InfoMagic Standards 1993 July

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Recommendation T.416 OPEN DOCUMENT ARCHITECTURE (ODA) AND INTERCHANGE FORMAT - CHARACTER CONTENT ARCHITECTURES1) CONTENTS 1 Scope 2 References 3 Definitions 4 General principles 4.1 Classes 4.2 Content 4.3 Presentation attributes 4.4 Control functions 4.5 Graphic characters 4.6 Space characters 4.7 Coding of content information 4.8 Internal structure 5 Character positioning 5.1 Basic concepts 5.2 Positioning of character images within a line box 5.3 Positioning of line boxes within a basic layout object 6 Character imaging 6.1 Emphasis 6.2 Font selection 6.3 Subscript and superscript 6.4 Character combinations 7 Definition of character presentation attributes 7.1 Shared presentation attributes 7.2 Layout presentation attributes 7.3 Logical presentation attributes 7.4 Content architecture class attributes 7.5 Interactions between presentation attributes and layout directives 8 Character content portion attributes 8.1 Common coding attributes 8.2 Other coding attributes 9 Formal definitions of character content architecture dependent data types 9.1 Introduction 9.2 Representation of presentation attributes 9.3 Representation of coding attributes 9.4 Representation of non-basic features and non-standard defaults 10 Graphic characters ─────── 1) This text is aligned with the final text of the corresponding International Standard ISO 8613-6. Fascicle VII.6 - Rec. T.416 1 11 Definition of control functions and the character SPACE 11.1 Shared control functions 11.2 Layout control functions 11.3 Logical control functions 11.4 Delimiters 11.5 SP - Space 12 Content layout process 12.1 Introduction 12.2 Content layout process for processable content 12.3 Content layout process for formatted processable form 12.4 Content layout process for formatted form 13 Content imaging process 13.1 Introduction 13.2 Content imaging process for formatted form 13.3 Content imaging process for formatted processable form 14 Interactions between presentation attributes and control functions 15 Definition of character content architecture classes 15.1 Formatted character content architecture class 15.2 Processable character content architecture class 15.3 Formatted processable character content architecture class Annex A - Summary of content architecture classes Annex B - Character content architecture levels Annex C - Coded representations of control functions Annex D - Summary of object identifiers 1 Scope 1.1 The purpose of the T.410 Series is to facilitate the interchange of documents. In the context of the T.410 Series, documents are considered to be items such as memoranda, letters, invoices, forms and reports, which may include pictures and tabular material. The content elements used within the documents may include graphic characters, geometric graphic elements and raster graphic elements, all potentially within one document. Note - The T.410 Series is designed to allow for extensions, including typographical features, colour, spreadsheets and additional types of content such as sound. 1.2 The T.410 Series of Recommendations applies to the interchange of documents by means of data communications or the exchange of storage media. The T.410 Series provides for the interchange of documents for either or both of the following purposes: - to allow presentation as intended by the originator; - to allow processing such as editing and reformatting. The composition of a document in interchange can take several forms: - formatted form, allowing presentation of the document; - processable form, allowing processing of the document; - formatted processable form, allowing both presentation and processing. 2 Fascicle VII.6 - Rec. T.416 The T.410 Series also provides for the interchange of ODA information structures used for the processing of interchanged documents. Furthermore, the T.410 Series allows for the interchange of documents containing one or more different types of content such as character text, images graphics and sound. 1.3 This Recommendation: - defines character content architectures that can be used in conjunction with the document architecture defined in Recommendation T.412; - defines the internal structure of content conforming to these character content architectures; - defines those aspects of positioning and imaging applicable to presentation of these character content architectures in a basic layout object; - defines the presentation attributes and control functions applicable to these character content architectures; - describes a content layout process which, together with the document layout process described in Recommendation T.412, determines the layout of character content in basic layout objects and the dimensions of these basic layout objects. 2 References The following Recommendations and Standards contain provisions which, through reference in this text, constitute provisions of the T.410 Series of Recommendations. At the time of publication, the editions indicated were valid. All Recommendations and Standards are subject to revision, and parties to agreements based on the T.410 Series of Recommendations are encouraged to investigate the possibility of applying the most recent editions of the Recommendations and Standards listed below. Members of CCITT, ISO and IEC maintain registers of currently valid Recommendations and Standards. - ISO 2022 (1986): Information processing - ISO 7-bit and 8-bit coded character sets - Code extension techniques. - ISO 6429 (1983): Information processing - ISO 7-bit and 8-bit coded character sets - Additional control functions for character imaging devices. - ISO 6937 (1983): Information processing - Coded character sets for text communication. - Rec. T.50 (1988): International alphabet No. 5. - Rec. X.208 (1988): Specification of abstract syntax notation one (ASN.1). - Rec. T.61 (1988): Character repertoire and coded character sets for the international Teletex service. - ISO 9541-52): Information processing - Font and character information interchange - Part 5: Font attributes and character model. ────── 2) To be published. Fascicle VII.6 - Rec. T.416 3 3 Definitions For the purpose of this Recommendation, the definitions given in Recommendation T.411 apply. 