Faces in Perception [1238] Perception, 1 993, volume 22, pages 257-262
Extracting prototypical facial images from exemplars
Philip J Benson'll, David I Perrett Department of Psychology, University of St Andrews, St Andrews, Fife KY1 6 9JU, UK Received 16 October 1 991 , in revised form 1 2 June 1992
Abstract. A computer graphic method for extracting a natural image of an individual's facial prototype, or average appearance, from a number of different images of that individual is presented. The process improves upon previous photographic and computational techniques. Synthesis of a person's average expression and pose from a sample of images is derived in an automatic and quantitative way.
Possible uses of composite faces produced in this manner in
psychological investigations of facial qualities (eg attractiveness) and in applied areas such as telecommunication are pointed out.
1 Introduction
The work of Galton
( 1878a,
1878b,
1879 ) , Taylor
( 1885 ),
Fletcher
( 1886 ),
and
Stoddard ( 1886, 1887 ) on producing composite photographic portraits stimulated much
interest in their day. Although their techniques were relatively primitive by today's standards the results were both remarkable and intriguing: Figure 1 illustrates a com posite of forty nine female students, and is striking because it looks like a real person.
Figure 1. Galton's (1878a) photographic composites were made by multiple exposures of a number of individual pictures scaled such that features were aligned as well as possible to minimise feature blurring. This figure shows a photographic composite portrait of forty nine female undergraduates of Smith College produced by Stoddard principles.
( 1886) with the
use of Galton's
Careful selection "of photographs of several persons alike in most respects, but
differing in minor details" (Galton 1878a, page 132) ensured that such large numbers of pictures could be combined reasonably effectively.
11 Address for correspondence: OXl 3PT, UK.
University Laboratory of P hysiology, Parks Road, Oxford
Faces in Perception [1239] P J Benson, D I Perrett
258
Galton's ( 1878b, 1888) reason for pursuing these 'trials' was to discover if it was possible to extract the typical characteristics of a number of faces from particular classes and types of people (eg criminals, the healthy and ill, and familial resemblances). Whereas such early composites of many people were a "portrait of a type [of person] and not of an individual" (Galton 1878a, page 133), blends of many instances of one person obtained by the same techniques, are. In his later work Galton ( 1879) concen trated on locating consistencies in the appearances of famous individuals.
It was
hoped that by combining several portraits of the same person produced by different artists a truer depiction of that person would result, by reducing stylistic and erro neous idiosyncrasies. Antiochus,
Composite portraits were fashioned of Alexander the Great,
Poliorcetes,
Cleopatra,
and
Nero.
Taylor
( 188 5)
similarly produced
composite images of George Washington in several perspective views by combining original images in the same pose. It was noticed from the outset that more often than not composites appeared younger and more attractive than constituents. For example, Galton ( 1878a, page 137) cited Austin's letter to Darwin claiming that in the case of some ladies' portraits the composite represented "a decided improvement in beauty". Jastrow ( 1885, page 168) noted that a composite of male faces made "a younger and handsomer man than the average of those whose faces enter into it". Perhaps with a little prejudice Galton noted of his blended Cleopatra that "the composite is as usual better looking than any of the components, none of which, however, give any indica tion of her reputed beauty; in fact, her features are not only plain, but to an ordinary English taste are simply hideous" (Galton 1879, page 164).
The increase in attrac
tiveness has been attributed to the average shape of the composite (Langlois and Roggman
1990) and a flatness or smoothing of skin tones (Benson and Perrett
1991a). These works highlighted a number of important issues which are relevant to current psychological studies of the perception and recognition of faces: faces as a homo geneous class of objects, and memory for particular familiar faces (Young et al 1987). Faces within the same cultural groups are highly similar in appearance. Individuating differences are on the whole only slight, yet our specialised perceptual mechanisms are able to identify these minute characteristics.
