GL_ARB_half_float_pixel |

- Name
- Name Strings
- Contributors
- Contact
- Status
- Version
- Number
- Dependencies
- Overview
- IP Status
- Issues
- New Tokens
- Additions to Chapter 2 of the OpenGL 2.0 Specification (OpenGL Operation)
- Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization)
- Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment Operations and the Framebuffer)
- Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions)
- Additions to Chapter 6 of the OpenGL 2.0 Specification (State and State Requests)
- Additions to the AGL/GLX/WGL Specifications
- GLX Protocol (Modification to the GLX 1.3 Protocol Encoding Specification)
- Errors
- New State
- New Implementation Dependent State
- Revision History

Name |

ARB_half_float_pixel

Name Strings |

GL_ARB_half_float_pixel

Contributors |

Pat Brown Jon Leech Rob Mace Brian Paul

Contact |

Dale Kirkland, 3Dlabs, Inc (dale.kirkland 'at' 3dlabs.com)

Status |

Complete. Appprove by the ARB on October 22, 2004.

Version |

Last Modified Date: October 1, 2004 Version: 6

Number |

ARB Extension #40

Dependencies |

This extension is written against the OpenGL 2.0 Specification but will work with the OpenGL 1.5 Specification. Based on the NV_half_float extension. This extension interacts with ARB_color_buffer_float.

Overview |

This extension introduces a new data type for half-precision (16-bit) floating-point quantities. The floating-point format is very similar to the IEEE single-precision floating-point standard, except that it has only 5 exponent bits and 10 mantissa bits. Half-precision floats are smaller than full precision floats and provide a larger dynamic range than similarly sized normalized scalar data types. This extension allows applications to use half-precision floating- point data when specifying pixel data. It extends the existing image specification commands to accept the new data type. Floating-point data is clamped to [0, 1] at various places in the GL unless clamping is disabled with the ARB_color_buffer_float extension.

IP Status |

SGI owns US Patent #6,650,327, issued November 18, 2003. SGI believes this patent contains necessary IP for graphics systems implementing floating point (FP) rasterization and FP framebuffer capabilities. SGI will not grant the ARB royalty-free use of this IP for use in OpenGL, but will discuss licensing on RAND terms, on an individual basis with companies wishing to use this IP in the context of conformant OpenGL implementations. SGI does not plan to make any special exemption for open source implementations. Contact Doug Crisman at SGI Legal for the complete IP disclosure.

Issues |

1. How is this extension different from the NV_half_float extension? This extension does not add new commands for specifying half- precision vertex data, and all imaging functions have been listed for supporting the "half" type. 2. What should the new data type be called? "half"? "hfloat"? RESOLVED: half . This convention builds on the convention of using the type "double" to describe double-precision floating- point numbers. Here, "half" will refer to half-precision floating-point numbers. Even though the 16-bit float data type is a first-class data type, it is still more problematic than the other types in the sense that no native programming languages support the data type. "hfloat/hf" would have reflected a second-class status better than "half/h". Both names are not without conflicting precedents. The name "half" is used to connote 16-bit scalar values on some 32-bit CPU architectures (e.g., PowerPC). The name "hfloat" has been used to describe 128-bit floating-point data on VAX systems. 3. Should half-precision data be accepted by commands in the imaging subset that accept pixel data? RESOLVED: Yes, all functions in the core OpenGL and the imaging subset that accept pixel data accept half-precision data. 4. Should the special representations NaN, INF, and denormal be supported? RESOLVED: Implementation dependent. The spec reflects that Nan and INF produce unspecified results. Denormalized numbers can be treated as a value of 0.

New Tokens |

Accepted by the <type> parameter of DrawPixels, ReadPixels, TexImage1D, TexImage2D, TexImage3D, GetTexImage, TexSubImage1D, TexSubImage2D, TexSubImage3D, GetHistogram, GetMinmax, ConvolutionFilter1D, ConvolutionFilter2D, GetConvolutionFilter, SeparableFilter2D, GetSeparableFilter, ColorTable, ColorSubTable, and GetColorTable: HALF_FLOAT_ARB 0x140B

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

Add a new Section 2.1.2, (p. 6): 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. (modify Table 2.2, p. 9) -- add new row Minimum GL Type Bit Width Description ------- --------- ----------------------------------- half 16 half-precision floating-point value encoded in an unsigned scalar Modify Section 2.14, (Colors and Coloring), p. 59 (modify Table 2.9, p. 59) Add new row to the table: GL Type Conversion ------- ---------- half c

Additions to Chapter 3 of the OpenGL 2.0 Specification (Rasterization) |

Modify Section 3.6.4, Rasterization of Pixel Rectangles (p. 126) (modify Table 3.5, p. 128 -- add new row) type Parameter Corresponding Special Token Name GL Data Type Interpretation -------------- ------------- -------------- HALF_FLOAT_ARB half No (modify Unpacking, p. 129) Data are taken from host memory as a sequence of signed or unsigned bytes (GL data types byte and ubyte), signed or unsigned integers (GL data types int and uint), or floating-point values (GL data types half and float).

Additions to Chapter 4 of the OpenGL 2.0 Specification (Per-Fragment Operations and the Framebuffer) |

Modify Section 4.3.2, Reading Pixels (p. 219) (modify Final Conversion, p. 222) For an index, if the type is not FLOAT or HALF_FLOAT_ARB, final conversion consists of masking the index with the value given in Table 4.6; if the type is FLOAT or HALF_FLOAT_ARB, then the integer index is converted to a GL float or half data value. (modify Table 4.7, p. 224 -- add new row) type Parameter GL Data Type Component Conversion Formula -------------- ------------ ---------------------------- HALF_FLOAT_ARB half c = f

Additions to Chapter 5 of the OpenGL 2.0 Specification (Special Functions) |

None

Additions to Chapter 6 of the OpenGL 2.0 Specification (State and State Requests) |

None

Additions to the AGL/GLX/WGL Specifications |

None

GLX Protocol (Modification to the GLX 1.3 Protocol Encoding Specification) |

Modify Appendix A, Pixel Data (p. 148) (Modify Table A.1, p. 149 -- add new row for HALF_FLOAT_ARB data) type Encoding Protocol Type nbytes ------------- -------- ------------- ------ HALF_FLOAT_ARB 0x140B CARD16 2

Errors |

None

New State |

None

New Implementation Dependent State |

None

Revision History |

Rev. Date Author Changes ---- -------- --------- -------------------------------------------- 1 12/15/03 Kirkland Initial version based on the NV_half_float specification. 2 2/26/04 Kirkland Changed NVIDIA_xxx to NV_xxx. Changed the issue resolution for INF and NaN. 3 3/11/04 Kirkland Updated language for float16 number handling. Added bit encodings for half values. Added issue dealing with name "half". 4 7/23/04 Kirkland Added alternative encodings options for float16 format. 5 9/17/04 Kirkland Updated to reference OpenGL 2.0 spec. 6 10/1/04 Kirkland Updated IP section. 6 10/1/04 Kirkland Updated IP section.

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