[Mp4-tech] [H.264] IPCM

[Gary Sullivan]

Make sure you're using a very very small value of QP.  For certain input sample value patterns, it can actually be prohibited to use any mode other than I_PCM when QP is sufficiently small (and a typical quantization scheme is applied), because there is a limit on the maximum total number of bits used to represent a macroblock.
Best Regards,
Gary Sullivan 
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org 
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of Dzung Hoang
+> Sent: Monday, January 15, 2024 9:21 AM
+> To: 'Sebastián López Suárez'; mp4-tech lists.mpegif.org
+> Subject: RE: [Mp4-tech] [H.264] IPCM 
+> 
+> Here are the steps to create a test image that forces IPCM.
+> 
+> 1) Input a block of high-valued coefficients into the 
+> inverse 4x4 integer
+> transform. Clip the output block to the range [0,255].
+> 2) Create an image that replicates the block computed in step 1.
+> 3) Encode the image.
+> 
+> The above recipe creates an input pattern that would require 
+> many bits to
+> compress using the integer transform. The JM encoder would 
+> then be forced to
+> use IPCM.
+> 
+> Regards,
+> - Dzung Hoang
+> 
+>  
+> -----Original Message-----
+> From: mp4-tech-bounces lists.mpegif.org
+> [mailto:mp4-tech-bounces lists.mpegif.org] On Behalf Of 
+> Sebastián López
+> Suárez
+> Sent: Monday, January 15, 2024 7:02 AM
+> To: mp4-tech lists.mpegif.org
+> Subject: [Mp4-tech] [H.264] IPCM 
+> 
+> Dear experts,
+> 
+> I am testing a propietary decoder and, within this process, 
+> I would like
+> to verify if the decoder supports properly the decoding of 
+> IPCM. For that
+> purpose I have generated several bitstreams with differents 
+> sequences and
+> coding conditions, enabling for all the cases the use of IPCM mode
+> (EnableIPCM = 1 at encoder.cfg). However, as the IPCM is an extremely
+> unusual mode, in all the sequences the number of IPCM MBs 
+> was equal to
+> zero.
+> 
+> Does anyone knows how to force the reference encoder to produce some
+> IPCM macroblocks in a coded sequence? It can be made by 
+> simply modifying
+> the configuration file? If not, which C files should be modified?
+> 
+> Thank in advance.
+> 
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