The Moving Picture Experts Group

Fixed point 8x8 DCT/IDCT

Part number: 
Activity status: 

Fixed-Point 8x8 IDCT and DCT


MPEG doc#: N7336
Date: July 2005
Author: Gary Sullivan


A number of visual coding related specifications (e.g., MPEG-1 Part 2, MPEG-2 Part 2 / H.262, MPEG-4 Part 2, JPEG-1, H.261, and H.263) include a requirement for decoders to implement an integer-output approximation of the 8x8 inverse discrete cosine transform (IDCT) function.  In addition, in practical terms, the implementation of encoders for these standards requires the implementation of approximations of both the IDCT and a forward discrete cosine transform (forward DCT, or simply DCT).

MPEG standards require decoders to use an approximation that is within a specified degree of precision relative to the integer values obtained from the ideal real-valued function definition of the IDCT followed by rounding to the nearest integer.

MPEG is considering the need for development of a new voluntary standard specifying a particular fixed-point approximation to the integerized ideal IDCT function, and possibly containing an additional (non-normative) example particular fixed-point approximation to the ideal forward DCT function.

The intended benefits of the development of this new normative IDCT standard are as follows.

  • Providing an example IDCT (and perhaps also an example forward DCT) method to ease the implementation community in their design of decoders and encoders.
  • To help ensure that decoders are implemented in conformance with the standard, as those decoders that are designed to use the specified method will be assured to conform to the IDCT conformance requirements of the relevant video coding standards.
  • To improve the quality of delivered video, as encoders designed to target their encoding process for the specified IDCT method can be assured that the decoding process will be free of drift on all decoders that conform to the new standard.

Conformance to the new IDCT specification standard would be voluntary (optional) for decoders that implement the relevant MPEG video coding standards.  The requirements for conformance to the relevant MPEG video coding standards will not be affected by the development of this specification.

Additionally, we believe it will be helpful for new specification to include an example forward DCT method.  The forward DCT description would be provided as an informative (non-normative) example only – as its only goal is to ease the effort required for implementation of an encoder.