ANSI CTA-708.1-2012 Digital Television (DTV) Closed Captioning 3D Extensions (Formerly ANSI CEA-708.1).pdf

1、 ANSI/CTA Standard Digital Television (DTV) Closed Captioning: 3D Extensions ANSI/CTA-708.1 R-2017 (Formerly ANSI/CEA-708.1) October 2012 NOTICE Consumer Technology Association (CTA) Standards, Bulletins and other technical publications are designed to serve the public interest through eliminating m

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11、740; Internet http:/; Email 2.2 Definitions For the purposes of this standard, the following definition applies: disparity difference between t he horizontal positions of a pixel representing the same point in space in the right and left views. 1Variable-length commands are defined in CEA-708 1 Sec

12、. 7.1.11.2. 2Code space organization is defined in CEA-708 1 Sec. 7.1. CTA-708.1 4Note: Positive disparity (horizontal right coordinate greater than horizontal left coordinate) creates a perception of a position behind the plane of display, and negative disparity creates a perception of a position i

13、n front of the display. 2.3 Symbols, Abbreviations and Acronyms EXT1 Character code 0x10, as specified in CTA-708 1. 2.4 Compliance Notation As used in this document “shall” and “must” denote mandatory provisions of the standard. “Should” denotes a provision that is recommended but not mandatory. “M

14、ay” denotes a feature whose presence does not preclude compliance, and implementation of which is optional. “Optional” denotes items that may or may not be present in a compliant device. 3 Caption Disparity Data The following sections specify the delivery of caption disparity data for use with CTA-7

15、08 caption services. Standard and Extended services shall be as defined in CTA-708 1. 3.1 Adjunct Service Caption disparity data for a given caption service shall be delivered in an “Adjunct Service.” An Adjunct Service shall be considered to be a caption service that provides additional commands an

16、d data for the caption service it is associated with. In CTA-708.1, Adjunct Service data to support 3D captioning shall be delivered in Standard caption service #6. Commands and data transported in Standard service #6 are considered to be effectively part of other identified services. Decoders proce

17、ssing closed caption commands for display on 3D video are expected to parse the Adjunct Service in Standard service #6 in addition to the selected Standard service #1-6 in order to access commands and data needed for 3D rendering. 3.2 SET DEPTH Name: SetDepth - Specify disparity for 3D captioning Fo

18、rmat: Variable-length Command Coding: b7 b6 b5 b4 b3 b2 b1 b0 0 0 0 1 0 0 0 0 EXT1 1 0 0 1 0 SL2 SL1 SL0 91h-96h 1 1 0 L4 L3 L2 L1 L0 caption_disparity_data() (variable length) Description: Caption disparity data shall be transported in a SetDepth command within an Adjunct Service transported in Sta

19、ndard service #6 using the syntax and semantics defined below. Each instance of the SetDepth command specifies the amount of horizontal offset between the left- and right-eye views (disparity) for one (or all) caption windows when captioning is used with stereoscopic 3D programming. Parameters: Serv

20、ice Linkage (SL) is a 3-bit unsigned integer in the range 1 to 6 that shall indicate the Standard caption service this SetDepth command is CTA-708.1 5associated with. EXT1- 0x91 indicates the SetDepth command provides disparity data for Standard caption service #1, EXT1- 0x92 indicates the SetDepth

21、command provides disparity data for Standard caption service #2, and so on. Length (L) is an unsigned integer that shall indicate the number of bytes following the header, in the range 2 to 27. 3.3 Caption Disparity Data The syntax of caption_disparity_data() shall be as shown in Table 1. No. of bit

22、s Format No. of Bytes caption_disparity_data() all_windows 1 bslbf 2 caption_window_id 3 uimsbf temporal_extension_flag 1 bslbf reserved 2 11 starting_disparity 9 simsbf if (temporal_extension_flag) alternate_disparity 9 simsbf 2 reserved 3 111 segment_count 4 uimsbf for (k=0; ksegment_count; k+) fr

23、ame_count 6 uimsbf 1.5 reserved 1 1 segment_slope_change 5 simsbf if (segment_count % 2)=1 reserved 4 1111 (0.5) Table 1 Caption Disparity Data Syntax all_windows a 1-bit flag that, when set to 1, shall indicate that the disparity data applies to all caption windows. When the flag is set to 0, the d

24、isparity data applies to the caption window identified in caption_window_id. caption_window_id a 3-bit unsigned integer that, when all_windows is set to 0, shall identify the Window ID in the corresponding service for which disparity data is being given. When all_windows is set to 1, caption_window_

25、id shall have no meaning. temporal_extension_flag a 1-bit flag that, when set to “1”, shall indicate data is included that identifies a time-varying disparity path. When set to “0” the flag shall indicate that the last field present in the command is starting_disparity. starting_disparity a 9-bit 2s

26、 complement signed integer that shall indicate the disparity value of the associated caption window,given in caption_window_id, relative to 1920 horizontal units. Value zero shall reference the screen plane. Negative values shall indicate depths in front of the screen plane. Positive values shall in

27、dicate depths behind the screen plane. The starting_disparity value indicates the CTA-708.1 6value of disparity to apply immediately upon reception of the command. Motion along a temporal path, if provided, shall start at this disparity at the time the command is executed. alternate_disparity a 9-bi

28、t 2s complement signed integer that shall be coded the same as starting_disparity. The alternate_disparity value is intended for use by decoders that are not able to render time-varying caption window paths. The alternate_disparity shall specify a constant disparity value applicable to the display o

29、f the current instance of the associated caption window(s). The alternate_disparity is typically authored to specify a disparity value that is the closest point to the viewer (most negative) of any point along the piecewise-linear path. segment_count a 4-bit unsigned integer in the range 1 to 15 tha

30、t shall indicate the number of segments to follow. frame_count a 6-bit unsigned integer in the range 0 to 63 that shall indicate the number of time increments, each having a duration of (1001/30) milliseconds, that occur prior to the application of the segment_slope_change field directly following.

