[M4IF Technotes] Round-off error in IDCT

[sunx]

MPEG-4 Part 10 also adopts the Integer DCT/IDCT.
On Wed, 25 Sep 2002, Kevan Ahmadi wrote:
> The MPEG 4 and MPEG 2 standards refer to an IEEE document which lists a
> series of tests that can be performed on an IDCT implementation to verify
> it's accuracy.  The implementation being tested is compared to a full
> precision floating point implementation and is allowed to deviate by a
> maximum of 2 in any case.
>
> The IEEE document is Std 1180-1990 and is titled "IEEE Standard
> Specification for the Implementation of 8x8 Inverse Discrete Cosine
> Transform".
>
> If you do get hold of that document be aware that both MPEG 2 and 4 specify
> changes to the wording of the tests which have the effect of relaxing the
> requirements.
>
> I believe the maximum input coefficient size can be represented in 12 bits
> (-2048 to +2047) and the output can be represented in 9 bits (-256 to +255).
>
> Good luck with your project.
>
> Kevan Ahmadi
> Leading Design Engineer
> PowerVR Technologies
> kevan.ahmadi   powervr.com
> www.powervr.com
>
>
> -----Original Message-----
> From: Morten Boysen [mailto:mortenb   mips.com]
> Sent: 25 September 2023 13:30
> To: technotes   lists.m4if.org
> Subject: [M4IF Technotes] Round-off error in IDCT
>
>
> Hi,
>
> The reference decoder uses floating point in computing the IDCT. As a
> part of a schoolproject, i have to rewrite the IDCT, so it only uses
> integers. I know this will cause round-off errors, but the problem is:
> How large are these round-off errors allowed to be according to the
> standard?
>
> Furthermore, i need the maximum allowable value of an IDCT coefficient,
> so i can save as many bits as possible.
>
> Thanks
>
> --
> --
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