Ambiguous image

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This figure can be seen as a young woman or an old woman; see My Wife and My Mother-in-Law. My Wife and My Mother-In-Law (Hill).svg
This figure can be seen as a young woman or an old woman; see My Wife and My Mother-in-Law.
Rubin's vase utilizes the concept of Negative space to create ambiguous images: the vase or two opposing faces. Face or vase ata 01.svg
Rubin's vase utilizes the concept of Negative space to create ambiguous images: the vase or two opposing faces.

Ambiguous images or reversible figures are visual forms that create ambiguity by exploiting graphical similarities and other properties of visual system interpretation between two or more distinct image forms. These are famous for inducing the phenomenon of multistable perception. Multistable perception is the occurrence of an image being able to provide multiple, although stable, perceptions.

Contents

One of the earliest examples of this type is the rabbit–duck illusion, first published in Fliegende Blätter, a German humor magazine. [1] Other classic examples are the Rubin vase, [2] and the "My Wife and My Mother-in-Law" drawing, the latter dating from a German postcard of 1888.

Ambiguous images are important to the field of psychology because they are often research tools used in experiments. [3] There is varying evidence on whether ambiguous images can be represented mentally, [4] but a majority of research has theorized that mental images cannot be ambiguous. [5]

Identifying and resolving ambiguous images

The rabbit-duck illusion Duck-Rabbit illusion.jpg
The rabbit–duck illusion

Middle vision is the stage in visual processing that combines all the basic features in the scene into distinct, recognizable object groups. This stage of vision comes before high-level vision (understanding the scene) and after early vision (determining the basic features of an image). When perceiving and recognizing images, mid-level vision comes into use when we need to classify the object we are seeing quickly. Whether perceived or actual, Negative space will play a role here.

Higher-level vision is used when the object classified must now be recognized as a specific member of its group. For example, through mid-level vision we perceive a face, then through high-level vision we recognize a face of a familiar person. Mid-level vision and high-level vision are crucial for understanding a reality that is filled with ambiguous perceptual inputs. [6]

Perceiving the image in mid-level vision

Rare example of an ambiguous image that can be interpreted in more than two ways: as the letters "KB," the mathematical inequality "1 < 13" or the letters "VD" with their mirror image. KB ambiguous image.png
Rare example of an ambiguous image that can be interpreted in more than two ways: as the letters "KB," the mathematical inequality "1 < 13" or the letters "VD" with their mirror image.

When we see an image, the first thing we do is attempt to organize all the parts of the scene into different groups. [8] To do this, one of the most basic methods used is finding the edges. Edges can include obvious perceptions such as the edge of a house, and can include other perceptions that the brain needs to process deeper, such as the edges of a person's facial features. When finding edges, the brain's visual system detects a point on the image with a sharp contrast of lighting. Being able to detect the location of the edge of an object aids in recognizing the object. In ambiguous images, detecting edges still seems natural to the person perceiving the image. However, the brain undergoes deeper processing to resolve the ambiguity. For example, consider an image that involves an opposite change in magnitude of luminance between the object and the background (e.g. From the top, the background shifts from black to white, and the object shifts from white to black). The opposing gradients will eventually come to a point where there is an equal degree of luminance of the object and the background. At this point, there is no edge to be perceived. To counter this, the visual system connects the image as a whole rather than a set of edges, allowing one to see an object rather than edges and non-edges. Although there is no complete image to be seen, the brain is able to accomplish this because of its understanding of the physical world and real incidents of ambiguous lighting. [6]

In ambiguous images, an illusion is often produced from illusory contours. An illusory contour is a perceived contour without the presence of a physical gradient. In examples where a white shape appears to occlude black objects on a white background, the white shape appears to be brighter than the background, and the edges of this shape produce the illusory contours. [9] These illusory contours are processed by the brain in a similar way as real contours. [8] The visual system accomplishes this by making inferences beyond the information that is presented in much the same way as the luminance gradient.