Certain differences may even be
difficult to verbalise. One face is seen from a great many perspectives and is continually undergoing elastic transformations during expression and speech. Our knowledge of the appearance of a familiar or famous face increases over time through experience of multiple encounters with that person, and allows recognition to become more tolerant of unusual viewing circumstances; the witness of an individual's face can be related to the essential and most useful components of their visage which underly potentially distracting and unexpected changes in pose, expression, feature contortions, or apparel. Although we are aware that abstractive mechanisms operate within the visual system, the psychological bases of this ability are unknown.
Different models have
been developed which simulate the effects of the generalisation process. parallel
distributed
processing
Rumelhart ( 1985 ) explicitly
(PDP )
models
extract prototypes,
such
as
that
Of these,
of McClelland
and
given multiple instances bearing
common features; Hintzman's ( 1986) model of schema abstraction does not explicitly code the abstraction, yet it is able to effect generalisation if so required. In this paper we also present an abstraction technique which operates over multiple images of given faces to synthesise an average or prototypical appearance.
Unlike the models
described above, the technique we describe here does not attempt to mimic cognitive processing but does serve to draw analogies insofar as the numerical procedure is able to locate consistent features and to dispense with miscellaneous details.
Faces in Perception [1240] 259
Extracting facial prototypes
Recognition also requires that a face be differentiated from others (Benson and Perrett 199 1b). This differentiation is not solely conducted at the level of individual features because faces are seen as a whole, processed in an holistic manner in which feature configuration is important (Carey and Diamond 1977). The failure of physi ognomists such as Lavater ( 1780) to notice this propagated many misleading ideas about social classes, types of person, and personality traits. The composite process we describe here abstracts not just the average shape of the features of a person or group, but also the average configuration of those features. 2 Methods
Careful selection of original photographs to ensure facial similarity is not sufficient to produce sharply defined composites.
For example, some faces will have wider or
narrower noses than others, which will cause ghosting of the feature in the final blend. Successful averaging of faces depends on the dimensions of the originals. A second difficulty lies in the multiple-exposure timing used by Galton and his followers. (t) Langlois and Roggman (1990) more recently attempted to improve upon this technique by digitally merging captured images of faces; as before, original portraits were scaled such that the pupils were as accurately aligned as possible and corresponding picture elements (pixels) numerically averaged to produce a high-quality composite.
The
results are an improvement upon the photographic technique, but still suffer from a lack of qualitative control. No facial features, not even the pupils, are aligned in any precise way since only two control points covering the estimated pupil centres exist. The computer composites produced by averaging corresponding pixel intensities in each facial image 'frame' produces blurring in the same way the original photographic portraits did (see figure 2b). Thus an improvement in processing with a decrease in blurring is desirable because an average face should, after all, be one which looks like
·
a face in every way and has well-defined characteristics. 2. 1
Computational procedure
We have previously described a process to distort facial images by computer in a systematic and controlled way (Benson and Perrett 199 1b, 199 1c).
Here we have
applied similar procedures to derive a highly realistic composite of one individual from a number of images. The same procedure can be applied to images of different people to produce a composite of a particular category of individual. On each of six digitised images of Margaret Thatcher 186 points describing the positions of her facial features were manually logged.
Identical numbers of points
described particular features and therefore a relationship between each of the original 'portraits' in different poses.
By averaging the x- y coordinates of matched feature
points across each image, the mean position of each feature can be found. The differ ence between the original and the average feature positions (and hence configuration) defines by how much Thatcher's face would have to change in each case in order to assume her 'normal' visage. That is, the difference in positions of the original feature points with respect to their average defines how each image of Thatcher deviates from (I) When producing composite faces, Galton exposed each original face for the same time to the new photographic plate ( Galton 1878a, page 133 ) . At very long ( and very short ) exposures the reciprocal relationship between settings of lens aperature and exposure time breaks down. When dealing with long
( multiple )
exposures Galton's composites would almost certainly have
been limited by such reciprocity failure; this, coupled with the inferior photographic plate chemistry, explains why many of the composites made by himself and others looked 'better' than they should have, and why some attempted replications failed. Procedural problems involving changes in lighting conditions and first or last exposure bias in the photochemical process can also account for the observation by Fletcher nant in a composite.
