A Survey on Story Generation Techniques for Authoring Computational Narratives <s> A. Constrained Plot and Space Modification <s> This paper describes an approach for managing the interaction of human users with computer-controlled agents in an interactive narrative-oriented virtual environment. In these kinds of systems, the freedom of the user to perform whatever action she desires must be balanced with the preservation of the storyline used to control the system's characters. We describe a technique, narrative mediation, that exploits a plan-based model of narrative structure to manage and respond to users' actions inside a virtual world. We define two general classes of response to situations where users execute actions that interfere with story structure: accommodation and intervention. Finally, we specify an architecture that uses these definitions to monitor and automatically characterize user actions, and to compute and implement responses to unanticipated activity. The approach effectively integrates user action and system response into the unfolding narrative, providing for the balance between a user's sense of control within the story world and the user's sense of coherence of the overall narrative. <s> BIB001 </s> A Survey on Story Generation Techniques for Authoring Computational Narratives <s> A. Constrained Plot and Space Modification <s> The ability to generate narrative is of importance to computer systems that wish to use story effectively for entertainment, training, or education. One way to generate narrative is to use planning. However, story planners are limited by the fact that they can only operate on the story world provided, which impacts the ability of the planner to find a solution story plan and the quality and structure of the story plan if one is found. We present a planning algorithm for story generation that can nondeterministically make decisions about the description of the initial story world state in a least-commitment fashion. <s> BIB002 </s> A Survey on Story Generation Techniques for Authoring Computational Narratives <s> A. Constrained Plot and Space Modification <s> In interactive plan-based narrative environments, users' actions must be monitored to ensure that conditions necessary for the execution of narrative plans are not compromised. In the Zocalo system, management of user actions has been performed on a reactionary basis by a process called mediation. In this paper, we describe an extension to this approach, proactive mediation, which calculates responses to user input in an anticipatory manner. A proactive mediation module accepts as input a plan describing the actions being performed by the user (generated by a plan recognition system) and identifies portions of that plan that jeopardize the causal structure of the overall narrative. Once these portions are identified, proactive mediation generates modifications to the narrative plan structure that avoid the unwanted interaction between user and story. This extension to the original mediation algorithm provides more responses to a user's actions and generates responses that are tailored to the user's activity. <s> BIB003 </s> A Survey on Story Generation Techniques for Authoring Computational Narratives <s> A. Constrained Plot and Space Modification <s> The authoring of fictional stories is considered a creative process. The purpose of most story authoring is not to invent a new style or genre of story that will be accepted by the population but to invent a single narrative that is novel enough to be tellable. Computational story generation systems are more limited than human authors in the space of narratives that can be considered because it is often the case that story generation systems are constrained to operate within a fixed representation of the story world. These limitations can impact whether a story generation system is considered creative or not. In this paper, we describe a story planning system, Fabulist. Fabulist however is constrained by the world model input by the system user. We present two algorithms that enable story planning systems such as Fabulist to break outside the bounds of the initial world model in order to search a larger space of narratives. <s> BIB004 </s> A Survey on Story Generation Techniques for Authoring Computational Narratives <s> A. Constrained Plot and Space Modification <s> In this paper we present our continuing efforts to generate narrative using a character-centric approach. In particular we discuss the advantages of explicitly representing the emergent event sequence in order to be able to exert influence on it and generate stories that ‘retell' the emergent narrative. Based on a narrative distinction between fabula, plot and presentation, we make a first step by presenting a model based on story comprehension that can capture the fabula, and show how it can be used for the automatic creation of stories. <s> BIB005
Since space and plot are both able to directly influence each other, the story resulting from any generation technique may suffer from issues of quality in either component. In this sense, some researchers argue that while generating plot, changes should be made to the space if this allows or improves the current plot generation BIB002 . This attempt at space automation according to plot constraints may be seen on a small scale in the initial state revision (ISR) algorithm implemented in the FABULIST BIB004 system. FABULIST is a planning-based plot generator, unique in its goal of merging both author and character goals, so that the resulting story would be satisfactory to the human author, while ensuring that the actions taken by the characters make sense to the reader. In terms of space, however, FABULIST uses the ISR algorithm in which, during the generation of the plot, the initial state of the space may be modified to fulfill certain aspects of the plot BIB002 . To allow this, the rules used for plot generation leave certain information as indeterminate. If these rules are used during the planning phase, then the system will modify the space in such a way that fulfills the missing information of the plot. The specific example provided by the authors involves the placement of a hidden weapon within the game world, so that one character is able to stealthily assassinate the other. The location of the weapon is left as indeterminate information, constrained by certain rules that state where the weapon may not be located. When generating the plot, the system then places the weapon in an appropriate location, automatically modifying the space in order to complete the plot. The space modifications provided by the FABULIST system show an application of limited automatic space generation, in which important space details related to the plot may be created automatically. Swartjes et al. also explored a similar space modification technique called late commitment  in their emergent VIRTUAL STORYTELLING system. Late commitment, inspired by improvisational theater, allows the characters themselves to modify or generate new content for the space in order to achieve certain goals, for example, a character may desire to fight another character and can propose spawning weapons to meet the preconditions of the fight event. The system will then check to make sure there are no consistency violations in the space after making this change, and if so, then, the weapons will be created. The late commitment approach use consistency as a constraint to ensure that each change is invisible, i.e., a reader perceiving the story should not be aware when modifications were made, it should always appear that the space was defined this way. The VIRTUAL STORYTELLER further makes use of a story director to help in the generation of plots BIB005 . The story director is an agent which contains a formal model of plot and a simulated understanding of storytelling and can selectively pick events within the simulated game world and combine them in such a way that the result is a story. The director also has a very specific influence over the game world and can introduce new characters or modify character goals in an effort to improve certain qualities of the resulting plot. Modifying space may be also of use in preventing or accommodating for the interactor's actions in interactive stories. MIMESIS BIB001 is an interactive story system that uses a technique called mediation to modify either the plot or the space in order to balance interactor action against the intended plot. MIMESIS uses two forms of mediation: reactive mediation BIB001 and proactive mediation BIB003 . Reactive mediation occurs at the moment the interactor makes an action that may render the current plot invalid. In these situations, the system may either replan the narrative to allow this action or force the interactor's action to fail, such as having a gun jam if the interactor is intending to shoot a main character in the story. Proactive mediation aims to predict future interactor actions and modify the space in advance to make the action impossible to attempt, for example, by locking the drawer containing a gun to prevent the interactor attempting to shoot any of the characters in the first place. TALE-SPIN  is an emergent storytelling system and is often considered one of the first story generation systems. Story generation in TALE-SPIN occurs by providing characters in the space with goals and personalities. In simulating the events surrounding the characters' attempts to achieve a goal, the system fashions basic stories. TALE-SPIN is a highly emergent system, relying almost entirely on stories emerging from the creation and simulation of character goals coupled with the unique personalities of each character. Its approach to space modeling is largely authored, although the space can be modified in order to ensure that the character goals could be achieved. TALE-SPIN is also an early system to stress how important elements of story, such as uniqueness and coherence, directly relate to the quality of the space, in terms of scale, depth of characters, and correctness (i.e., the space authored should accurately represent the space the author envisions). A poor quality space will inevitably lead to dull or even nonsensical and strange plots, nicknamed mis-spun tales by its creator.