Tree and Branching Structures

Trees and Branching Structures have been used extensively to enable choice to occur in a variety of mediums including books, websites, computer software and hypertext. In its simplest and most basic form a Tree involves a choice between two alternatives (or possibly more) and from that different paths originate.

Figure 2.5: Basic Branching Structure

In this section I will be looking at a selection of possible tree structures, and how they can influence choice. This list is taken from a variety of sources, the first is Samsel and Wimberley’s Writing for Interactive Media (1998), the second is Glassner’s Interactive Storytelling (2004) and the third is Ryan’s Avatars of Story (2006). There is also Claude Berge’s work analysing the writings of Oulipo which is also instructive (in Motte, 1986, p.115-125), but as his work largely does not name these structures and only visually represents them without detailed explanation, he will not be discussed here.

An Extended Branching Tree

An Extended Branching Tree (Samsel and Wimberley p.26, 1998) offers the fullest most complex variation of narrative within any of the branching narrative variations. Each choice leads to multiple different paths, with broad narrative differences and a wide variety of endings. Glassner refers to this mechanism as a fully populated tree (p.241-242, 2004). Glassner and Samsel and Wimberley (1998) both comment on how difficult and complex this

structure can be to achieve. An extended branching tree is not impossible but it does require a lot of forethought and planning.

Within an Extended Branching Tree, the narrative can exist in two forms with regards to the world of the story. The first form lets the choices take place and impact upon the same narrative world. The second and even rarer (and which requires yet more planning) is when each choice takes place within a different narrative world. This is difficult to achieve because each world must be carefully defined and constructed.

Figure 2.6: Extended Branching Tree Structure

An example of the first form from the CYOA Series is The Secret Treasure of Tibet (Book 36) written by Richard Brightfield (1984) in which YOU (the second-person hero of the story) are tasked with finding a lost city in Tibet. Each choice leads to a genuine new exploration of different parts of the world and story. As a result many of the actions change according to the different choices and certain characters exist only in certain stories. However internal consistency remains high, and characters who commit bad deeds do so in all versions of the storyline and the initial goal and broader goal are both achievable within the frame of the story.

An examples of the second form is another CYOA Book , Mystery of the Secret Room (Book 63) written by Ellen Kushner (1986) which integrates 3 different worlds into its storylines. Two are variations of our existing world (one in the present, another in the future). The third world is a fantasy world unlike our own. This type of structure was more prevalent in the high choice/high ending phase of the CYOA series but disappeared as the series progressed.

An Unbalanced Tree

An Unbalanced Tree as identified by Glassner (2004) features a series of choices but often only a few paths lead to long and satisfying narratives. This structure is by the far the most common structure found in existing Choice-Based Narratives, particularly in printed books. An unbalanced tree often has two or more stories that are well developed. It is also common for this structure to have multiple story paths that are wildly divergent (yet all paths

ostensibly take place in the same narrative world) with many different endings.

Figure 2.7: An Unbalanced Tree Structure

There are many examples of this type of narrative structure because it is by far the most typical found in Choice-Based Narratives. An example can be found in the Choose Your Own Adventure Super Adventure – Danger Zones (Book 2 in the series) written by R.A.

Montgomery (1987) in which there are three story pathways but only two are developed in detail.

An Unbalanced Tree as a form of branching should be compared with the Pruned Tree and the Partial Bulging Tree.

The Pruned Tree (Branching with Forced Paths)

In the Pruned Tree structure, the creator favours one storyline over many others, so that although choice is available, the branches are pruned until we only receive the longest storyline that has been developed. As a diagram it represents a tree whose branches have been pruned. Samsel and Wimberley refer to this as Branching with Forced Paths (p.27-28, 1998) as often the choices forces the audience into a specific course of action.

There may be cul-de-sacs (Samsel and Wimberley, p.25-26 an p.93-100, 1998) diversions, merges and switches (see Sub Mechanisms section below), they will all eventually lead back to the central path. A Pruned Tree is different from an Unbalanced Tree because it focuses on one story at the expense of the others. There may be other stories in a Pruned Tree but they will be smaller and less significant than the central story.

Figure 2.8: Pruned Tree Structure (Branching with Forced Paths)

Examples of this structure can be found in the later books in the CYOA series. As the series progressed the number of endings and choices decreased. This meant that many of the authors focused on longer storylines that developed into a single primary storyline. Two examples are the Edward Packard books Superbike (Book 124, 1992) and Fugitive (Book 182, 1998).

Pruned Trees can be disappointing for an audience because their curiosity and interest must lie in the story being told. Agency is also limited in this structure as most choices will not allow movement away from the central story.

