Chapter 10 Perceiving Depth and Size I Cue approach to depth perception what information is contained in this two dimensional image that enables us to perceive depth in a scene a The cue approach the depth perception focuses on identifying information in the retinal image that is correlated with depth in the scene ex if one object partially covers another object the object that is partially covered must be at a greater distance than the object covering it b This situation is called a occlusion a signal or cue that one object is in front of another according to the cue theory we learn the connection between this cue and depth through our experiences with the environment c Three major groups of cues that signal depth in a scene i Oculomotor cues based on our ability o sense the position of our eyes and the tension of our eye muscles ii Monocular cues that work with one eye iii Binocular cues that depend on two eyes II Oculomotor Cues a Oculomotor cues are created by 1 convergence the inward movement of the eyes that occur when we look at nearby objects and 2 accommodation the change in the shape of the lends that occurs when we focus on objects at various distances i The idea behind these cues is that we can feel the inward movemtn of the eyes that occurs when the eyes converge to look at nearby objects and we feel the tightening of eye muscles that change the shape of lens to focus on a nearby object III Monocular Cues a Monocular cues work with only one eye i They include accommodation pictorial cues depth information that can be depicted in a two dimensional picture and movement based cues based on depth information created by movement b Pictorial Cues in a picture i Pictorial cues sources of depth information that can be depicted ii Occlusion However this does not give us information on the object s absolute distance only the relative distance we know that the object behind another is farther away but not how much farther iii Relative Height objects that are below the horizon and have their bases higher in the field of view are usually seen as being more distant 1 When objects are above the horizon like the clouds being lower in the field of view indicates more distance 2 Looking straight out at an object high in the visual field near the horizon indicates greater depth than looking down as you would for a lower object in the visual field iv Relative Size when two objects are of equal size the one that is farther away will take up less of your field of view than the one that is closer v Perspective Convergence when parallel lines extend out from an observer they are perceived as converging becoming closer together as distance increases vi Familiar Size used to judge distance based on our prior knowledge of the sizes of objects 1 Ex Epstein showed equal sized photographs of a dime quarter and half dollar and positioned the same distance from the observer they looked at it with one eye RESULTS the observer judged the dome as closest quarter as farther than the dime and the half dollar was the farthest 2 This did NOT occur when viewing with two eyes vii Atmospheric perspective occurs when more distant objects appear less sharp and often have a slight blue tint the farther away an object is the more air and particles we have to look through making objects look less sharp and more blue viii Texture Gradient elements that are equally spaced in a scene appear to be more closely packed as distance increases ix Shadows can provide information regarding the locations of these objects above below also enhance the three dimensionality of objects looking spherical not round c Motion Produced Cues move more slowly i Motion Parallax occurs when as we move objects appear to 2 1 ex when you look out the side window of a moving train or care nearby objects appear to speed by in a blur but objects on the horizon may appear to be moving slowly looking at a closer object the object moves across the retina a longer distance so it appears to move rapidly as the observer moves the image of the far object ravels a much smaller distance across the retina so it appears to move more slowly as the observer moves ii Deletion and Accretion uncovering 3 1 As an observer moves sideways some things become covered and others become uncovered 2 Occur when overlapping surfaces appear to move relative to one another especially effective for detecting the differences in the depths of two surfaces IV Binocular Depth Information a Binocular Disparity i Binocular disparity the difference in the images in the left and right eyes ii When you switch from looking with your left eye to your right you probably notices that your finger appeared to move to the left relative to the far object this occurred because you were looking right at both objects so their images would fall on the foveas iii When the right eye was open the image of the far object still fell on the fovea because you were looking at it but the image of the finger was now off to the side iv The difference between the images in the left and right eyes creates binocular disparity to describe how it works we introduce corresponding retinal points the places on each retina that would overlap if one retina could be slid on top of the other v Horopter an imaginary surface that passes through the point of fixation and indicates the location of objects that fall on corresponding points on the two retinas vi Noncorresponding points none of the points vii the difference between where Carol s image falls on the right eye Cr and the corresponding point is called the angle of disparity in this ex is about 26 degrees is the absolute angle of disparity or simple the absolute disparity when the lifeguard is looking at Frieda 1 absolute disparity gives us information about the distances of objects the amount of absolute disparity indicates how for an object is from the horopter 2 one of the properties of absolute disparity is that it changes every time the observer changes where he or she is looking viii disparity information that remains the same no matter where the observer is looking is called relative disparity the difference between two object s absolute disparity 1 when looking at Frieda the absolute disparity of her was 0 and of Carole is 26 then when looking at Carole hers is 0 and Frieda s is 26 the difference still remains the same b Connecting Disparity Information and the Perception of Depth i So we have seen that both absolute and relative disparity information