4 General principles 4.1 Classes Three classes of character content architecture are distinguished: - a character content architecture for formatted content which allows for document content to be presented (e.g. printed or displayed) as intended by the originator. Formatted content can be used in any basic component; - a character content architecture for processable content which allows for document content to be processed (e.g. edited or formatted). Processable content can be used in any basic logical component; - a character content architecture for formatted processable content which allows for document content to be processed and also to be presented as intended by the originator. Formatted processable content can be used in any basic component. 4.2 Content The content of a basic component that conforms to a character content architecture is a character string. This character string is formed by concatenating the character strings in the content portions of the basic component. The content character string consists of a combination of graphic characters, control functions and space characters. 4.3 Presentation attributes Presentation attributes are applicable to basic logical and layout components. They contain information that specifies the initial conditions relating to the layout, the imaging and the selection of graphic characters of the content of these basic components. Some of these conditions can be changed by control functions contained within the content. Presentation attributes are classified as follows: - logical presentation attributes which can be associated with processable and formatted processable character content. These attributes take effect during the content layout process but are ignored during the content imaging process; - layout presentation attributes which can be associated with formatted and formatted processable character content. These attributes take effect during the content imaging process. They are generated either by a content layout process or by a process that creates or edits the formatted or formatted processable content; - shared presentation attributes which can be associated with all character content architecture classes. These attributes take effect during either or both the content layout and imaging processes. Note - Presentation attributes can be applied to the content of a basic component in one of three ways. They can be specified directly in an object or object class description or they can be specified directly by means of a presentation style associated with the object or object class description. Alternatively, they can be indirectly applied to an object or object class by means of a default value list (see Recommendation T.412). 4 Fascicle VII.6 - Rec. T.416 4.4 Control functions Control functions with zero or more parameters may specify information relating to the layout or imaging of subsequent graphic characters. A control function can also be used to extend or replace the set of graphic characters being used. The scope of all control functions is limited to the basic component in which they occur. Classification of control functions is similar to that of presentation attributes: - logical control functions which can be used in processable and formatted processable character content. These control functions take effect during the content layout process but are ignored during the content imaging process; - layout control functions which can be used in formatted and formatted processable character content. These control functions take effect during the content imaging process. They are generated by the content layout process. Alternatively, they may be inserted by a process (not described in the T.410 Series of Recommendations) that creates of edits the formatted or formatted processable content; - shared control functions which can be used in all character content architecture classes. These control functions take effect during either or both the content layout and imaging processes. In addition, formatted processable content may contain control functions known as delimiters. These delimiters are used to indicate a string of one or more graphic characters and/or control functions that have been inserted as the result of a content layout process (see 12). The delimited graphic characters and/or control functions take effect only during the content imaging process. The delimiters take effect during the content layout process by deleting them and the enclosed character sequence. 4.5 Graphic characters The set of graphic characters used in the content of a basic component, and their coded representation, are specified by presentation attributes and code extension control functions (see 10 and 11.1.17). Any set or sets of graphic characters may be used in the content of basic components, subject to the restrictions associated with the particular content architecture in use and subject to proper designation and invocation in accordance with ISO 2022. Any non-spacing characters included in a graphic character set are not to be used in isolation but only in combination with spacing characters. 4.6 Space characters The character SPACE (SP) is considered both as a logical control function and as a graphic character. As a graphic character, it has a graphical representation consisting of the absence of a graphic symbol. As a control function, it indicates a potential line break point (see 12.2.1.3.2). Note - NBSP (No Break SPace) and any fixed-width space characters, such as "digit space", "em space" and "en space" are regarded as graphic characters i.e. are not regarded as line break points. 