8.2.7 MPEG-4 AVC/H.264 High Profile / Level 4.1 Video Compression

MPEG-4 AVC/H.264 High Profile / Level 4.1 corresponds to what is commonly known as HDTV (‘High Definition Television’). DICOM provides a mechanism for supporting the use of MPEG-4 AVC/H.264 Image Compression through the Encapsulated Format (see PS 3.3). Annex A defines a Transfer Syntax that references the MPEG-4 AVC/H.264 Standard.

Note: MPEG-4 AVC/H.264 compression @ High Profile compression is inherently lossy. The context where the usage of lossy compression of medical images is clinically acceptable is beyond the scope of the DICOM Standard. The policies associated with the selection of appropriate compression parameters (e.g. compression ratio) for MPEG-4 AVC/H.264 HiP@Level4.1 are also beyond the scope of this standard.

The use of the DICOM Encapsulated Format to support MPEG-4 AVC/H.264 compressed pixel data requires that the Data Elements which are related to the Pixel Data encoding (e.g. Photometric Interpretation, Samples per Pixel, Planar Configuration, Bits Allocated, Bits Stored, High Bit, Pixel Representation, Rows, Columns, etc.) shall contain values that are consistent with the characteristics of the compressed data stream, with some specific exceptions noted here. The Pixel Data characteristics included in the MPEG-4 AVC/H.264 bit stream shall be used to decode the compressed data stream.

Notes: 1. These requirements are specified in terms of consistency with what is encapsulated, rather than in terms of the uncompressed pixel data from which the compressed data stream may have been derived.

2. When decompressing, should the characteristics explicitly specified in the compressed data stream be inconsistent with those specified in the DICOM Data Elements, those explicitly specified in the compressed data stream should be used to control the decompression. The DICOM data elements, if inconsistent, can be regarded as suggestions as to the form in which an uncompressed data set might be encoded.

The requirements are:

Planar Configuration (0028,0006) shall be 0

Samples per Pixel (0028,0002) shall be 3

Photometric Interpretation (0028,0004) shall be YBR_PARTIAL_420

Bits Allocated (0028,0100) shall be 8

Bits Stored (0028,0101) shall be 8

High Bit (0028,0102) shall be 7

Pixel Representation (0028,0103) shall be 0

The value of MPEG-4 AVC/H.264 sample aspect_ratio_idc shall be 1 in the encapsulated MPEG-4 AVC/H.264 bit stream if aspect_ratio_info_present_flag is 1.

Pixel Aspect Ratio (0028,0034) shall be absent. This corresponds to a 'Sampling Aspect Ratio' (SAR) of 1:1.

The possible values for Rows (0028,0010), Columns (0028,0011), Cine Rate (0018,0040) , and Frame Time (0018,1063) or Frame Time Vector (0018,1065) depend on the used transfer syntax.

Table 8-4 Values permitted for MPEG-4 AVC/H.264 BD-compatible High Profile / Level 4.1

Rows Columns Frame rate Video Type Progressive or Interlace
1080 1920 25 25 Hz HD I
1080 1920 29.97 30 Hz HD I
1080 1920 24 24 Hz HD P
1080 1920 23.976 24 Hz HD P
720 1280 50 50 Hz HD P
720 1280 59.94, 60 Hz HD P
720 1280 24 24 Hz HD P
720 1280 23.976 24 Hz HD P

Notes: 1. The value of Planar Configuration (0028,0006) is irrelevant since the manner of encoding components is specified in the MPEG-4 AVC/H.264 standard, hence it is set to 0.

2. The limitation on rows and columns are to maximize interoperability between software environments and commonly available hardware MPEG-4 AVC/H.264 encoder/decoder implementations. Source pictures that have a lower value should be re-formatted by scaling and/or pixel padding prior to MPEG-4 AVC/H.264 encoding.

3. The frame rate of the acquiring camera for ’30 Hz HD’ MPEG-4 AVC/H.264 may be either 30 or 30/1.001 (approximately 29.97) frames/sec. Similarly, the frame rate in the case of 60 Hz may be either 60 or 60/1.001 (approximately 59.94) frames/sec. This may lead to small inconsistencies between the video timebase and real time. The relationship between frame rate and frame time is shown in Table 8-5.

4. The Frame Time (0018,1063) may be calculated from the frame rate of the acquiring camera. A frame rate of 29.97 frames per second corresponds to a frame time of 33.367 ms.

5. The value of chroma_format for this profile and level is defined by MPEG as 4:2:0.

6. Example screen resolutions supported by MPEG-4 AVC/H.264 High Profile / Level 4.1 can be taken from Table 8-4. Frame rates of 50 Hz and 60 Hz (progressive) at the maximum resolution of 1080 by 1920 are not supported by MPEG-4 AVC/H.264 High Profile / Level 4.1. Interlace at the maximum resolution is supported at a field rate of 50 Hz or 60 Hz, which corresponds to a frame rate of 25 Hz or 30 Hz respectively. Smaller resolutions may be used as long as they comply with the square pixel aspect ratio. An example is XGA resolution with an image resolution of 768 by 1024 pixels. For smaller resolutions there are higher frame rates possible. For example it may be up to 80 Hz for XGA.

7. The display aspect ratio is defined implicitly by the pixel resolution of the video picture. Only square pixel aspect ratio is allowed. MPEG-4 AVC/H.264 BD-compatible High Profile / Level 4.1 will only support resolutions that result in a 16:9 display aspect ratio

8. The permitted screen resolutions for the MPEG-4 AVC/H.264 BD-compatible High Profile / Level 4.1 are listed in Table 8-4. Only HD resolutions and no progressive frame rates for 25 or 29.97 frames per seconds are supported. Frame rates of 50 Hz and 60 Hz (progressive) at the maximum resolution of 1080 by 1920 are not supported.

Table 8-5 MPEG-4 AVC/H.264 High Profile / Level 4.1 IMAGE TRANSFER SYNTAX FRAME RATE ATTRIBUTES

Video Type Spatial resolution layer Frame Rate (see Note 2) Frame Time (see Note 3)
30 Hz HD Single level, Enhancement 30 33.33 ms
25 Hz HD Single level, Enhancement 25 40.0 ms
60 Hz HD Single level, Enhancement 60 16.17 ms
50 Hz HD Single level, Enhancement 50 20.00 ms

One fragment shall contain the whole MPEG-4 AVC/H.264 bit stream.

Note: If a video stream exceeds the maximum length of one fragment (approximately 4 GB), it may be sent as multiple SOP Instances.

The PTS/DTS of the transport stream shall be used in the MPEG coding. Audio components shall be interleaved in either LPCM or AC-3 audio format and shall comply with the following restrictions:

Note: AC-3 is standardized in ETSI TS 102 366