title: A simple algorithm for medial axis transform computation

authors: Au, Chi Kit

subjects: medial axis

abstract: Although medial axis transform is introduced as a shape description for many engineering applications, the computational algorithm is still challenging. This is especially true for the shape with free form boundary. This paper presents an algorithm for medial axis transform computation from a perspective of minimum distance between the points in a two-dimensional shape and its boundary. The minimum distance is given by a resultant distance function which is a superposition of the individual distance function between a point within a shape and each boundary point. By elaborating the resultant distance function, the medial axis transform will be obtained naturally. The distance function is modeled as a solid cone and the superposition is equivalent to the union Boolean set operation. The implementation of the approach is simplified using a solid modeling kernel. Several examples of two-dimensional shapes with free form boundaries are raised to illustrate the concept and algorithm.

text: A simple algorithm for medial axis transform computation Although medial axis transform is introduced as a shape description for many engineering applications, the computational algorithm is still challenging. This is especially true for the shape with free form boundary. This paper presents an algorithm for medial axis transform computation from a perspective of minimum distance between the points in a two-dimensional shape and its boundary. The minimum distance is given by a resultant distance function which is a superposition of the individual distance function between a point within a shape and each boundary point. By elaborating the resultant distance function, the medial axis transform will be obtained naturally. The distance function is modeled as a solid cone and the superposition is equivalent to the union Boolean set operation. The implementation of the approach is simplified using a solid modeling kernel. Several examples of two-dimensional shapes with free form boundaries are raised to illustrate the concept and algorithm.

year: 2013

title: Visual Perception and Analysis of an Approaching Train at Railway Level Crossings in New Zealand

authors: Clark, Helen Elizabeth

subjects: vision science

abstract: The prevalence of railway level crossing accidents in New Zealand is a high profile issue that has warranted close scrutiny over the last 10 years. However, the incident rate has not decreased. This research examined the possibility that visual illusions and perceptual errors contribute to an underestimation of a train‟s arrival time by motorists. The first experiment was designed to analyze whether the Size-Arrival Effect, a theory that states that large-far objects are judged to arrive earlier than small-nearer objects, was applicable to trains. Participants were shown a computer simulation of a moving vehicle (train, motorcar or motorcycle) and asked to indicate when the point was reached where they would no longer cross in front of the approaching vehicle. Approach speeds were systematically varied (60 km/h, 70 km/h, 80 km/h, 100 km/h and 120km/h). Results found that participants adopted the greatest safety gap distance to cross for the train. However, there was no adjustment for velocity when adopting safety gaps for the train and the motorcycle with observers using the same gap distance, regardless of the approach speed. The second experiment sought to examine the Leibowitz hypothesis (Leibowitz, 1985), which proposes an illusory bias; a large object seems to be moving more slowly than a small object travelling at the same speed. Experiment 2 measured participants' ability to make a direct comparison between the speed of an approaching motorcar and a train. Participants were asked to judge which of the vehicles appeared faster, with the distance from the observer varied (far, middle or near). Participants significantly underestimated the speed of the train as compared to the car, in both the 'middle' and 'near' conditions, with the magnitude of underestimation greatest in the 'middle' condition. The overall findings offered support for both theories, which indicates that a combination of distance and speed perceptual errors may at least partly contribute to the high rates of level crossing collisions.

text: Visual Perception and Analysis of an Approaching Train at Railway Level Crossings in New Zealand The prevalence of railway level crossing accidents in New Zealand is a high profile issue that has warranted close scrutiny over the last 10 years. However, the incident rate has not decreased. This research examined the possibility that visual illusions and perceptual errors contribute to an underestimation of a train‟s arrival time by motorists. The first experiment was designed to analyze whether the Size-Arrival Effect, a theory that states that large-far objects are judged to arrive earlier than small-nearer objects, was applicable to trains. Participants were shown a computer simulation of a moving vehicle (train, motorcar or motorcycle) and asked to indicate when the point was reached where they would no longer cross in front of the approaching vehicle. Approach speeds were systematically varied (60 km/h, 70 km/h, 80 km/h, 100 km/h and 120km/h). Results found that participants adopted the greatest safety gap distance to cross for the train. However, there was no adjustment for velocity when adopting safety gaps for the train and the motorcycle with observers using the same gap distance, regardless of the approach speed. The second experiment sought to examine the Leibowitz hypothesis (Leibowitz, 1985), which proposes an illusory bias; a large object seems to be moving more slowly than a small object travelling at the same speed. Experiment 2 measured participants' ability to make a direct comparison between the speed of an approaching motorcar and a train. Participants were asked to judge which of the vehicles appeared faster, with the distance from the observer varied (far, middle or near). Participants significantly underestimated the speed of the train as compared to the car, in both the 'middle' and 'near' conditions, with the magnitude of underestimation greatest in the 'middle' condition. The overall findings offered support for both theories, which indicates that a combination of distance and speed perceptual errors may at least partly contribute to the high rates of level crossing collisions.

year: 2010

title: Genre and authentic written discourse in Maori and their relevance to the education of students in upper secondary and tertiary Maori-medium educational settings

authors: Houia-Roberts, Ngaere

subjects: Māori medium education

abstract: I report here on part of a research project whose primary aim was to investigate a range of authentic Māori texts in terms of the search for commonly occurring organisational and cognitive patterns, and to consider the pedagogic implications of this investigation in terms of the teaching of writing skills in Māori-medium educational settings. An examination of a range of text segments written by educated and highly proficient users of Māori suggests that, in terms of textual relationships, there are characteristic patterns of organisation that characterise the three genres that appear to be most commonly required of student writers in Māori-medium upper secondary and tertiary educational contexts, that is, the genres of arguing, explaining and describing. Another part of the project – an examination of whole texts in terms of overall, structuring – is reported in the next issue of this journal.

text: Genre and authentic written discourse in Maori and their relevance to the education of students in upper secondary and tertiary Maori-medium educational settings I report here on part of a research project whose primary aim was to investigate a range of authentic Māori texts in terms of the search for commonly occurring organisational and cognitive patterns, and to consider the pedagogic implications of this investigation in terms of the teaching of writing skills in Māori-medium educational settings. An examination of a range of text segments written by educated and highly proficient users of Māori suggests that, in terms of textual relationships, there are characteristic patterns of organisation that characterise the three genres that appear to be most commonly required of student writers in Māori-medium upper secondary and tertiary educational contexts, that is, the genres of arguing, explaining and describing. Another part of the project – an examination of whole texts in terms of overall, structuring – is reported in the next issue of this journal.

year: 2003-09