( 1886 )
that one individual's face appeared domi
·
Faces in Perception [1241] P J Benson, D I Perrett
260
a prototypical Thatcher configuration. W ith the feature markers employed as reference points, each of the six images was 'warped' ( Benson and Perrett 199lc ) into the shape
defined by the averaged coordinates such that each assumed an identical average facial shape. F igure 2c was produced by averaging each corresponding pixel intensity
(c)
(b)
(a)
Figure 2. Upper two rows: Original photographs of Margaret Thatcher. Bottom row:
( a ) The
composite of all six original unnormalised images; the image appears as an unrecognisable blur.
( b) A
composite
( produced
digitally
}
according to the procedures defined by Galton; three of
the images have been rotated and magnified to align the pupils horizontally, and two images were mirrored vertically to change the left-right pose.
(
( c ) The )
blended computer composite.
The automatic process of generating the composite see text deals with differences in expression and pose by normalising the original images before blending. Here we deliberately chose base images which varied a great deal in a number of ways to determine the power of the technique.
Faces in Perception [1242] 261
Extracting facial prototypes
in the six distorted images.
The blend is well-focused though 'soft' in contrast, and
maintains various features such as drooping eyelids, hairline, and narrow nose. Blending all six images together without intermediary transformation produces an almost unrecognisable composite with extreme ghosting of features (eg six mouths; figure 2a). Figure 2b shows a composite made according to Galton's method. 3 Discussion
The process we have described in this paper does in fact produce an accurate and faithful generic face when averaging different faces.
We do not intend the graphic
process to be seen as an explanation of psychological processes underlying recogni tion. Rather, the technique demonstrates that salient visually distinctive features can be extracted automatically from disparate images. Facial shape and feature configura tion are maintained over large samples and only a pervasive blandness of skin textures is introduced. Although the process is restricted to images of faces in preprofile view it does provide a reliable means of normalisation. Whatever photographic or computational process is used to create facial composites, it is important to bear in mind a fundamental phenomenon being procured-that of extraction of averages or prototypes from a set of exemplars. Formation of category prototypes has been shown to be useful for recognising objects at the psychological level (Tversky 1977; Rosch 1978) by providing a level of abstraction which seeks to maximise the amount of information about a category with the least cognitive effort. In a sense the averaging process we present here also maximises the information about the category to which the face belongs; a normalised appearance is one which may be more likely to display perceptual similarity to the target individual and conse quently evoke recognition, more so than perhaps an unusual 'caught unaware' snapshot would.
It remains to be seen whether the average appearance of an individual is a
better likeness than a caricature for example (Benson and Perrett 1991b, 1992), or even whether a caricature of the computed prototype is in turn better still. Prototypes are not only useful for determining whether new stimuli belong to a familiar category but they also enable reliable judgements of the relative distinctive ness of another class of exemplars to be made (Bruce et al 1991; Ferrara 199 1).
Valentine and
Of course, abstractions of this nature also allow predictions to be
made about the appearance and veracity of novel items belonging to one or another phenomenal category. It is worth noting that Galton ( 1879) had contemplated the biological implications of his technique.
W ithout the evidence of experimental psychology which was to
come a century later he believed that, like his composite photographs, human memory for categories of faces must also be blurred. He argued that even if people were able to form some kind of facial prototype it would be ill-defined far in excess of his portraits, owing to the sheer number of acquaintances involved. He reasoned that the "human mind is therefore a most
imperfect
apparatus for the elaboration of general
ideas" and "the criterion of a perfect mind would lie in its capacity of always creating images of a truly generic nature ... " (Galton 1879, page 169).