Gestalt grouping rules

"Kanizsa Triangle". These spatially separate fragments give the impression of illusory contours (also known as modal completion) of a triangle. Kanizsa triangle.svg
"Kanizsa Triangle". These spatially separate fragments give the impression of illusory contours (also known as modal completion) of a triangle.

In mid-level vision, the visual system utilizes a set of heuristic methods, called Gestalt grouping rules, to quickly identify a basic perception of an object that helps to resolve an ambiguity. [3] This allows perception to be fast and easy by observing patterns and familiar images rather than a slow process of identifying each part of a group. This aids in resolving ambiguous images because the visual system will accept small variations in the pattern and still perceive the pattern as a whole. The Gestalt grouping rules are the result of the experience of the visual system. Once a pattern is perceived frequently, it is stored in memory and can be perceived again easily without the requirement of examining the entire object again. [6] For example, when looking at a chess board, we perceive a checker pattern and not a set of alternating black and white squares.

Good continuation

The principle of good continuation provides the visual system a basis for identifying continuing edges. This means that when a set of lines is perceived, there is a tendency for a line to continue in one direction. This allows the visual system to identify the edges of a complex image by identifying points where lines cross. For example, two lines crossed in an "X" shape will be perceived as two lines travelling diagonally rather than two lines changing direction to form "V" shapes opposite to each other. An example of an ambiguous image would be two curving lines intersecting at a point. This junction would be perceived the same way as the "X", where the intersection is seen as the lines crossing rather than turning away from each other. Illusions of good continuation are often used by magicians to trick audiences. [10]

Similarity

The rule of similarity states that images that are similar to each other can be grouped together as being the same type of object or part of the same object. Therefore, the more similar two images or objects are, the more likely it will be that they can be grouped together. For example, two squares among many circles will be grouped together. They can vary in similarity of colour, size, orientation and other properties, but will ultimately be grouped together with varying degrees of membership. [6]

Proximity, common region, and connectedness

Law of Proximity Gestalt proximity.svg
Law of Proximity

The grouping property of proximity (Gestalt) is the spatial distance between two objects. The closer two objects are, the more likely they belong to the same group. This perception can be ambiguous without the person perceiving it as ambiguous. For example, two objects with varying distances and orientations from the viewer may appear to be proximal to each other, while a third object may be closer to one of the other objects but appear farther.

Objects occupying a common region on the image appear to already be members of the same group. This can include unique spatial location, such as two objects occupying a distinct region of space outside of their group's own. Objects can have close proximity but appear as though part of a distinct group through various visual aids such as a threshold of colours separating the two objects.

Additionally, objects can be visually connected in ways such as drawing a line going from each object. These similar but hierarchical rules suggest that some Gestalt rules can override other rules. [6]

Texture segmentation and figure-ground assignments

The visual system can also aid itself in resolving ambiguities by detecting the pattern of texture in an image. This is accomplished by using many of the Gestalt principles. The texture can provide information that helps to distinguish whole objects, and the changing texture in an image reveals which distinct objects may be part of the same group. Texture segmentation rules often both cooperate and compete with each other, and examining the texture can yield information about the layers of the image, disambiguating the background, foreground, and the object. [11]

Size and surroundedness

When a region of texture completely surrounds another region of texture, it is likely the background. Additionally, the smaller regions of texture in an image are likely the figure. [6]

Parallelism and symmetry

Parallelism is another way to disambiguate the figure of an image. The orientation of the contours of different textures in an image can determine which objects are grouped together. Generally, parallel contours suggest membership to the same object or group of objects. Similarly, symmetry of the contours can also define the figure of an image. [6]

Extremal edges and relative motion

Schroeder's stairs Schroeder's stairs.svg
Schroeder's stairs

An extremal edge is a change in texture that suggests an object is in front of or behind another object. This can be due to a shading effect on the edges of one region of texture, giving the appearance of depth. Some extremal edge effects can overwhelm the segmentations of surroundedness or size. The edges perceived can also aid in distinguishing objects by examining the change in texture against an edge due to motion. [6]

