Question about decoders

Comatose · 18th July 2008, 18:55

Are they supposed to produce identical output, if 100% standard compliant?

Guest · 18th July 2008, 18:58

No. For example, MPEG2 defines the legal syntax of the bitstream and how it is decoded, but not how the bitstream is to be generated. For example, a given stream might be encoded as frames or as fields; neither is mandated by any spec while both are legal according to spec. Another example is the use of different quantization matrices for encoding. The spec doesn't specify the actual values of the quant matrices to be used for encoding (some defaults are defined, however). Then there's also GOP length, frame type, etc., etc.

Comatose · 18th July 2008, 19:03

Ah, okay, thanks =)

Sergey A. Sablin · 18th July 2008, 20:49

Quote:

Originally Posted by neuron2

No. For example, MPEG2 defines the legal syntax of the bitstream and how it is decoded, but not how the bitstream is to be generated. For example, a given stream might be encoded as frames or as fields; neither is mandated by any spec while both are legal according to spec. Another example is the use of different quantization matrices for encoding. The spec doesn't specify the actual values of the quant matrices to be used for encoding (some defaults are defined, however). Then there's also GOP length, frame type, etc., etc.

Don, I guess you didn't read the question and/or title:

Quote:

Question about decoders:
Are they supposed to produce identical output, if 100% standard compliant?

the answer is dependent on specification used - for mpeg-2/4, h261/3 (and any other which involves not bit exact calculations like DCT) might produce different results, while still be standard compliant. For such formats there are restrictions on the accuracy of calculations.
For specifications with all operations defined using bit exact formulas like H.264, VC-1 etc outputs from standard compliant decoders shall be the same (not counting post-processing of course)

Guest · 18th July 2008, 20:59

I read it perfectly well, and I don't know why you are nitpicking me.

No specification that the user could reasonably be referring to specifies all encoding choices! The AVC spec doesn't specify that you have to choose Frame vs. MBAFF vs. PAFF encoding. And it doesn't specify whether to choose frame or field for a given frame or macroblock.

Sergey A. Sablin · 18th July 2008, 21:27

Quote:

Originally Posted by neuron2

I read it perfectly well, and I don't know why you are nitpicking me.

no you didn't.

Quote:

Question about decoders:
Are they supposed to produce identical output, if 100% standard compliant?

Guest · 18th July 2008, 23:28

Oops, brain fart.

You could have just said that in your first post.

Dark Shikari · 18th July 2008, 23:43

Answer: yes.

Guest · 19th July 2008, 00:05

Quote:

Originally Posted by Dark Shikari

Answer: yes.

Not always yes, as Sergey explained. For example, MPEG2 decoders can use different IDCT algorithms, producing slightly different outputs.

Dark Shikari · 19th July 2008, 00:06

Quote:

Originally Posted by neuron2

Not always yes, as Sergey explained. For example, MPEG2 decoders can use different IDCT algorithms, producing slightly different outputs.

Look up. This is the MPEG-4 AVC forum.

Comatose · 19th July 2008, 00:15

>:3

Get in the car!

Gabriel_Bouvigne · 20th July 2008, 10:07

For AVC decoders, the general answer is : YES, outputs should be 100% identical

...

except if

...

the stream includes some specific SEI messages of which interpretation is not mandatory, like fgm or output_before_deblocking

18th July 2008, 18:55	#1 \| Link
Comatose Registered User Join Date: Dec 2007 Posts: 639	Question about decoders Are they supposed to produce identical output, if 100% standard compliant?

18th July 2008, 18:58	#2 \| Link
Guest Guest Join Date: Jan 2002 Posts: 21,868	No. For example, MPEG2 defines the legal syntax of the bitstream and how it is decoded, but not how the bitstream is to be generated. For example, a given stream might be encoded as frames or as fields; neither is mandated by any spec while both are legal according to spec. Another example is the use of different quantization matrices for encoding. The spec doesn't specify the actual values of the quant matrices to be used for encoding (some defaults are defined, however). Then there's also GOP length, frame type, etc., etc. Last edited by Guest; 18th July 2008 at 20:14.

18th July 2008, 23:43	#8 \| Link
Dark Shikari x264 developer Join Date: Sep 2005 Posts: 8,664	Answer: yes. __________________ Follow x264 development progress \| akupenguin quotes \| x264 git status ffmpeg and x264-related consulting/coding contracts \| Doom10

18th July 2008, 19:03	#3 \| Link
Comatose Registered User Join Date: Dec 2007 Posts: 639	Ah, okay, thanks =)

18th July 2008, 20:59	#5 \| Link
Guest Guest Join Date: Jan 2002 Posts: 21,868	I read it perfectly well, and I don't know why you are nitpicking me. No specification that the user could reasonably be referring to specifies all encoding choices! The AVC spec doesn't specify that you have to choose Frame vs. MBAFF vs. PAFF encoding. And it doesn't specify whether to choose frame or field for a given frame or macroblock.

18th July 2008, 23:28	#7 \| Link
Guest Guest Join Date: Jan 2002 Posts: 21,868	Oops, brain fart. You could have just said that in your first post.

19th July 2008, 00:15	#11 \| Link
Comatose Registered User Join Date: Dec 2007 Posts: 639	>:3 Get in the car!

20th July 2008, 10:07	#12 \| Link
Gabriel_Bouvigne L.A.M.E. developer Join Date: Dec 2001 Location: Paris - France Posts: 276	For AVC decoders, the general answer is : YES, outputs should be 100% identical ... except if ... the stream includes some specific SEI messages of which interpretation is not mandatory, like fgm or output_before_deblocking