4.7 Coding of content information The coded representation of the content information within a content portion is in accordance with the rules specified in ISO 2022. Coded representations of control functions are defined in ISO 6429 and are summarized in Annex C. Fascicle VII.6 - Rec. T.416 5 4.8 Internal structure 4.8.1 Formatted content Formatted content is content for which all the necessary information relating to the layout and imaging of that content has been specified. Content in this form is intended to be imaged as specified and is not intended to be revised by an editing process or to be reformatted. The content of a basic component conforming to a formatted character content architecture consists of one or more lines of characters. Each pair of successive lines is separated by a hard line terminator. The last (or only) line may or may not be terminated by a hard line terminator; the end of the content of a basic component implicitly terminates the last line. 4.8.2 Processable content Processable content is content which has not been laid out. Content in this form is suitable for revision by an editing process. Note - The editing process is implementation dependent and is not described in the T.410 Series of Recommendations. In order to image content in this form, it is necessary to apply a content layout process (see 12) to the content which converts the processable content into formatted content (see 4.8.1) or into formatted processable content (see 4.8.3). To assist the processing (i.e. editing or layout processes) of processable content, a number of logical presentation attributes and control functions have been defined (see 7 and 11). In addition, the character SPACE is regarded as both a graphic character and as a control function that indicates where a line break may occur when the content is laid out. The content of a basic component conforming to a processable character content architecture consists of one or more sequences of characters. Each pair of successive character sequences is separated by a hard line terminator control function. The last (or only) character sequence may or may not be terminated by a hard line terminator. If the hard line terminator is omitted at the end of the content of a basic logical component to which another basic logical component is concatenated (see Recommendation T.412), then the last character sequence continues into the content of the next basic logical component. In all other cases, the end of the content of the basic logical component implicitly terminates the last character sequence. The division into character sequences represents the internal structure of the processable content of a basic logical component. Each character sequence is anonymous, in that no name or identifier is associated with it, and no relationship exists among character sequences except that of sequence. 4.8.3 Formatted processable content Formatted processable content is content that is structured such that it contains both the formatted content and the processable content as subsets. It is identical in structure to the processable content, except that it may contain additional control functions and graphic characters that have been added as a result of the content layout process. It is identical in structure to the formatted content, except that it may contain logical control functions and delimiters. Thus, formatted processable content can be converted to processable content by deleting (or ignoring) all layout control functions, all occurrences of the delimiters and all control functions and characters within those delimiters. 6 Fascicle VII.6 - Rec. T.416 Alternatively, formatted processable content can be converted to formatted content by deleting (or ignoring) all logical control functions and the delimiters but retaining the control functions and characters within the delimiters. Note - The conversion of formatted processable content to processable content is a reversible process (providing the same layout constraints are applicable to the content layout process) but converting formatted processable content to formatted content is irreversible. The formatted view of a basic component conforming to a formatted processable character content architecture consists of one or more lines of characters. Each pair of successive lines is separated by either a hard or soft line terminator. The last (or only) line may or may not be terminated by a hard line terminator; the end of the content of a basic layout component implicitly terminates the last line. The processable view of a basic component conforming to a formatted processable character content architecture consists of one or more sequences of characters. Each pair of successive character sequences is separated by a hard line terminator. The last (or only) character sequence may or may not be terminated by a hard line terminator. If the hard line terminator is omitted at the end of the content of a basic logical component to which another basic logical component is concatenated (see Recommendation T.412), then the last character sequence continues into the content of the next basic logical component. In all other cases, the end of the content of the basic logical component implicitly terminates the last character sequence. Soft line terminators are used as separators between lines within a character sequence. The division into character sequences represents the internal structure of the content of a basic logical component. Each character sequence is anonymous, in that no name or identifier is associated with it, and no relationship exists among character sequences except that of sequence. 5 Character positioning This section specifies how characters are to be positioned within a basic layout object. The intention is to aid understanding of the presentation attributes and control functions that relate to character positioning. This section provides for the positioning of any font that is defined in accordance with ISO 9541-5. This section also caters for the positioning of characters pertaining to different fonts within the same basic layout object. 