[Mp4-tech] [H.264] Inverse 4x4 transform and the 1-D transform order

[Ronny Pedersen]

Hmmm, seems like I should probably get that latest version of the
spec ;-) It seems like I have an old draft. I wonder why this has
changed though? Well, anyway, thank you for your help.
-Ronny
On 10/9/06, Gary Sullivan <garysull windows.microsoft.com> wrote:
>
> Ronny (et al),
>
> You're not correct about what the text of the standard says.  Refer to
> subclause 8.5.10, which says:
>
> "First, each (horizontal) row of scaled transform coefficients is
> transformed using a one-dimensional inverse transform as follows." ...
> "Then, each (vertical) column of the resulting matrix is transformed
> using the same one-dimensional inverse transform as follows."
>
> Perhaps you're looking at an old draft.  Make sure you have the actual
> current standard.  That can save you some headaches.  To get the correct
> document, use the following process:
>
> Step 1: Register for the "3 Free" program at
>  http://ecs.itu.ch/cgi-bin/ebookshop
>
> Step 2: Download the spec from
>  http://www.itu.int/rec/T-REC-H.264
>
> Best Regards,
>
> Gary Sullivan
>
> +> -----Original Message-----
> +> From: mp4-tech-bounces lists.mpegif.org
> +> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of
> +> Ronny Pedersen
> +> Sent: Friday, October 06, 2023 8:21 AM
> +> To: mp4-tech lists.mpegif.org
> +> Subject: [Mp4-tech] [H.264] Inverse 4x4 transform and the
> +> 1-D transform order
> +>
> +> Hi,
> +>
> +> The H.264 specification states that first each column of the 4x4
> +> block should be processed with a 1-d inverse transform and then the
> +> rows should be processed with the same inverse transformations, but
> +> as I can see it the H.264/AVC reference decoder processes
> +> rows and columns
> +> in the opposite order.
> +>
> +> I guess this will make a difference in the result since the
> +> 1-d transforms
> +> performes a left shift (divide by two) on two of the input
> +> coefficients?
> +>
> +> What is the correct order in which to apply the 1-d  inverse
> +> transforms to
> +> the 4x4 block?
> +>
> +> Regards,
> +> Ronny
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