Using ambiguous images to hide in the real world: camouflage

In nature, camouflage is used by organisms to escape predators. This is achieved through creating an ambiguity of texture segmentation by imitating the surrounding environment. Without being able to perceive noticeable differences in texture and position, a predator will be unable to see their prey. [6]

Occlusion

Many ambiguous images are produced through some occlusion, wherein an object's texture suddenly stops. An occlusion is the visual perception of one object being behind or in front of another object, providing information about the order of the layers of texture. [6] The illusion of occlusion is apparent in the effect of illusory contours, where occlusion is perceived despite being non-existent. Here, an ambiguous image is perceived to be an instance of occlusion. When an object is occluded, the visual system only has information about the parts of the object that can be seen, so the rest of the processing must be done deeper and must involve memory.

Accidental viewpoints

An accidental viewpoint is a single visual position that produces an ambiguous image. The accidental viewpoint does not provide enough information to distinguish what the object is. [12] Often, this image is perceived incorrectly and produces an illusion that differs from reality. For example, an image may be split in half, with the top half being enlarged and placed further away from the perceiver in space. This image will be perceived as one complete image from only a single viewpoint in space, rather than the reality of two separate halves of an object, creating an optical illusion. Street artists often use tricks of point-of-view to create two-dimensional scenes on the ground that appear three-dimensional.

Recognizing an object through high-level vision

The Necker Cube: a wire frame cube with no depth cues. Necker cube.svg
The Necker Cube: a wire frame cube with no depth cues.

Figures drawn in a way that avoids depth cues may become ambiguous. Classic examples of this phenomenon are the Necker cube, [6] and the rhombille tiling (viewed as an isometric drawing of cubes).

To go further than just perceiving the object is to recognize the object. Recognizing an object plays a crucial role in resolving ambiguous images, and relies heavily on memory and prior knowledge. To recognize an object, the visual system detects familiar components of it, and compares the perceptual representation of it with a representation of the object stored in memory. [8] This can be done using various templates of an object, such as "dog" to represent dogs in general. The template method is not always successful because members of a group may significantly differ visually from each other, and may look much different if viewed from different angles. To counter the problem of viewpoint, the visual system detects familiar components of an object in 3-dimensional space. If the components of an object perceived are in the same position and orientation of an object in memory, recognition is possible. [6] Research has shown that people that are more creative in their imagery are better able to resolve ambiguous images. This may be due to their ability to quickly identify patterns in the image. [13] When making a mental representation of an ambiguous image, in the same way as normal images, each part is defined and then put onto the mental representation. The more complex the scene is, the longer it takes to process and add to the representation. [14]

Using memory and recent experience

Our memory has a large impact on resolving an ambiguous image, as it helps the visual system to identify and recognize objects without having to analyze and categorize them repeatedly. Without memory and prior knowledge, an image with several groups of similar objects will be difficult to perceive. Any object can have an ambiguous representation and can be mistakenly categorized into the wrong groups without sufficient memory recognition of an object. This finding suggests that prior experience is necessary for proper perception. [15] Studies have been done with the use of Greebles to show the role of memory in object recognition. [6] The act of priming the participant with an exposure to a similar visual stimulus also has a large effect on the ease of resolving an ambiguity. [15]

Verbeek's strips could be seen differently when viewed upside down. This image will flip upside-down automatically. Verbeek-rocanoe.gif
Verbeek's strips could be seen differently when viewed upside down. This image will flip upside-down automatically.