This argument is in
appropriate since it is based on the assumption that the brain works in a way analogous to the photographic technology of the Victorian era and therefore would be limited by the blurring of successive images. Presumably the reason for the brain 'wanting' to derive such generic descriptions is to be able to make decisions based upon perceived normality or canonicalness. Any abstraction of facial information to provide descriptions of typicality or normality must make explicit information about distinctiveness in order to discriminate between individuals, but also to capture the subtleties of age, gender, attractiveness, ethnicism, health, etc. In the interests of efficiency and breadth of knowledge we should not be
Faces in Perception [1243] 262
P J Benson, D I Perrett
restricted to one concept of average structure and dimension;
rather we should
assume reference to a variety of norms and dimensions along which to judge faces (Valentine and Endo
1992).
The ability to deform and normalise face shapes may play an important role in the development of telecommunication procedures for transmitting many facial images of an individual at reduced bandwidth (see also Aizawa et al
1989).
Prototypical images
created in the manner described here could also be used in studies of the perception of facial attribute information-for example in the study of attractiveness and the characteristic dimensions of age, gender, identity, and expression of particular types of faces. Acknowledgements.
T his work was supported by an award from the Science and Engineering Research Council ( GR/F 97591) . We thank T he Guardian, Rex Features, T he Daily Express, and The Dail Mail for allowing us to publish the original photographs of Mrs T hatcher.
References Aizawa K, Harashima H,
Saito T, 1989 "Model-based analysis synthesis image coding ( MBASIC) system for a person's face" Signal Processing: Image Communication 1139-152 Benson PJ, Perrett D I, 1991a "Computer averaging and manipulation of faces" in Photovideo Ed. P Wombell ( London: Rivers Oram Press ) Benson PJ, Perrett D I, 1991b "Perception and recognition of photographic quality facial caricatures: implications for the recognition of natural images" European Journal of Cognitive Psychology 3 105-135 Benson PJ, Perrett D I, 1991c "Synthesising continuous-tone caricatures" Image and Vision Computing9 123-129 Benson PJ, Perrett D I, 1992 "Face to face with the perfect image" New Scientist ( 1809, 22 February ) 32-35 Bruce V, Doyle T, Dench N, Burton M, 1991 "Remembering facial configurations" Cognition 38 109-144 Carey S, Diamond R, 1977 "From piecemeal to configurational representation of faces" Science 195 312-327 F letcher A C, 1886 "Composite portraits of American Indians" Science 7 408-409 Galton F, 1878a "Composite portraits" Journal of the Anthropological Institute of Great Britain and Ireland8 132-142 Galton F, 1878b "Composite portraits" Nature (London) 18 97-100 Galton F, 1879 "Generic images" Proceedings of the Royal Institution9 161-170 Galton F, 1888 Inquiries into Human Faculty and its Development ( London: J M Dent and Co) Hintzman D L, 1986 '"Schema abstraction' in a multiple-trace memory model" Psychological Review93 411-428 JastrowJ, 1885 "Composite portraiture" Science 6 165-168 LangloisJ H, Roggman L A, 1990 "Attractive faces are only average" Psychological Science 1 115-121 LavaterJ C, 1780 Essays on Physiognomy ( London: Ward, Lock and Co ) McClellandJ L, Rumelhart DE, 1985 "Distributed memory and representations of general and specific information" Journal of Experimental Psychology: General114 159-188 RoschE, 1978 "Principles of categorisation" in Cognition and Categorisation EdsE Rosch, B B Lloyd ( Hillsdale, NJ: LawrenceErlbaum Associates) pp 27-48 Stoddard J T, 1886 "Composite portraiture" Science 8 89-91 Stoddard J T, 1887 "Composite photography" Century 33 750-757 Taylor W C, 1885 "Three new portraits of Washington" Science 6 facing page 528 Tversky A, 1977 "Features of similarity" Psychological Review 84 327-352 Valentine T, Endo M, 1992 "Towards an exemplar model of face processing: the effects of race and distinctiveness" Quarterly Journal of Experimental Psychology A 44 671-703 Valentine T, Ferrara A, 1991 "Typicality in categorisation, recognition and identification: evidence from face recognition" British Journal of Psychology82 87-102 Young A W, Hellawell D, Hay D C, 1987 "Configurational information in face perception" Perception 16 747-759
p
© 1993 a Pion publication printed in Great Britain
Extracting prototypical facial images from exemplars
Philip J Benson'll, David I Perrett Department of Psychology, University of St Andrews, St Andrews, Fife KY1 6 9JU, UK Received 16 October 1 991 , in revised form 1 2 June 1992
Abstract. A computer graphic method for extracting a natural image of an individual's facial prototype, or average appearance, from a number of different images of that individual is presented. The process improves upon previous photographic and computational techniques. Synthesis of a person's average expression and pose from a sample of images is derived in an automatic and quantitative way.
