

void PolygonOffsetEXT(float factor, | |

float bias); | |

New Tokens | |

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

by the <pname> parameter of GetBooleanv, GetIntegerv, GetFloatv, and | |

GetDoublev: | |

POLYGON_OFFSET_EXT 0x8037 | |

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

GetFloatv, and GetDoublev: | |

POLYGON_OFFSET_FACTOR_EXT 0x8038 | |

POLYGON_OFFSET_BIAS_EXT 0x8039 | |

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

None | |

Additions to Chapter 3 of the GL Specification (Rasterization) | |

The changes to the GL Specification are limited to the description of | |

polygon rasterization, specifically while the polygon mode (specified | |

by calling PolygonMode) is FILL. The GL Specification requires that | |

fragment Z values during such rasterization be computed using the | |

equation | |

Z = (a * Za) + (b * Zb) + (c * Zc) | |

where a, b, and c are the barycentric coordinates of the fragment | |

center, and Za, Zb, and Zc are the Z values of the triangle's vertices. | |

When POLYGON_OFFSET_EXT is enabled, this extension modifies this | |

equation as follows: | |

Z' = (a * Za) + (b * Zb) + (c * Zc) + (factor * maxdZ) + bias | |

/ 0 Z' < 0 | |

Z = < Z' 0 <= Z' <= 1 | |

\ 1 Z' > 1 | |

where factor and bias are the polygon offset factor and bias as | |

specified by PolygonOffsetEXT, and maxdZ is the maximum positive change | |

in Z for a unit-step in the X,Y plane. MaxdZ can be approximated as | |

the larger of the absolute values of dZ/dX and dZ/dY, the rates of | |

change of Z in the positive X and Y directions. The equations for | |

maxdZ, dZ/dX, and dZ/dY are: | |

2 2 | |

maxdZ = sqrt[ (dZ/dX) + (dZ/dY) ] | |

((Yc - Yb) * (Zb - Za)) - ((Yb - Ya) * (Zc - Zb)) | |

dZ/dX = ------------------------------------------------- | |

area | |

((Xb - Xa) * (Zc - Zb)) - ((Xc - Xb) * (Zb - Za)) | |

dZ/dY = ------------------------------------------------- | |

area | |

area = ((Xb - Xa) * (Yc - Yb)) - ((Xc - Xb) * (Yb - Ya)) | |

To simplify the calculations, maxdZ may be approximated by | |

maxdZ = maximum( |dZ/dX| , |dZ/dY| ) | |

Note that all these equations treat window coordinate Z values as | |

ranging from 0.0 through 1.0, regardless of their actual representation | |

(refer to Controlling the Viewport in Chapter 2 of the GL Specification). | |

POLYGON_OFFSET_EXT is enabled and disabled using Enable and Disable with | |

parameter <cap> specified as POLYGON_OFFSET_EXT. | |

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 Specification | |

None | |

GLX Protocol | |

A new GL rendering command is added. The following command is sent to the | |

server as part of a glXRender request: | |

PolygonOffsetEXT | |

2 12 rendering command length | |

2 4098 rendering command opcode | |

4 FLOAT32 factor | |

4 FLOAT32 bias | |

Errors | |

INVALID_OPERATION is generated if PolygonOffsetEXT is called between | |

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

New State | |

Initial | |

Get Value Get Command Type Value Attrib | |

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

POLYGON_OFFSET_EXT IsEnabled B False polygon/enable | |

POLYGON_OFFSET_FACTOR_EXT GetFloatv R 0 polygon | |

POLYGON_OFFSET_BIAS_EXT GetFloatv R 0 polygon | |

New Implementation Dependent State | |

None |