Disorders in perception

Prosopagnosia is a disorder that causes a person to be unable to identify faces. The visual system undergoes mid-level vision and identifies a face, but high-level vision fails to identify who the face belongs to. In this case, the visual system identifies an ambiguous object, a face, but is unable to resolve the ambiguity using memory, leaving the affected unable to determine who they are seeing. [6]

In media

From 1903 to 1905 Gustave Verbeek wrote his comic series The Upside Downs of Little Lady Lovekins and Old Man Muffaroo. These comics were made in such a way that one could read the 6-panel comic, flip the book and keep reading. He made 64 such comics in total. In 2012 a remake of a selection of the comics was made by Marcus Ivarsson in the book 'In Uppåner med Lilla Lisen & Gamle Muppen'. ( ISBN   978-91-7089-524-1)

The use of the ambiguous image phenomena can be seen in select works of M.C. Escher and Salvador Dalí. The children's book, Round Trip, by Ann Jonas used ambiguous images in the illustrations, where the reader could read the book front to back normally at first, and then flip it upside down to continue the story and see the pictures in a new perspective. [16]

See also

Related Research Articles

<span class="mw-page-title-main">Perception</span> Interpretation of sensory information

Perception is the organization, identification, and interpretation of sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous system, which in turn result from physical or chemical stimulation of the sensory system. Vision involves light striking the retina of the eye; smell is mediated by odor molecules; and hearing involves pressure waves.

An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people.

<span class="mw-page-title-main">Optical illusion</span> Visually perceived images that differ from objective reality

In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.

<span class="mw-page-title-main">Gestalt psychology</span> Theory of perception

Gestalt psychology, gestaltism, or configurationism is a school of psychology that emerged in the early twentieth century in Austria and Germany as a theory of perception that was a rejection of basic principles of Wilhelm Wundt's and Edward Titchener's elementalist and structuralist psychology.

<span class="mw-page-title-main">Figure–ground (perception)</span>

Figure–ground organization is a type of perceptual grouping that is a vital necessity for recognizing objects through vision. In Gestalt psychology it is known as identifying a figure from the background. For example, black words on a printed paper are seen as the "figure", and the white sheet as the "background".

<span class="mw-page-title-main">Ehrenstein illusion</span> Optical illusion

The Ehrenstein illusion is an optical illusion of brightness or colour perception. The visual phenomena was studied by the German psychologist Walter H. Ehrenstein (1899–1961) who originally wanted to modify the theory behind the Hermann grid illusion. In the discovery of the optical illusion, Ehrenstein found that grating patterns of straight lines that stop at a certain point appear to have a brighter centre, compared to the background.

<span class="mw-page-title-main">Visual language</span> System of communication using visual elements

A visual language is a system of communication using visual elements. Speech as a means of communication cannot strictly be separated from the whole of human communicative activity which includes the visual and the term 'language' in relation to vision is an extension of its use to describe the perception, comprehension and production of visible signs.

Geons are the simple 2D or 3D forms such as cylinders, bricks, wedges, cones, circles and rectangles corresponding to the simple parts of an object in Biederman's recognition-by-components theory. The theory proposes that the visual input is matched against structural representations of objects in the brain. These structural representations consist of geons and their relations. Only a modest number of geons are assumed. When combined in different relations to each other and coarse metric variation such as aspect ratio and 2D orientation, billions of possible 2- and 3-geon objects can be generated. Two classes of shape-based visual identification that are not done through geon representations, are those involved in: a) distinguishing between similar faces, and b) classifications that don’t have definite boundaries, such as that of bushes or a crumpled garment. Typically, such identifications are not viewpoint-invariant.

<span class="mw-page-title-main">Rubin vase</span>

Rubin's vase is a famous set of ambiguous or bi-stable two-dimensional forms developed around 1915 by the Danish psychologist Edgar Rubin.

<span class="mw-page-title-main">Illusory contours</span> Visual illusions

Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann is often credited with the discovery of illusory contours around the beginning of the 20th century, but they are present in art dating to the Middle Ages. Gaetano Kanizsa’s 1976 Scientific American paper marked the resurgence of interest in illusory contours for vision scientists.

<span class="mw-page-title-main">Filling-in</span>

In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis. Classical demonstrations of perceptual filling-in involve filling in at the blind spot in monocular vision, and images stabilized on the retina either by means of special lenses, or under certain conditions of steady fixation. For example, naturally in monocular vision at the physiological blind spot, the percept is not a hole in the visual field, but the content is “filled-in” based on information from the surrounding visual field. When a textured stimulus is presented centered on but extending beyond the region of the blind spot, a continuous texture is perceived. This partially inferred percept is paradoxically considered more reliable than a percept based on external input..