A Bulging Tree

A Bulging Tree (Glassner, 2004, p.244) is when a series of choices allow exploration within the same story world but all of the choices will eventually return the audience to a predetermined ending. So although choice is possible, it leads to a set ending. Both Ryan (2006, p.104-105) and Samsel and Wimberley (1998, p.25-26) looks at this form slightly differently by viewing the Bulging Tree as one in a chain that represents different forms of progress within a narrative. Ryan identifies this as a Flowchart while Samsel and Wimberley identify it as Branching with Bottlenecking. In Ryan’s version it is possible for the audience to skip to a later Bulging Tree through the choices made (see also A Partial Bulging Tree), whereas in Samsel and Wimberley this option is not available. These permutations though offering different outcomes in the final narrative are ostensibly the same form as described by Glassner.

Figure 2.9: Bulging Tree Structure (Flowchart/Branching with Bottle-necking)

An example of a Bulging Tree is the Time Machine series of books published by Bantam books from 1984 to 1989. There were 25 books in the series and each book features a single mission that was to be completed by YOU the second person hero. There is only one ending in this book and it is only through exploration of the various choices within the time period that was the setting (e.g. the Spanish Inquistion or World War II) that the final ending could be reached.

By extension modern video games often utilise the Ryan/Samsel and Wimberley model to control the action and story in the form of chapters. This device is particularly used in first- person and third-person shooters, platform games and adventure games, where each mission can be separated into a single chapter each leading to the same endpoint rather than existing in one large continuous world.

A Partial Bulging Tree

Glassner identifies a Partial Bulging Tree (2004, p.244) as differing from a standard Bulging Tree in that it has multiple endings but only one ending that is considered the true ending. A Partial Bulging Tree is slightly different to a Pruned Tree as the shaping of the Pruned Tree allows for one main story and one or two smaller side stories to coexist. In a Partial Bulging Tree all choices that do not result in the true ending lead instead to false or negative endings. Whereas a Pruned Tree can be disappointing to the spectator for the lack of choice and agency, a Partial Bulging Tree is slightly more successful because the audience from the outset assumes only one true ending and so is primed to continue searching for that ending.

Figure 2.10: Partial Bulging Tree

An example of this is the Be an Interplanetary Spy series published by Bantam Books from 1983 to 1985. In this series ‘YOU’ were a spy tasked with various missions across the galaxy. Each book was a specific mission which held only one true ending which was always located

spatially at the end of the book, so the reader knew to continue progressing in this direction to complete the mission. Although there were other endings they always represented a failure in the completion of the set mission (and often featured a gruesome death). Much Interactive Fiction such as the Zork series (Infocom, 1981-1982) and Planetfall (Infocom, 1983) and Adventure Games such as Under a Killing Moon (Access Software, 1994) and Blade Runner (Westwood, 1997) also uses this structure.

Modular Tree

A Modular Tree represents a more complicated concept than the previous trees and branching described above. There are several points at which the audience can start the experience and it allows for multiple contexts to be achieved depending on the selections made by the audience. This means that the scenes or nodes can have multiple meanings and context depending on their placement. So that in one version of the narrative certain scenes will have different meanings and context than in another version.

Figure 2.11: Modular Tree Structure

The Melete Effect is an example of a Modular Tree. Depending on the choices made by the

audience throughout this experience they will see various scenes from the character Mary Melete’s life as a journalist. Various scenes and nodes are designed to take on different meanings depending on which of the three storiylines is represented.

Many of the above examples of Trees and Branching also feature sub-mechanisms that help the creator to move the narrative in a particular direction or otherwise encourage the audience to move in a specific direction. These mechanisms although limiting the agency of the audience can be necessary to maintain cohesion and immersion within a narrative. The four sub-mechanisms are as follows:

Diversions

A diversion is when a choice leads the audience into a short detour but then they are returned to a main story path quickly. This diversion provides a little bit more detail about the story and the characters, but the rest of the narrative remains the same.

Merges

A merge is a longer and more significant diversion within a story that then shifts the audience to another branch of the storyline that they may not have chosen previously. This results in closing one particular storyline into another storyline. The original storyline cannot be returned to; it has been merged with the new storyline.

Loopbacks

A Loopback is a return to a place where the audience has already been. In most cases the loopback represents the same location and time as when the audience visited originally, there is no change. This can lead to the audience feeling as though they are stuck in a narrative loop from which they cannot escape. Occasionally however a loopback can return to the same location but at a different time, serving to move the story in direction or to examine the spaces in which the narrative is set.

Switches

A Switch is a sub-mechanism that leads to the abandonment of the current story to take you to the beginning of another major storyline. It is in effect a resetting of the choices made by the audience to lead them back to begin another narrative path. However it may be possible to switch back to original storyline at a later stage.

It should be noted these four sub-mechanisms are common to all Choice-Based Narratives regardless of their medium. These mechanisms are used predominantly to control the size and complexity of any given Choice Structure.