Possible uses of composite faces produced in this manner in
psychological investigations of facial qualities (eg attractiveness) and in applied areas such as telecommunication are pointed out.
1 Introduction
The work of Galton
( 1878a,
1878b,
1879 ) , Taylor
( 1885 ),
Fletcher
( 1886 ),
and
Stoddard ( 1886, 1887 ) on producing composite photographic portraits stimulated much
interest in their day. Although their techniques were relatively primitive by today's standards the results were both remarkable and intriguing: Figure 1 illustrates a com posite of forty nine female students, and is striking because it looks like a real person.
Figure 1. Galton's (1878a) photographic composites were made by multiple exposures of a number of individual pictures scaled such that features were aligned as well as possible to minimise feature blurring. This figure shows a photographic composite portrait of forty nine female undergraduates of Smith College produced by Stoddard principles.
( 1886) with the
use of Galton's
Careful selection "of photographs of several persons alike in most respects, but
differing in minor details" (Galton 1878a, page 132) ensured that such large numbers of pictures could be combined reasonably effectively.
11 Address for correspondence: OXl 3PT, UK.
University Laboratory of P hysiology, Parks Road, Oxford
Faces in Perception [1239] P J Benson, D I Perrett
258
Galton's ( 1878b, 1888) reason for pursuing these 'trials' was to discover if it was possible to extract the typical characteristics of a number of faces from particular classes and types of people (eg criminals, the healthy and ill, and familial resemblances). Whereas such early composites of many people were a "portrait of a type [of person] and not of an individual" (Galton 1878a, page 133), blends of many instances of one person obtained by the same techniques, are. In his later work Galton ( 1879) concen trated on locating consistencies in the appearances of famous individuals.
It was
hoped that by combining several portraits of the same person produced by different artists a truer depiction of that person would result, by reducing stylistic and erro neous idiosyncrasies. Antiochus,
Composite portraits were fashioned of Alexander the Great,
Poliorcetes,
Cleopatra,
and
Nero.
Taylor
( 188 5)
similarly produced
composite images of George Washington in several perspective views by combining original images in the same pose. It was noticed from the outset that more often than not composites appeared younger and more attractive than constituents. For example, Galton ( 1878a, page 137) cited Austin's letter to Darwin claiming that in the case of some ladies' portraits the composite represented "a decided improvement in beauty". Jastrow ( 1885, page 168) noted that a composite of male faces made "a younger and handsomer man than the average of those whose faces enter into it". Perhaps with a little prejudice Galton noted of his blended Cleopatra that "the composite is as usual better looking than any of the components, none of which, however, give any indica tion of her reputed beauty; in fact, her features are not only plain, but to an ordinary English taste are simply hideous" (Galton 1879, page 164).
The increase in attrac
tiveness has been attributed to the average shape of the composite (Langlois and Roggman
1990) and a flatness or smoothing of skin tones (Benson and Perrett
1991a). These works highlighted a number of important issues which are relevant to current psychological studies of the perception and recognition of faces: faces as a homo geneous class of objects, and memory for particular familiar faces (Young et al 1987). Faces within the same cultural groups are highly similar in appearance. Individuating differences are on the whole only slight, yet our specialised perceptual mechanisms are able to identify these minute characteristics.