<span class="mw-page-title-main">Chubb illusion</span> Optical illusion

The Chubb illusion is an optical illusion or error in visual perception in which the apparent contrast of an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed. These visual illusions are of particular interest to researchers because they may provide valuable insights in regard to the workings of human visual systems.

<span class="mw-page-title-main">Visual perception</span> Ability to interpret the surrounding environment using light in the visible spectrum

Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.

<span class="mw-page-title-main">Watercolor illusion</span> Optical illusion in which a white area takes on a pale tint

The watercolor illusion, also referred to as the water-color effect, is an optical illusion in which a white area takes on a pale tint of a thin, bright, intensely colored polygon surrounding it if the coloured polygon is itself surrounded by a thin, darker border. The inner and outer borders of watercolor illusion objects often are of complementary colours. The watercolor illusion is best when the inner and outer contours have chromaticities in opposite directions in color space. The most common complementary pair is orange and purple. The watercolor illusion is dependent on the combination of luminance and color contrast of the contour lines in order to have the color spreading effect occur.

<span class="mw-page-title-main">Neon color spreading</span> Optical illusion

Neon color spreading is an optical illusion in the category of transparency effects, characterized by fluid borders between the edges of a colored object and the background in the presence of black lines. The illusion was first documented in 1971 and was eventually rediscovered in 1975 by Van Tuijl.

Geometrical-optical illusions are visual illusions, also optical illusions, in which the geometrical properties of what is seen differ from those of the corresponding objects in the visual field.

Form perception is the recognition of visual elements of objects, specifically those to do with shapes, patterns and previously identified important characteristics. An object is perceived by the retina as a two-dimensional image, but the image can vary for the same object in terms of the context with which it is viewed, the apparent size of the object, the angle from which it is viewed, how illuminated it is, as well as where it resides in the field of vision. Despite the fact that each instance of observing an object leads to a unique retinal response pattern, the visual processing in the brain is capable of recognizing these experiences as analogous, allowing invariant object recognition. Visual processing occurs in a hierarchy with the lowest levels recognizing lines and contours, and slightly higher levels performing tasks such as completing boundaries and recognizing contour combinations. The highest levels integrate the perceived information to recognize an entire object. Essentially object recognition is the ability to assign labels to objects in order to categorize and identify them, thus distinguishing one object from another. During visual processing information is not created, but rather reformatted in a way that draws out the most detailed information of the stimulus.

<span class="mw-page-title-main">Phantom contour</span>

A phantom contour is a type of illusory contour. Most illusory contours are seen in still images, such as the Kanizsa triangle and the Ehrenstein illusion. A phantom contour, however, is perceived in the presence of moving or flickering images with contrast reversal. The rapid, continuous alternation between opposing, but correlated, adjacent images creates the perception of a contour that is not physically present in the still images. Quaid et al. have also authored a PhD thesis on the phantom contour illusion and its spatiotemporal limits which maps out limits and proposes mechanisms for its perception centering around magnocellularly driven visual area MT.

<span class="mw-page-title-main">Amodal completion</span>

Amodal completion is the ability to see an entire object despite parts of it being covered by another object in front of it. It is one of the many functions of the visual system which aid in both seeing and understanding objects encountered on an everyday basis. This mechanism allows the world to be perceived as though it is made of coherent wholes. For example, when the sun sets over the horizon it is still perceived as a full circle, despite occlusion causing it to appear as a semi-circle. Another example of this is a cat behind a picket fence. Amodal completion allows the cats to be seen as a full animal continuing behind each picket of the fence. Essentially amodal completion allows for sensory stimulation from any parts of an occluded object we can not directly see.

<span class="mw-page-title-main">Accidental viewpoint</span> Ambiguous image or illusion

An accidental viewpoint is a singular position from which an image can be perceived, creating either an ambiguous image or an illusion. The image perceived at this angle is viewpoint-specific, meaning it cannot be perceived at any other position, known as generic or non-accidental viewpoints. These view-specific angles are involved in object recognition. In its uses in art and other visual illusions, the accidental viewpoint creates the perception of depth often on a two-dimensional surface with the assistance of monocular cues.