5.1 Basic concepts 5.1.1 Character fonts In the context of this part of ISO 8613, the term graphic character is used in its abstract sense; that is, this term refers to a number of a set of graphic symbols used for the representation of information. The term character image is then used to refer to the rendition of a graphic character on a presentation medium. A font is a set of character images, normally with a common design and size. A set of font attributes is associated with the font as a whole and a set of character attributes is associated with each individual character. These attributes are defined in ISO 9541-5. The main purpose of the font attributes is for the recipient to identify the font used by the originator and, in case the specified font is not available, the font and character attributes serve as a guidance for the recipient to find an appropriate substitute font among those available. Fascicle VII.6 - Rec. T.416 7 Further information concerning the designation and invocation of different fonts within a basic object is given in 6. 5.1.2 Directions In the context of this Recommendation, all directions are expressed as counter-clockwise angles of rotation (in degrees) relative to a specified reference direction (an example is given in Figure 1/T.416). The character path is the direction of progression of successive character images within a line box (defined in 5.1.7) and is expressed as a direction relative to the horizontal direction of the layout object (see Figure 4/T.416). The line progression is the direction of progression of successive line boxes within the basic layout object and is expressed as a direction relative to the character path (see Figure 11/T.416). The character orientation is the direction of the character baseline (defined in 5.1.3) relative to the character path. Only one value for the character path, line progression and character orientation may be specified for a basic component. FIGURE 1/T.416 Example of direction 5.1.3 Character image model The position point is a reference point associated with a character image (see Figure 2/T.416). It is used for the positioning of the character image within a line box. The escapement point is a reference point associated with a character image (see Figure 2/T.416). It is used for the positioning of the next character image. FIGURE 2/T.416 Illustration of kerns The character baseline is an imaginary line across a character image, for the purpose of defining the character orientation. The character baseline is a horizontal lien with the character image is in its intended viewing orientation (see Figure 3/T.416). 8 Fascicle VII.6 - Rec. T.416 FIGURE 3/T.416 Reference points for character image positioning A position point and escapement point must be defined for each character orientation which is intended to be used (see Figure 3/T.416); i.e., "writing modes" corresponding to the required character orientation must be defined in the font description, or fall-backs must be defined in document application profiles. A kern is that part of the character image that extends beyond its position and escapement points (see Figure 2/T.416). 5.1.4 Character spacing The concept of character spacing is only applicable when a constant spacing font is selected. It is used (in conjunction with the inter-character space) to determine the distance between character images within a line box as defined in 5.2.1. The character spacing is equal to the distance between the position points of successive character images when the inter-character space is zero. The character spacing is independent of the distance between the position point and escapement point of character images. 5.1.5 Active position The active position is an abstraction of an imaging device concept such as a cursor. This concept is used in the definitions of control functions (see 11) where a sequential method of processing a character string is assumed. The active position indicates the point, within the positioning area of a basic layout object, at which the action specified by the next character (graphic character or control function) is to be effected. If the next character is a graphic character, its character image is positioned with the position point at the active position and the active position is advanced in the direction of the character path by the amount of spacing defined in 5.2.1. If the next character is a control function, this may cause the active position to move to another point within the positioning area. 5.1.6 Positioning area A positioning area is a rectangular area, wholly contained within a basic layout object, within which position points and escapement points are to be positioned (see Figure 4/T.416). Kerns of character images are permitted to extend beyond the positioning area but are not permitted to extend beyond the edges of the basic layout object. Fascicle VII.6 - Rec. T.416 9 Two of the edges of the positioning area are referred to as the start edge and end edge (see Figure 4/T.416). The start edge and the end edge are defined such that the direction from the start edge to the end edge is in the direction of the character path. The other two edges of the positioning area are referred to as the top edge and bottom edge (see Figure 11/T.416). The top edge and the bottom edge are defined such that direction from the top edge to the bottom edge is in the direction of line progression. The start and end edges are indented from the corresponding edges of the basic layout object by a distance referred to as the kerning offset (see Figure 4/T.416). The kerning offset specified depends upon the fonts of the characters to be positioned in the positioning area. Its value is chosen such that no part of any character image with kerns will extend beyond the boundary of the basic layout object when sequences of character images are positioned within it. FIGURE 4/T.416 Illustration of character position concepts 5.1.7 Line boxes Within the positioning area, a sequence of character images is positioned within an area, a sequence of character image is positioned within an area called a line box (see Figure 11/T.416). Each line box is a rectangular area that extends from the start edge to the end edge of the positioning area. Each line box contains a reference point called the line home position (see Figure 4/T.416). This point is used for positioning the line box within the basic layout object. It also serves as the active position for the first graphic character or control function in each line. Each line box contains an imaginary line called a reference line (see Figure 4/T.416). The reference line passes through the line home position in the direction of the character path. It extends from the start edge to the end edge within the line box and is used for the alignment of character images. The length of the line box is equal to the distance between the start and end edges. The width (or height) of a line box is equal to the sum of the line box forward extent and the line box backward extent (see Figure 4/T.416). The line box backward extent is the distance between the reference line and the edge of the lien box in the direction opposite to the direction of line progression. The line box forward extent is the distance between the reference line and the edge of the line box in the direction of line progression. 