depth & size perception
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what does the term ‘depth’ customarily signify
judgements of an object’s 3D spatial properties (e.g. how wide and what sort of general 3D configuration it has) &/or its relative separation (distance from the fixation plane i.e. horopter or distance from an object from another object in the fov) from other objects or surfaces in the field of view, with ‘distance’ reserved for absolute judgements of how far away an object is from us, the viewer.
what does the visual system make up a variety of
cues (source of information)
what do cues judge
the depths and distances of objects from us
what are the two types of cues
• monocular cues: available to one eye alone - binocular cues: based on different views of the two eyes
what does the horizontal separation of our 2 eyes result in
our eyes to be in slightly different positions in our heads which then have slightly different views of the world
name the two categories of monocular perceptual cues
relative depth & absolute distance
list the points useful for relative depth (as monocular perceptual cues)
• relative image size • textural gradients • interposition/occlusion • light & shade • motion parallax
list the points useful for absolute distance (as monocular perceptual cues)
• retinal image size • texture gradients • linear perspective • atmospheric perspective (outdoors) • height-in-scene
which multiple monocular perceptual cues are useful for both relative depth & absolute distance
relative image size & textured gradients
which monocular perceptual cues don’t tell us much about absolute distance
• interposition/occlusion • light & shade • motion parallax
what are all the multiple monocular perceptual cues known as (except motion parallax), and have been exploited by artists for millennia for depict 3D in 2D images
learned pictorial cues
give an example of relative image size
two balloons, larger one looks closer and smaller one looks further away as the larger one forms a larger image on the retina
why is retinal image size perceived this way
because retinal image size is known from learned experience to be proportionally related to the viewing distance
what is textural gradient based on
systemic reductions in the relative size of the individual texture elements, which are assumed to be identical
give an example of textural gradient
circles at the top which are also smaller, seem further away and circles at the bottom which are larger and shows more detail, seem closer by
what is interposition/occlusion
where one object blocks/occludes the view of the one (or more) further behind it, so it is assumed to be nearer e.g. the visual system does not assume it is a circle with crescents above it, stacked on top of each other, it assumes that they’re all circles
give an example of light & shadow
light coming from the left shows the concavity of a crater as it is in shadow i.e. it is not flat
what gives off the effects of atmospheric perspective
because the distant objects have lower contrast and a blue tinge due to scatter (short wave light) in the atmosphere, it seems further away in the scene
what is height in scene
things lower in the scene seem nearer than things that are higher in the scene
what is not a learned pictorial cue, i.e. is not in conscious awareness
motion parallax: formed by head motion
how does motion parallax: formed by head motion work
e.g. when the head moves/turns to the right, the far object also moves to the right with the observer, but nearer objects moves to the left, against the observer (doesn’t matter where your fixated)
what is motion parallax generated by
self motion i.e. motion of the observer
what is the problem with monocular cues
they’re ambiguous and require assumptions
give an example of a problem with retinal image size
it is an ambiguous distance cue, unless the physical size of the object is already known (familiar) e.g. are the two balloons at different distances, or are they actually the same distance but different sizes…..? they are actually the same distance but different sizes, as they are against a flat screen (can be at different distances if you know the circumstances at which you are viewing these images, otherwise you can’t tell)