Certain differences may even be
difficult to verbalise. One face is seen from a great many perspectives and is continually undergoing elastic transformations during expression and speech. Our knowledge of the appearance of a familiar or famous face increases over time through experience of multiple encounters with that person, and allows recognition to become more tolerant of unusual viewing circumstances; the witness of an individual's face can be related to the essential and most useful components of their visage which underly potentially distracting and unexpected changes in pose, expression, feature contortions, or apparel. Although we are aware that abstractive mechanisms operate within the visual system, the psychological bases of this ability are unknown.
Different models have
been developed which simulate the effects of the generalisation process. parallel
distributed
processing
Rumelhart ( 1985 ) explicitly
(PDP )
models
extract prototypes,
such
as
that
Of these,
of McClelland
and
given multiple instances bearing
common features; Hintzman's ( 1986) model of schema abstraction does not explicitly code the abstraction, yet it is able to effect generalisation if so required. In this paper we also present an abstraction technique which operates over multiple images of given faces to synthesise an average or prototypical appearance.
Unlike the models
described above, the technique we describe here does not attempt to mimic cognitive processing but does serve to draw analogies insofar as the numerical procedure is able to locate consistent features and to dispense with miscellaneous details.
Faces in Perception [1240] 259
Extracting facial prototypes
Recognition also requires that a face be differentiated from others (Benson and Perrett 199 1b). This differentiation is not solely conducted at the level of individual features because faces are seen as a whole, processed in an holistic manner in which feature configuration is important (Carey and Diamond 1977). The failure of physi ognomists such as Lavater ( 1780) to notice this propagated many misleading ideas about social classes, types of person, and personality traits. The composite process we describe here abstracts not just the average shape of the features of a person or group, but also the average configuration of those features. 2 Methods
Careful selection of original photographs to ensure facial similarity is not sufficient to produce sharply defined composites.
For example, some faces will have wider or
narrower noses than others, which will cause ghosting of the feature in the final blend. Successful averaging of faces depends on the dimensions of the originals. A second difficulty lies in the multiple-exposure timing used by Galton and his followers. (t) Langlois and Roggman (1990) more recently attempted to improve upon this technique by digitally merging captured images of faces; as before, original portraits were scaled such that the pupils were as accurately aligned as possible and corresponding picture elements (pixels) numerically averaged to produce a high-quality composite.
The
results are an improvement upon the photographic technique, but still suffer from a lack of qualitative control. No facial features, not even the pupils, are aligned in any precise way since only two control points covering the estimated pupil centres exist. The computer composites produced by averaging corresponding pixel intensities in each facial image 'frame' produces blurring in the same way the original photographic portraits did (see figure 2b). Thus an improvement in processing with a decrease in blurring is desirable because an average face should, after all, be one which looks like
·
a face in every way and has well-defined characteristics. 2. 1
Computational procedure
We have previously described a process to distort facial images by computer in a systematic and controlled way (Benson and Perrett 199 1b, 199 1c).
Here we have
applied similar procedures to derive a highly realistic composite of one individual from a number of images. The same procedure can be applied to images of different people to produce a composite of a particular category of individual. On each of six digitised images of Margaret Thatcher 186 points describing the positions of her facial features were manually logged.
Identical numbers of points
described particular features and therefore a relationship between each of the original 'portraits' in different poses.
By averaging the x- y coordinates of matched feature
points across each image, the mean position of each feature can be found. The differ ence between the original and the average feature positions (and hence configuration) defines by how much Thatcher's face would have to change in each case in order to assume her 'normal' visage. That is, the difference in positions of the original feature points with respect to their average defines how each image of Thatcher deviates from (I) When producing composite faces, Galton exposed each original face for the same time to the new photographic plate ( Galton 1878a, page 133 ) . At very long ( and very short ) exposures the reciprocal relationship between settings of lens aperature and exposure time breaks down. When dealing with long
( multiple )
exposures Galton's composites would almost certainly have
been limited by such reciprocity failure; this, coupled with the inferior photographic plate chemistry, explains why many of the composites made by himself and others looked 'better' than they should have, and why some attempted replications failed. Procedural problems involving changes in lighting conditions and first or last exposure bias in the photochemical process can also account for the observation by Fletcher nant in a composite.