10 Fascicle VII.6 - Rec. T.416 The values of the forward and backward extents depend on the maximum extents (measured perpendicular to the reference line) of the character fonts used in the line box. Determination of the forward and backward extents take into account any displacements of character images perpendicu- lar to the character path, e.g. for subscripts, superscripts and parallel annotation. Note - As an example, for a Latin font with character path 0 degrees, line progression 270 degrees and when a single font is used in the line box, then the forward and backward extents are equal to the maximum right and maximum left extents for that font as defined in ISO 9541-5. 5.2 Positioning of character images within a line box Successive character images are positioned within a line box in the direction of the character path. The position points of the character images are lined on the reference line unless the characters are imaged as subscripts, superscripts or parallel annotation. There are several factors which affect the positioning of character images along the reference line: - spacing between characters; - alignment; - tabulation; - character ordering; - parallel annotation; - subscript/superscript; - pairwise kerning; - first line offset; - itemization. 5.2.1 Spacing between characters The inter-character space is an additional amount of spacing between the position points of successive character images, in the direction of the character path (see 5.1.4). A negative value indicates a reduction in the spacing between successive character images. The distance between the position points of successive character images may be constant or variable depending upon the font as follows: - For fonts with constant spacing, the distance between the position points of successive character images is independent of the characters and is the sum of the character spacing (as specified by presentation attributes and control functions) and the inter-character space (see Figure 5/T.416). - For fonts with variable spacing, the distance between the position points of successive character images is dependent upon the character i.e. normally the distance between the position point and the escapement point of a character, and is the sum of the net escapement of the character (as specified by the font) and the inter-character space (see Figure 6/T.416). Fascicle VII.6 - Rec. T.416 11 FIGURE 5/T.416 Spacing for a constant spacing font FIGURE 6/T.416 Spacing for a variable spacing font The space width, i.e. the width of the SP (space) character image, is determined as follows: - for any SP that follows a soft line terminator and precedes the first graphic character of a line, or precedes a line terminator and follows the last graphic character of a line, the width is equal to zero; - in a constant spacing font, the default width equals the character spacing; - in a variable spacing font, the width is implicitly defined by the font; - for all fonts, the width may be specified by a control function. 5.2.2 Alignment The character images are positioned within a line box in accordance with the alignment attribute as follows: - start-aligned: the position point of the first character image is placed at the line home position; - end-aligned: the escapement point of the last character image is placed at the end edge of the positioning area; - centred: the distance from the line home position to the position point of the first character image is approximately equal to the distance from the escapement point of the last character image to the end edge of the positioning area; - justified: the position point of the first character image is placed at the line home position and the escapement point of the last character image is coincident with the end edge of the positioning area by appropriately setting the space width and/or the inter- character space. 12 Fascicle VII.6 - Rec. T.416 5.2.3 Tabulation The position of character images along a reference line can be controlled by means of a set of tabulation stops. Each tabulation stop specifies a point along a reference line relative to the start edge the positioning area. A string of character images can be placed at a tabulation stop by means of a control function embedded in the text. The string may be start-aligned, end-aligned, centred or aligned around one or more specified characters within that string as follows (see Figure 7/T.416). - start-aligned: the position point of the first character image of the string is placed at the tabulation stop; - end-aligned: the escapement point of the last character image of the string is placed at the tabulation stop; - centred: the string is placed such that the position point of the first character image and the escapement point of the last character image of the string are approximately equidistant from the tabulation stop; - aligned around: the position point of the first character image of the first instance of the specified group of characters in that string is positioned at the tabulation stop. If the specified group or characters does not appear in the text associated with that tabulation stop, then the alignment defaults to end-aligned as defined above. FIGURE 7/T.416 Tabulation 5.2.4 Character ordering Within the content of a basic object, the interchange order is always the reading order of the language used. In the case of certain languages, e.g. Arabic and Hebrew, where the alphanumeric text is read from right to left and the numeric text is read