DICOM PS3.5 2021d - Data Structures and Encoding

F Encapsulated Images As Part of A DICOM Message (Informative) (Current)
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# F Encapsulated Images As Part of A DICOM Message (Informative)

The following remarks apply generally to communicating an encoded image within a message structure according to the DICOM Standard:

a) In the course of including an encoded image in a DICOM message, the encoding is not changed. The encoded data stream is merely segmented and encapsulated according to the protocols of the DICOM Standard. After unpacking the DICOM message, the encoded data stream can be fully reconstructed at the receiving node.

b) The object definition of the DICOM Standard is always determining format and other choices that a specific encoding implementation may offer. The encoded image must be consistent with the definition of the object of which the encoded image is part. For example:

1) If the object is defined to contain 10-bit pixel data, it is assumed that the encoding process is one that accepts at least 10-bit data. Hence, there is no need for defining separate Transfer Syntaxes, e.g., for 8-bit or 12-bit implementations. Any 12-bit implementation is assumed to operate in an 8-bit process if the object is defined to contain 8-bit data.

2) If the image of an object is interleaved, the encoding process must reproduce the interleaving.

c) Specifications in the encoding file header must be consistent with the DICOM Message header, e.g., regarding the number of rows and columns.

d) The byte order specification of an encoded file is not altered in the course of encapsulating it in a DICOM message.

## F.1 Encapsulated JPEG Encoded Images

The International Standards Organization (ISO/IEC JTC1/SC2/WG10) has prepared an International Standard, ISO 10918-1 (JPEG Part 1) and International Draft Standard ISO 10918-2 (JPEG Part 2), for the digital compression and coding of continuous-tone still images. This standard is collectively known as the JPEG Standard.

Part 1 of the JPEG Standard sets out requirements and implementation guidelines for the coded representation of compressed image data to be interchanged between applications. The processes and representations are intended to be generic in order to support the broad range of applications for color and grayscale still images for the purpose of communications and storage within computer systems. Part 2 of the JPEG Standard defines tests for determining whether implementations comply with the requirements of the various encoding and decoding processes specified in Part 1 of the JPEG Standard.

The JPEG Standard specifies lossy and lossless code processes. The lossy coding is based on the discrete cosine transform (DCT), permitting data compression with an adjustable compression ratio. The lossless coding employs differential pulse code modulation (DPCM).

The JPEG Standard permits a variety of coding processes for the coder and decoder. These processes differ in coding schemes for the quantified data and in sample precision. The coding processes are consecutively numbered as defined in the International Draft Standard ISO 10918-2 (JPEG Part 2), and are summarized in Table F.1-1. The simplest DCT-based coding process is referred to as Baseline Sequential with Huffman Coding for 8-bit Samples.

Table F.1-1. JPEG Modes of Image Coding

No. | Description | Lossy LY Lossless LL | Non-Hierarchical NH Hierarchical H | Sequential S Progressive P | Transform | Coding | Accepted Bits
--- | --- | --- | --- | --- | --- | --- | ---
1 2 4 | Baseline Extended Extended | LY LY LY | NH NH NH | S S S | DCT DCT DCT | Huffman Huffman Huffman | 8 8 12
14 | Lossless | LL | NH | S | DPCM | Huffman | 2-16

The different coding processes specified in the JPEG Standard are closely related. By extending the capability of an implementation, increasingly more 'lower level' processes can also be executed by the implementation. This is shown in Table F.1-2 for Huffman Coding.

Inclusion of a JPEG-coded image in a DICOM message is facilitated by the use of specific Transfer Syntaxes that are defined in Annex A. Independent of the JPEG coding processes, the same syntax applies. The only distinction for different processes in the syntax (apart from different SOF marker segments in the JPEG bit stream) is the UID value. Table F.1-5 lists the UID values in the Transfer Syntax for the various JPEG coding processes for reference.

Table F.1-2. Relationship Between the Lossy JPEG Huffman Coding Processes

Process | 1 | 2 | 4
--- | --- | --- | ---
1 | * | * | *
2 |  | * | *
4 |  |  | *

* Coding process of column can execute coding process of row

Table F.1-5. Identification of JPEG Coding Processes in DICOM

DICOM Transfer Syntax UID | JPEG process | JPEG description | capable of decoding
--- | --- | --- | ---
1.2.840.10008.1.2.4.50 | 1 | baseline | 1
1.2.840.10008.1.2.4.51 | 2,4 | extended | 1,2,4 (see Note)
1.2.840.10008.1.2.4.57 | 14 | lossless NH | 14
1.2.840.10008.1.2.4.70 | 14 Selection Value 1 | lossless NH, first-order prediction |

### Note

Though the coding processes (2, 4) described in ISO 10918-1 are capable of decoding the other listed process (1), the bit stream uses different SOF marker segments. I.e., the baseline JPEG process 1 used with the 1.2.840.10008.1.2.4.50 Transfer Syntax uses the SOF0 marker, whereas the extended process 2 used with the 1.2.840.10008.1.2.4.51 Transfer Syntax uses the SOF1 marker. Accordingly, even though both bit streams encode 8 bit images using DCT and Huffman coding, the bit streams are not identical.

ISO 10918-2 describes compliance tests for decoders, and requires that implementations of specific extended processes (such as 2 and 4) be capable of decoding bit streams of related baseline processes (such as 1) (ISO 10918-2 Section 7.4 Compliance tests for DCT-based sequential mode decoding processes). The converse is not true for encoders however, and the presence of SOF marker segments not defined by the specific process is not compliant (ISO 10918-2 Section 5.1.1 Non-hierarchical coding processes syntax compliance test Tables 1 and 2).

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DICOM PS3.5 2021d - Data Structures and Encoding