[Mp4-tech] [H.264] A question about the scaling list syntax

[Gary Sullivan]

It sounds like you may be misunderstanding the purpose of the "Flat"
arrays.  We never change the values in those arrays.  Those arrays just
determine the value that we put into the scaling list when
seq_scaling_matrix_present_flag is equal to 0.  Nothing in the bitstream
ever affects the entries in the "Flat" arrays.  It may be helpful to
read the reference software source code to understand this.
Best Regards,
Gary Sullivan
________________________________
	From: mp4-tech-bounces lists.mpegif.org
[mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Tan Rui --
Huawei
	Sent: Saturday, September 30, 2023 1:39 AM
	To: mpeg 4 mail-list
	Subject: [Mp4-tech] [H.264] A question about the scaling list
syntax
	Hi, all experts, when I read the protocol about the
seq_scaling_matrix_present_flag, I find some questions as the following:
	seq_scaling_matrix_present_flag equal to 1 specifies that the
flags seq_scaling_list_present_flag[ i ] for i = 0..7 are
	present. seq_scaling_matrix_present_flag equal to 0 specifies
that these flags are not present and the sequence-level
	scaling list specified by Flat_4x4_16 shall be inferred for i =
0..5 and the sequence-level scaling list specified by
	Flat_8x8_16 shall be inferred for i = 6..7. When
seq_scaling_matrix_present_flag is not present, it shall be inferred to
	be equal to 0.
	The scaling lists Flat_4x4_16 and Flat_8x8_16 are specified as
follows:
	Flat_4x4_16[ i ] = 16, with i = 0..15, ( 7-6)
	Flat_8x8_16[ i ] = 16, with i = 0..63. ( 7-7)
	[ Question : In the first paragraph, we can get the following
understanding: 
	    for(i=0; i<16; i++) {
	        if seq_scaling_list_present_flag[i] == 1 {
	            if(i<6) {
	                //According to each
seq_scaling_list_present_flag,
	                //There will be 16 scaling list elements, i.e.,
	                //Flat_4x4_16[i], with i = 0..15.
	               //In the end, there will be 6x16=96 Flat_4x4_16
elements for all. Is that true?
	            }
	            else {
	                //There will be 64 scaling list elements, i.e.,
	                //Flat_8x8_16[i], with i = 0..63
	            }
	        }
	        else {
	            //There will not be any Flat_8x8_16 and Flat_4x4_16
elements.
	        }
	    }
	But I can not find any further usage of the Flat_4x4_16 and
Flat_8x8_16. Can you give me some clue?
	By the way, I find some syntax comments in the 7.3.2.1.1 just
like the following:
	scaling_list( scalingList, sizeOfScalingList,
useDefaultScalingMatrixFlag ) {
	    lastScale = 8
	    nextScale = 8
	    for( j = 0; j < sizeOfScalingList; j++ ) {
	        if( nextScale != 0 ) {
	            delta_scale
	            nextScale = ( lastScale + delta_scale + 256 ) % 256
	            useDefaultScalingMatrixFlag = ( j = = 0 && nextScale
= = 0 )
	        }
	        scalingList[ j ] = ( nextScale = = 0 ) ? lastScale :
nextScale
	        lastScale = scalingList[ j ]
	    }
	}
	I wonder that the assignment for the useDefaultScalingMatrixFlag
variable. It mean that when i==0 and nextScale==0,
	the useDefaultScalingMatrixFlag will be 1, doesn't it? When
useDefaultScalingMatrixFlag is equal to 1, which scaling
	list variable will use the default value? I can not find any
assignments in the protocol.
	] 
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