from left to right, the interchanged stream must indicate the change in presentation direction at the appropriate point(s). This is necessary since control functions in character content architectures are defined to operate sequentially according to their position in the character stream. When a string of characters with reversed presentation direction is embedded in the text with normal presentation direction, the image of the last character of the string with reversed presentation direction is positioned adjacent to the image of the last character of the preceding string with normal presentation direction (see Figure 8/T.416). Note - In Figure 8/T.416 the terms "first" and "last" are used in relation to the interchange order and the terms "normal" and "reversed" in relation to the direction of the character path. Fascicle VII.6 - Rec. T.416 13 FIGURE 8/T.416 Character ordering 5.2.5 Parallel annotation Two sequential character strings may be delimited such that the second string is used to indicate the pronunciation and interpretation of the first. It is intended to be used in the Japanese language to provide pronunciation and interpretation information (Ruby) in the form of Kana character(s) for one or more Kanji characters. This is indicated in the formatted text by the Kana character(s) being centered either above or to the right of the Kanji character(s), for character image orientations orthogonal and parallel to the character path respectively (see Figure 9/T.416). Where centring would result in Kana characters being positioned outside the positioning area, then the Kana character string is positioned such that it is start-aligned or end-aligned with the edge of the available area. 5.2.6 Subscript/superscript Subscript rendition allows for the active position to be displaced from the reference line in the direction of line progression. Superscript rendition allows for the active position to be displaced from the reference line in the direction opposite to that of line progression. The combined effect of all subscript/superscript renditions within a line box must be such that the active position is returned to the reference line before the occurrence of a hard or soft line terminator. 14 Fascicle VII.6 - Rec. T.416 Where: HPB Character position backward; HPR Character position relative; VPB Line position backward; VPR Line position relative; xx,xw'xw',yy User specified. The current reference point moves from "a", "b", "c" and returns to "a". FIGURE 9/T.416 Illustration of parallel annotation 5.2.7 Pairwise kerning Pairwise kerning allows for the moving of the active position from that defined by the preceding character. The distance and direction depend both on the character being imaged and the preceding character. In the case of a constant spacing font, pairwise kerning is ignored. In the case of a variable spacing font, the actual distance between the escapement point of one character image and the position point of the next character image is modified by the kerning information as defined in the character attributes of the font. 5.2.8 First line offset First line offset allows for character imaging of the first line of a basic component to start at a position displaced from the line home position. The offset is either in the direction of the character path (producing first line indentation) or in the direction opposite to the character path (producing overhang) as illustrated in Figure 10/T.416. Fascicle VII.6 - Rec. T.416 15 FIGURE 10/T.416 Illustration of itemization and first line offset 5.2.9 Itemization Itemization allows for imaging of an item identifier on the first line of a basic component in positions which are not constrained by the line home position and the first line offset (see Figure 10/T.416). An item identifier is a string of characters that precedes and is separated from the remainder of the first line of a basic component. A start offset and an end offset are defined relative to the line home position. These offsets determine the location of the item identifier and the separation between the item identifier and the line home position. 16 Fascicle VII.6 - Rec. T.416 The item identifier may be positioned on the first line in accordance with the item identifier alignment attribute as follows: - start-aligned: the position point of the first character image of the item identifier is place at the start offset; - end-aligned: the escapement point of the last character image of the item identifier is placed at the end offset. 5.3 Positioning of line boxes within a basic layout object For positioning of line boxes in a basic layout object, the area of that object is independent of any adjoining areas. No part of the image is permitted to extend beyond the boundaries of the basic layout object. The initial point is the point relative to which all line boxes are positioned within the basic layout object (see 7.2.2 and Figure 11/T.416). The line home position of the first line box is at the initial point of the basic layout object. Subsequent line home positions are located on a line through the initial point in the direction of line progression. The distance between the line home positions of two successive line boxes is determined as follows: - when proportional line spacing is to be performed, the distance between the reference lines of two successive line boxes is evaluated by an implementation dependent algorithm not defined in this Recommendation; - when proportional line spacing is not to be performed, the distance between the reference lines of two successive line boxes is equal to the current line spacing as specified by presentation attributes and control functions. FIGURE 11/T.416 Illustration of line box positioning concepts 6 Character imaging Four groups of specifications may apply to the imaging of graphic character elements in a basic object, namely those relating to: - emphasis; - font selection; - subscript and superscript; - character combinations. These groups of specifications are defined below. Note 1 - Emphasis (by "weight", "posture" etc.) and subscript/superscript rendition may be achieved by font selection. Fascicle VII.6 - Rec. T.416 17