31、segment_slope_change a 5-bit 2s complement signed integer in the range -16 to +15 that shall indicate the amount of change in slope to apply at the increment indicated in frame_count. Slopes are specified in number of units of disparity over 32 frames. A slope value of zero shall be assumed at the s

32、tarting point of the time-varying path. 3.4 Time-varying Disparity Example Caption disparity data that changes over time may be specified by use of the temporal extension syntax. Figure 1 shows the time line resulting from the following sequence of frame_count and segment_slope_change parameters: (2

33、,-7),(10,-9),(8,+9),(10,+7). Figure 1 Time-varying Disparity Example The slope of the disparity curve always starts at zero. In the example, after two frames, the slope changes to -7 disparity units per 32 frames. After 10 frames, it changes again to 16 disparity units per 32 frames. Eight frames la

34、ter it changes back to -7, and 10 frames after that it changes back to zero. Figure 1 shows the computed disparity value at each breakpoint, assuming the starting value of starting_disparity to be 0. CTA-708.1 73.5 Proper Order of Inclusion of Disparity Data CTA-708 1 Section 8.11 discusses recommen

35、ded command sequences for roll-up, paint-on, and pop-on style captioning. For captioning of 3D content, it is recommended to include a SetDepth command immediately following the DefineWindow command within the sequence. This recommendation applies to any of the three captioning styles. 3.6 Descripti

36、on and Use of Alternate Disparity As described below in Annex A, caption decoders in receivers may vary in their capability to process the SetDepth command defined in CTA-708.1. While all decoders are expected to parse the entire command properly, lower-end units may not implement a full feature set

37、. In particular, some implementations may not have the computational power in their processing or graphics engines to move a caption window smoothly in depth over time according to the temporal extension data provided in the command. Such an implementation is called a “limited-capability” decoder in

38、 Annex A. This section describes the purpose of the alternate_disparity parameter and provides an illustrative example. The author of 3D captioning data must ensure that the z-axis position of each caption window is positioned comfortably in front of any object in the scene during the entire time th

39、at window is visible. If, due to an incorrect disparity setting, an object in the scene is perceived to be in front of the caption window, a “depth violation” occurs and the 3D effect is broken. In view of the likely presence of some limited-capability receivers, the caption author must formulate th

40、e SetDepth command such that the viewing optimum experience is afforded to all decoder types. To accommodate limited-capability receivers, the caption author uses alternate_disparity to set the z-axis position of each caption window such that it stays in front of objects in the scene during the full

41、 duration of display of that window. To stay “in front” typically means that the value of disparity used for alternate_disparity corresponds to the most-negative value of disparity of any point on the temporal curve delivered for the benefit of the fully-capable receivers. Figure 2 Alternate Dispari

42、ty Example Figure 2 illustrates a SetDepth command which specifies both a temporal curve and a value of alternate_disparity. It should be noted that the caption author has chosen a value for alternate_disparity that corresponds to the most-negative value of disparity occurring during the duration of

43、 the caption window. In this example, the author could have safely chosen a value for alternate_disparity slightly less negative than the most-negative peak of the temporal path, if desired. Disparity0+CTA-708.1 8ANNEX A Decoder Performance Levels (informative) It is anticipated that not all decoder

44、s will have the same level of capabilities relative to interpreting the SetDepth command. The cases below present various levels of performance. For the purposes of this annex, the following definition applies: 3D Rendering: The left and right views of the caption window are rendered at a total disp

45、lacement from one another of a number of units of disparity given in the given disparity value, relative to a 1920-unit horizontal grid. Both views should be displaced from the nominal (2D) placement of the caption window by an approximately equal amount (within one pixel). A.1 Full-Capability Decod

46、er The full-capability decoder is expected to render the SetDepth command as follows: a. When temporal_extension_flag is 0 (no temporal extension is present), the left and right views of the associated caption window are rendered with 3D Rendering using starting_disparity as the disparity value. b.

47、When temporal_extension_flag is 1 (temporal extension is present), the left and right views are rendered using 3D Rendering, with a disparity value that starts at starting_disparity and that changes frame-by-frame to follow the path defined by the line segments given in the command. The z-axis motio

48、n of the caption window is synchronized to begin when the caption window changes to the “visible” state, or if already visible, when the SetDepth command is received. The disparity at each frame is computed precisely using binary arithmetic carrying five binary digits to the right of the decimal poi

49、nt (precision of 1/32 disparity unit). Receivers that do not support 1/32 disparity unit precision in rendering are expected to round the results at each frame to whatever rendering precision they do support, such as 1/16, 1/8, 1/4 pixel, etc. To avoid accumulated error, the computation of the disparity at each frame is expected to be performed at full precision (1/32 unit), with rounding (if any) performed afterwards. The window ID in the SetDepth command applies