References

  1. ↑ Fliegende Blätter Oct. 23, 1892, p. 147
  2. ↑ Parkkonen, L.; Andersson, J.; Hämäläinen, M.; Hari, R. (2008). "Early visual brain areas reflect the percept of an ambiguous scene". Proceedings of the National Academy of Sciences of the United States of America. 105 (51): 20500–20504. Bibcode:2008PNAS..10520500P. doi: 10.1073/pnas.0810966105 . PMC   2602606 . PMID   19074267.
  3. 1 2 Wimmer, M.; Doherty, M. (2011). "The development of ambiguous figure perception: Vi. conception and perception of ambiguous figures". Monographs of the Society for Research in Child Development. 76 (1): 87–104. doi:10.1111/j.1540-5834.2011.00595.x.
  4. ↑ Mast, F.W.; Kosslyn, S.M. (2002). "Visual mental images can be ambiguous: Insights from individual differences in spatial transformation abilities". Cognition. 86 (1): 57–70. doi:10.1016/S0010-0277(02)00137-3. PMID   12208651. S2CID   37046301.
  5. ↑ Chambers, D.; Reisberg, D. (1985). "Can mental images be ambiguous?". Journal of Experimental Psychology: Human Perception and Performance. 11 (3): 317–328. doi:10.1037/0096-1523.11.3.317. S2CID   197655523.
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  7. ↑ Postic, Guillaume; Ghouzam, Yassine; Chebrek, Romain; Gelly, Jean-Christophe (2017). "An ambiguity principle for assigning protein structural domains". Science Advances. 3 (1): e1600552. Bibcode:2017SciA....3E0552P. doi:10.1126/sciadv.1600552. ISSN   2375-2548. PMC   5235333 . PMID   28097215.
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  9. ↑ Bradley, D.R.; Dumais, S.T. (1975). "Ambiguous cognitive contours". Nature. 257 (5527): 582–584. Bibcode:1975Natur.257..582B. doi:10.1038/257582a0. PMID   1165783. S2CID   4295897.
  10. ↑ Bamhart, A.S. (2010). "The exploitation of gestalt principles by magicians". Perception. 39 (9): 1286–1289. doi:10.1068/p6766. PMID   21125955. S2CID   8016846.
  11. ↑ Tang, Xiangyu (2005). A model for figure-ground segmentation by self-organized cue integration (Thesis). University of Southern California Digital Library (USC.DL). doi:10.25549/usctheses-c16-597264. ProQuest   621577763.
  12. ↑ Koning, A.; van Lier, R. (2006). "No symmetry advantage when object matching involves accidental viewpoints". Psychological Research. 70 (1): 52–58. doi:10.1007/s00426-004-0191-8. PMID   15480756. S2CID   35284032.
  13. ↑ Riquelme, H (2002). "Can people creative in imagery interpret ambiguous figures faster than people less creative in imagery?". The Journal of Creative Behavior. 36 (2): 105–116. doi:10.1002/j.2162-6057.2002.tb01059.x.
  14. ↑ Kosslyn, S.M.; Reiser, B.J.; Farah, M.J.; Fliegel, S.L. (1983). "Generating visual images: Units and relations". Journal of Experimental Psychology: General. 112 (2): 278–303. doi:10.1037/0096-3445.112.2.278. PMID   6223974.
  15. 1 2 Daelli, V.; van Rijsbergen, N.J.; Treves, A. (2010). "How recent experience affects the perception of ambiguous objects". Brain Research. 1322: 81–91. doi:10.1016/j.brainres.2010.01.060. PMID   20122901. S2CID   45388116.
  16. ↑ Higgins, Carter (29 May 2013). "Round Trip – Design Of The Picture Book". Design of the Picture Book. Retrieved 2 December 2020.
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