skia / external / github.com / KhronosGroup / OpenGL-Registry / b30fdddf61ace3c3455a1918329b7a4bf5a402b8 / . / extensions / OES / OES_vertex_half_float.txt

Name | |

OES_vertex_half_float | |

Name Strings | |

GL_OES_vertex_half_float | |

Contact | |

Aaftab Munshi (amunshi@ati.com) | |

Notice | |

Copyright (c) 2005-2013 The Khronos Group Inc. Copyright terms at | |

http://www.khronos.org/registry/speccopyright.html | |

Specification Update Policy | |

Khronos-approved extension specifications are updated in response to | |

issues and bugs prioritized by the Khronos OpenGL ES Working Group. For | |

extensions which have been promoted to a core Specification, fixes will | |

first appear in the latest version of that core Specification, and will | |

eventually be backported to the extension document. This policy is | |

described in more detail at | |

https://www.khronos.org/registry/OpenGL/docs/update_policy.php | |

IP Status | |

None. | |

Status | |

Ratified by the Khronos BOP, July 22, 2005. | |

Version | |

April 22, 2007 (version #3) | |

Number | |

OpenGL ES Extension #38 | |

Dependencies | |

This extension is written against the OpenGL 2.0 specification | |

Overview | |

This extension adds a 16-bit floating pt data type (aka half float) | |

to vertex data specified using vertex arrays. The 16-bit floating-point | |

components have 1 sign bit, 5 exponent bits, and 10 mantissa bits. | |

The half float data type can be very useful in specifying vertex attribute | |

data such as color, normals, texture coordinates etc. By using half floats | |

instead of floats, we reduce the memory requirements by half. Not only does | |

the memory footprint reduce by half, but the memory bandwidth required for | |

vertex transformations also reduces by the same amount approximately. | |

Another advantage of using half floats over short/byte data types is that we | |

do not needto scale the data. For example, using SHORT for texture coordinates | |

implies that we need to scale the input texture coordinates in the shader or | |

set an appropriate scale matrix as the texture matrix for fixed function pipeline. | |

Doing these additional scaling operations impacts vertex transformation | |

performance. | |

Issues | |

1. Should there be a half-float version of VertexAttrib{1234}[v] functions | |

RESOLUTION: No. | |

There is no reason to support this, as these functions are not | |

performance or memory footprint critical. It is much more important that the | |

vertex data specified using vertex arrays be able to support half float data | |

format. | |

New Procedures and Functions | |

None | |

New Tokens | |

Accepted by the <type> parameter of VertexPointer, NormalPointer, | |

ColorPointer, SecondaryColorPointer, IndexPointer, FogCoordPointer, | |

TexCoordPointer, and VertexAttribPointer | |

HALF_FLOAT_OES 0x8D61 | |

Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation) | |

Add a new section 2.1.2. This new section is copied from the | |

ARB_texture_float extension. | |

2.1.2 16-Bit Floating-Point Numbers | |

A 16-bit floating-point number has a 1-bit sign (S), a 5-bit | |

exponent (E), and a 10-bit mantissa (M). The value of a 16-bit | |

floating-point number is determined by the following: | |

(-1)^S * 0.0, if E == 0 and M == 0, | |

(-1)^S * 2^-14 * (M / 2^10), if E == 0 and M != 0, | |

(-1)^S * 2^(E-15) * (1 + M/2^10), if 0 < E < 31, | |

(-1)^S * INF, if E == 31 and M == 0, or | |

NaN, if E == 31 and M != 0, | |

where | |

S = floor((N mod 65536) / 32768), | |

E = floor((N mod 32768) / 1024), and | |

M = N mod 1024. | |

Implementations are also allowed to use any of the following | |

alternative encodings: | |

(-1)^S * 0.0, if E == 0 and M != 0, | |

(-1)^S * 2^(E-15) * (1 + M/2^10), if E == 31 and M == 0, or | |

(-1)^S * 2^(E-15) * (1 + M/2^10), if E == 31 and M != 0, | |

Any representable 16-bit floating-point value is legal as input | |

to a GL command that accepts 16-bit floating-point data. The | |

result of providing a value that is not a floating-point number | |

(such as infinity or NaN) to such a command is unspecified, but | |

must not lead to GL interruption or termination. Providing a | |

denormalized number or negative zero to GL must yield predictable | |

results. | |

Modifications to section 2.8 (Vertex Arrays) | |

Add HALF_FLOAT_OES as a valid <type> value in Table 2.4. | |

For <type> the values BYTE, SHORT, INT, FLOAT, and DOUBLE indicate | |

types byte, short, int, float, and double, respectively; and the values | |

UNSIGNED_BYTE, UNSIGNED_SHORT, and UNSIGNED_INT indicate types ubyte, | |

ushort, and uint, respectively. A <type> value of HALF_FLOAT_OES represents | |

a 16-bit floating point number with 1 sign bits, 5 exponent bits, | |

and 10 mantissa bits. | |

Errors | |

None | |

New State | |

None | |

Revision History | |

June 15, 2005 Aaftab Munshi First draft of extension. | |

June 22, 2005 Aaftab Munshi Renamed HALF_FLOAT token to HALF_FLOAT_OES | |

#3, 2007/04/22 Jon Leech Add version date and extension number. |