| DICOM PS3.5 2024c - Data Structures and Encoding |
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MPEG2 Main Profile / High Level corresponds to what is commonly known as HDTV ('High Definition Television'). DICOM provides a mechanism for supporting the use of MPEG2 Main Profile / High Level Video Compression through the Encapsulated Format. Annex A defines Non-Fragmentable and Fragmentable Encapsulated Transfer Syntaxes that reference the MPEG2 Main Profile / High Level Standard.
MPEG2 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 MPEG2 Main Profile / High Level are also beyond the scope of this Standard.
The use of the DICOM Encapsulated Format to support MPEG2 Main Profile / High Level compressed pixel data requires that the Data Elements that 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 MPEG2 Main Profile / High Level bit stream shall be used to decode the compressed data stream.
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.
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, subject to the general and IOD-specific rules for uncompressed Photometric Interpretation and Planar Configuration, which may require that decompressed data be converted to one of the permitted forms.
If MPEG2 Compressed Pixel Data is decompressed and re-encoded in Native (uncompressed) form, then the Data Elements that are related to the Pixel Data encoding are updated accordingly. If color components are converted from YBR_PARTIAL_420 to RGB during decompression and Native re-encoding, the Photometric Interpretation will be changed to RGB in the Data Set with the Native encoding.
Photometric Interpretation (0028,0004) shall be YBR_PARTIAL_420 or MONOCHROME2
Columns (0028,0011) shall be 1280 if Rows is 720, or shall be 1920 if Rows is 1080.
The value of MPEG2 aspect_ratio_information shall be 0011 in the encapsulated MPEG2 data stream corresponding to a 'Display Aspect Ratio' (DAR) of 16:9.
The DICOM Attribute Pixel Aspect Ratio (0028,0034) shall be absent. This corresponds to a 'Sampling Aspect Ratio' (SAR) of 1:1.
Cine Rate (0018,0040) and Frame Time (0018,1063) or Frame Time Vector (0018,1065) shall be consistent with the limitations of Main Profile / High Level, as specified in Table 8-2.
The requirements on rows and columns are to maximize interoperability between software environments and commonly available hardware MPEG2 encoder/decoder implementations. Should the source picture have a lower value, it should be re-formatted accordingly by scaling and/or pixel padding prior to MPEG2 encoding.
The frame rate of the acquiring camera for '30 Hz HD' MPEG2 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 Frame Time (0018,1063) may be calculated from the frame rate of the acquiring camera. A frame time of 33.367 ms corresponds to 29.97 frames per second.
The value of chroma_format for this profile and level is defined by MPEG as 4:2:0.
Examples of screen resolutions supported by MPEG2 Main Profile / High Level are shown in Table 8-y. Frame rates of 50 Hz and 60 Hz (progressive) at the maximum resolution of 1080 by 1920 are not supported by Main Profile / High Level. 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 as described in Table 8-y.
An MPEG2 Main Profile / High Level decoder is able to decode bit streams conforming to lower levels. These include the 1080 by 1440 bit streams of MP@H-14, and the Main Level bit streams used in the existing MPEG2 Main Profile / Main Level Transfer Syntax in the Visible Light IOD.
The restriction of DAR to 16:9 is required to ensure interoperability because of limitations in commonly available hardware chip set implementations for MPEG2 Main Profile / High Level.
For the Non-Fragmentable Encapsulated Transfer Syntax, one Fragment shall contain the whole MPEG2 bit stream.
For the Fragmentable Encapsulated Transfer Syntax, the stream may be segmented into multiple Fragments.
If a video stream exceeds the maximum length of one fragment (2^32-2 bytes), it may be sent using a Fragmentable Encapsulated Transfer Syntax. Alternatively, it may be sent using a Non-Fragmentable Encapsulated Transfer Syntax as multiple SOP Instances, but each SOP Instance will contain an independent and playable bit stream, and not depend on the encoded bit stream in other (previous) instances. The manner in which such separate instances are related is not specified in the Standard, but mechanisms such as grouping into the same Series, and references to earlier instances using Referenced Image Sequence may be used.
Fragmentable Encapsulated Transfer Syntaxes allow for streams of essentially unlimited length; the only limit imposed is the maximum Number of Frames (0028,0008), which is 2^31-1 frames (largest positive Value in an Integer String VR).
The Basic Offset Table in the Pixel Data (7FE0,0010) shall be empty (present but zero length).
The Basic Offset Table is not used because MPEG2 contains its own mechanism for describing navigation of frames. To enable decoding of only a part of the sequence, MPEG2 manages a header in any group of pictures (GOP) containing a time_code - a 25-bit integer containing the following: drop_frame_flag, time_code_hours, time_code_minutes, marker_bit, time_code_seconds and time_code_pictures.
The container format for the video bit stream is not constrained. For example, it may MPEG-2 Transport Stream (MPEG-TS), MPEG-2 Program Stream (MPEG-PS), MPEG-2 Elementary Stream (MPEG-ES), MPEG-2 Packetized Elementary Stream (MPEG-PES) (see [ISO/IEC 13818-1]) or MPEG-4 (MP4) container (see [ISO/IEC 14496-12] and [ISO/IEC 14496-14]).
Any audio components present within the MPEG2 Main Profile / High Level bit stream shall comply with the restrictions as for MPEG2 Main Profile / Main Level as stated in Section 8.2.5.
| DICOM PS3.5 2024c - Data Structures and Encoding |
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