( 1886 )
that one individual's face appeared domi
·
Faces in Perception [1241] P J Benson, D I Perrett
260
a prototypical Thatcher configuration. W ith the feature markers employed as reference points, each of the six images was 'warped' ( Benson and Perrett 199lc ) into the shape
defined by the averaged coordinates such that each assumed an identical average facial shape. F igure 2c was produced by averaging each corresponding pixel intensity
(c)
(b)
(a)
Figure 2. Upper two rows: Original photographs of Margaret Thatcher. Bottom row:
( a ) The
composite of all six original unnormalised images; the image appears as an unrecognisable blur.
( b) A
composite
( produced
digitally
}
according to the procedures defined by Galton; three of
the images have been rotated and magnified to align the pupils horizontally, and two images were mirrored vertically to change the left-right pose.
(
( c ) The )
blended computer composite.
The automatic process of generating the composite see text deals with differences in expression and pose by normalising the original images before blending. Here we deliberately chose base images which varied a great deal in a number of ways to determine the power of the technique.
Faces in Perception [1242] 261
Extracting facial prototypes
in the six distorted images.
The blend is well-focused though 'soft' in contrast, and
maintains various features such as drooping eyelids, hairline, and narrow nose. Blending all six images together without intermediary transformation produces an almost unrecognisable composite with extreme ghosting of features (eg six mouths; figure 2a). Figure 2b shows a composite made according to Galton's method. 3 Discussion
The process we have described in this paper does in fact produce an accurate and faithful generic face when averaging different faces.
We do not intend the graphic
process to be seen as an explanation of psychological processes underlying recogni tion. Rather, the technique demonstrates that salient visually distinctive features can be extracted automatically from disparate images. Facial shape and feature configura tion are maintained over large samples and only a pervasive blandness of skin textures is introduced. Although the process is restricted to images of faces in preprofile view it does provide a reliable means of normalisation. Whatever photographic or computational process is used to create facial composites, it is important to bear in mind a fundamental phenomenon being procured-that of extraction of averages or prototypes from a set of exemplars. Formation of category prototypes has been shown to be useful for recognising objects at the psychological level (Tversky 1977; Rosch 1978) by providing a level of abstraction which seeks to maximise the amount of information about a category with the least cognitive effort. In a sense the averaging process we present here also maximises the information about the category to which the face belongs; a normalised appearance is one which may be more likely to display perceptual similarity to the target individual and conse quently evoke recognition, more so than perhaps an unusual 'caught unaware' snapshot would.
It remains to be seen whether the average appearance of an individual is a
better likeness than a caricature for example (Benson and Perrett 1991b, 1992), or even whether a caricature of the computed prototype is in turn better still. Prototypes are not only useful for determining whether new stimuli belong to a familiar category but they also enable reliable judgements of the relative distinctive ness of another class of exemplars to be made (Bruce et al 1991; Ferrara 199 1).
Valentine and
Of course, abstractions of this nature also allow predictions to be
made about the appearance and veracity of novel items belonging to one or another phenomenal category. It is worth noting that Galton ( 1879) had contemplated the biological implications of his technique.
W ithout the evidence of experimental psychology which was to
come a century later he believed that, like his composite photographs, human memory for categories of faces must also be blurred. He argued that even if people were able to form some kind of facial prototype it would be ill-defined far in excess of his portraits, owing to the sheer number of acquaintances involved. He reasoned that the "human mind is therefore a most
imperfect
apparatus for the elaboration of general
ideas" and "the criterion of a perfect mind would lie in its capacity of always creating images of a truly generic nature ... " (Galton 1879, page 169).
