chapter 20: natural language generation presented by: anastasia gorbunova ling538: computational...
TRANSCRIPT
Chapter 20: Natural Language Generation
Presented by: Anastasia Gorbunova
LING538: Computational Linguistics, Fall 2006
Speech and Language Processing (Jurafsky & Martin)
Natural Language Generation(NLG)
The process of constructing natural language outputs from non-linguistic inputs - maps from meaning to text
Concerns: - choice
Natural Language Understanding(NLU)
The process of producing non-linguistic outputs from natural language inputshypothesis management - maps from text to meaning
Concerns: - ambiguity - under-specification - ill-formed input
Both must represent a range of lexical and grammatical forms required for the application domain
NLG
Choice Issues Architecture
Communicative Goal Knowledge Base
Discourse Planner Mechanisms for building Discourse Structures: - Text schemata; Rhetorical Relations Content Selection
Surface Realizer Approaches: - Systemic Grammar; - Functional Unification Grammar
Natural Language Output
Microplanning
Lexical Selection
Context Selection
Discourse Structure
Sentence Structure-referring expressions
-aggregation
Output from DP
Input to SR
Surface Realizer Produces ordered sequences of words as constrained by the rules of lexicon and grammar.
Approaches:
Systemic Grammar Functional Unification Grammar
• Treats language as resource for expressingmeaning in context • Represents sentences as collections of functions and maintains rules for mapping those functions onto explicit grammatical forms • Expressed as a system network
Surface Realizer Produces ordered sequences or words as constrained by the rules of lexicon and grammar.
Approaches:
Systemic Grammar Functional Unification Grammar
• Treats language as resource for expressingmeaning in context • Represents sentences as collections of functions and maintains rules for mapping those functions onto explicit grammatical forms • Expressed as a system network
• Builds generational grammar as a feature structure with potential alternations • Then unifies it with input specification built using the same sort of feature structure• Expressed as an attribute-value matrix
ThemeTextual
TransitivityIdeational
MoodInterpersonalThe system will save the document
LayersMeta-
functions
ThemeTextual
TransitivityIdeational
MoodInterpersonalThe system will save the document
LayersMeta-
functions
rhemetheme
goalprocessactor
objectpredicatorfinitesubject
rhemetheme
goalprocessactor
objectpredicatorfinitesubject
Surface Realizer Produces ordered sequences or words as constrained by the rules of lexicon and grammar.
Approaches:
Systemic Grammar Functional Unification Grammar
Take input at different levels• Treats language as resource for expressingmeaning in context • Represents sentences as collections of functions and maintains rules for mapping those functions onto explicit grammatical forms • Expressed as a system network
Both grammars• use functional categorizations
- input is functionally rather than syntactically specified
• support multiple levels that are entered recursively during the generation process
• Builds generational grammar as a feature structure with potential alternations • Then unifies it with input specification built using the same sort of feature structure• Expressed as an attribute-value matrix• Input represented as a functional description:
ThemeTextual
TransitivityIdeational
MoodInterpersonalThe system will save the document
LayersMeta-
functions
ThemeTextual
TransitivityIdeational
MoodInterpersonalThe system will save the document
LayersMeta-
functions
rhemetheme
goalprocessactor
objectpredicatorfinitesubject
rhemetheme
goalprocessactor
objectpredicatorfinitesubject
NLG
Choice Issues Architecture
Communicative Goal Knowledge Base
Discourse Planner Mechanisms for building Discourse Structures: - Text schemata; Rhetorical Relations Content Selection
Surface Realizer Approaches: - Systemic Grammar; - Functional Unification Grammar
Natural Language Output
Microplanning
Lexical Selection
Context Selection
Discourse Structure
Sentence Structure-referring expressions
-aggregation
Output from DP
Input to SR
Discourse PlannerKeeps track of the focus and the local topic of discourse; considers relationships between
sentences. Also responsible for content selection and lexical selection
Mechanisms for building discourse structures: Text Schemata• Useful if a discrete set of consistent patternsand expressions can be found and encoded • May be represented as an augmented transition network
• Problems: - impractical when text calls for structural variety and richness of expression - resulting discourse structure includes no higher-level structure relating sentences together
S0
S1
S2
Є
Add Precondition
Express the Action
Recursively Add Sub-Step
Add Side-Effect
Rhetorical Relations •Rhetorical Structure Theory – text organization based on relationships between parts of text• RST relations: elaboration, contrast, condition, purpose, result, etc.• Used when the text being generated calls for variation • Can develop own schema based on a particular situation
NLG
Choice Issues Architecture
Communicative Goal Knowledge Base
Discourse Planner Mechanisms for building Discourse Structures: - Text schemata; Rhetorical Relations Content Selection
Surface Realizer Approaches: - Systemic Grammar; - Functional Unification Grammar
Natural Language Output
Microplanning
Lexical Selection
Context Selection
Discourse Structure
Sentence Structure-referring expressions
-aggregation
Output from DP
Input to SR
Microplanning
The link between the discourse planner output and the surface realizer input
Two main areas of concern:
Referring expressions - determine those aspects of an entity that should be used when referring to it in a particular context
Aggregation- apportioning the content from the knowledge base into phrase, clause, and sentence-sized chunks
NLG
Choice Issues Architecture
Communicative Goal Knowledge Base
Discourse Planner Mechanisms for building Discourse Structures: - Text schemata; Rhetorical Relations Content Selection
Surface Realizer Approaches: - Systemic Grammar; - Functional Unification Grammar
Natural Language Output
Microplanning
Lexical Selection
Context Selection
Discourse Structure
Sentence Structure-referring expressions
-aggregation
Output from DP
Input to SR