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

[Gary Sullivan]

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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