This argument is in
appropriate since it is based on the assumption that the brain works in a way analogous to the photographic technology of the Victorian era and therefore would be limited by the blurring of successive images. Presumably the reason for the brain 'wanting' to derive such generic descriptions is to be able to make decisions based upon perceived normality or canonicalness. Any abstraction of facial information to provide descriptions of typicality or normality must make explicit information about distinctiveness in order to discriminate between individuals, but also to capture the subtleties of age, gender, attractiveness, ethnicism, health, etc. In the interests of efficiency and breadth of knowledge we should not be
Faces in Perception [1243] 262
P J Benson, D I Perrett
restricted to one concept of average structure and dimension;
rather we should
assume reference to a variety of norms and dimensions along which to judge faces (Valentine and Endo
1992).
The ability to deform and normalise face shapes may play an important role in the development of telecommunication procedures for transmitting many facial images of an individual at reduced bandwidth (see also Aizawa et al
1989).
Prototypical images
created in the manner described here could also be used in studies of the perception of facial attribute information-for example in the study of attractiveness and the characteristic dimensions of age, gender, identity, and expression of particular types of faces. Acknowledgements.
T his work was supported by an award from the Science and Engineering Research Council ( GR/F 97591) . We thank T he Guardian, Rex Features, T he Daily Express, and The Dail Mail for allowing us to publish the original photographs of Mrs T hatcher.
References Aizawa K, Harashima H,
Saito T, 1989 "Model-based analysis synthesis image coding ( MBASIC) system for a person's face" Signal Processing: Image Communication 1139-152 Benson PJ, Perrett D I, 1991a "Computer averaging and manipulation of faces" in Photovideo Ed. P Wombell ( London: Rivers Oram Press ) Benson PJ, Perrett D I, 1991b "Perception and recognition of photographic quality facial caricatures: implications for the recognition of natural images" European Journal of Cognitive Psychology 3 105-135 Benson PJ, Perrett D I, 1991c "Synthesising continuous-tone caricatures" Image and Vision Computing9 123-129 Benson PJ, Perrett D I, 1992 "Face to face with the perfect image" New Scientist ( 1809, 22 February ) 32-35 Bruce V, Doyle T, Dench N, Burton M, 1991 "Remembering facial configurations" Cognition 38 109-144 Carey S, Diamond R, 1977 "From piecemeal to configurational representation of faces" Science 195 312-327 F letcher A C, 1886 "Composite portraits of American Indians" Science 7 408-409 Galton F, 1878a "Composite portraits" Journal of the Anthropological Institute of Great Britain and Ireland8 132-142 Galton F, 1878b "Composite portraits" Nature (London) 18 97-100 Galton F, 1879 "Generic images" Proceedings of the Royal Institution9 161-170 Galton F, 1888 Inquiries into Human Faculty and its Development ( London: J M Dent and Co) Hintzman D L, 1986 '"Schema abstraction' in a multiple-trace memory model" Psychological Review93 411-428 JastrowJ, 1885 "Composite portraiture" Science 6 165-168 LangloisJ H, Roggman L A, 1990 "Attractive faces are only average" Psychological Science 1 115-121 LavaterJ C, 1780 Essays on Physiognomy ( London: Ward, Lock and Co ) McClellandJ L, Rumelhart DE, 1985 "Distributed memory and representations of general and specific information" Journal of Experimental Psychology: General114 159-188 RoschE, 1978 "Principles of categorisation" in Cognition and Categorisation EdsE Rosch, B B Lloyd ( Hillsdale, NJ: LawrenceErlbaum Associates) pp 27-48 Stoddard J T, 1886 "Composite portraiture" Science 8 89-91 Stoddard J T, 1887 "Composite photography" Century 33 750-757 Taylor W C, 1885 "Three new portraits of Washington" Science 6 facing page 528 Tversky A, 1977 "Features of similarity" Psychological Review 84 327-352 Valentine T, Endo M, 1992 "Towards an exemplar model of face processing: the effects of race and distinctiveness" Quarterly Journal of Experimental Psychology A 44 671-703 Valentine T, Ferrara A, 1991 "Typicality in categorisation, recognition and identification: evidence from face recognition" British Journal of Psychology82 87-102 Young A W, Hellawell D, Hay D C, 1987 "Configurational information in face perception" Perception 16 747-759
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