[Mp4-tech] MPEG4 Simple Profile question. Is DC/AC saturation in the range [-2048, 2047] redundant provided that bits_per_pixel is 8?

[Gary Sullivan]

Shevach Riabtsev et al,
I think the clipping is necessary.  The first thing to remember is that the standard specifies the decoding process and the constraints on bitstream conformance.  It does not specify how encoding is done.  So if you are trying to base your rationale on what you assume that an encoder must be doing, you need to give up and stop looking at things from that perspective.
Section A.1 was replaced a year or two ago - part of the purpose of that replacement was to clarify what the standard was trying to say.  It would be best to make sure that you're looking at the latest version of that section if you want to refer to something in it.  And make sure that you're aware of the difference between normative and non-normative aspects of the standard.  My guess is that you're trying to refer to the forward DCT operation, which is not normative, since that is something that happens (if it happens) only in an encoder - and encoding methods are not normatively specified by the standard.  Encoders are not actually required to use a forward DCT.
I think you are correct that dc_scaler is always at least 8 for Simple profile.
There is an equation at the beginning of 7.4.3.2 that specifies how QFx[0][0] is computed using PQFx[0][0], dc_scaler, and Fa[0][0] or Fc[0][0].  So to determine the range of values, prior to the clipping specified at the beginning of 7.4.3.4, you should examine the ranges of values that those four variables can have, and then assume an "evil" worst case combination of them for the equation in 7.4.3.2.  Again, don't assume that the encoder is doing anything friendly.  Assume that the encoder has been designed to try to force a violation of the clipping range without regard to whether you would consider what it is doing to be sensible.
Best Regards,
Gary Sullivan
________________________________
From: mp4-tech-bounces lists.mpegif.org [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Shevach Riabtsev
Sent: Wednesday, April 16, 2024 6:55 AM
To: mp4-tech lists.mpegif.org
Subject: [Mp4-tech] MPEG4 Simple Profile question. Is DC/AC saturation in the range [-2048, 2047] redundant provided that bits_per_pixel is 8?
Dear experts
I have a question on MPEG4 Simple Profile related to DC/AC quantized coefficient saturation.
According to 7.4.3.4
The quantized coefficients resulting from the DC and AC Prediction are saturated to lie in the range [*2048, 2047].
In MPEG4 Simple Profile bits_per_pixel value is 8, I don't see a case when a quantized DC or AC coefficient can exceed [-2048,2047] range.
For example,  DC obtains the maximal value equal 8*255 = 2040 provided that all pixels in a 8x8 block are 255, see A.1. The minimal DC quantization scaler is 8 (see Table 7-1). Thus the maximal quantized DC magnitude is 2040/8=255. Hence DC residual lies in the interval [-255..255].  The same reasoning can be applied to AC case.
Thus I conjecture that AC/DC coefficients satuaration is redundant in Simple Profile. Am I right?
Regards, Shevach
Broadcom
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