Problem Set #2- Explanations Flashcards
Explain why texture gradient is only a relative metrical depth cues
Texture gradient does not indicate the absolute distance to an object. It does, however, indicate the relative distances to objects, (e.g., that one object is twice as far away as another). For instance, given any picture, if you made all the objects in the picture twice as far away and twice as big, the texture gradient would remain the same.
Explain why vergence is an absolute metrical depth cues.
ecause the angle of rotation of our eyes provides a direct and accurate measure of the distance of an object from our eyes without requiring any assumptions or prior knowledge about the scene.
Suppose that light looks white when it activates all cone types equally. (a) Is there a single wavelength of light that looks white to a normal human observer?
No. There is no single wavelength that activates all cone types equally
Suppose that light looks white when it activates all cone types equally. Are there lights that have different spectral power distributions, but that look exactly the same shade of white to a normal human observer?
Yes. There are lights that have different power spectra, but stimulate all three cone
types equally, so they all look white
Why does it mean that we need more than one type of cone in order to perceive colour?
Without multiple types of cones, we wouldn’t be able to perceive color because the same response could be generated by different wavelengths. With more than two cone types, the cone responses convey information about the wavelength of light, allowing the visual system to perceive color.
What is univariance?
Univariance means that a photoreceptor’s response is independent of the wavelength of the photon that stimulated it.
Explain why binocular disparity and motion parallax provide very similar information
about the depth of an object
Binocular disparity and motion parallax both exploit the small differences between the
view of the world from slightly different viewpoints.
Why do some lizards do pushups when you approach
them?
Lizards with eyes on opposite sides of their heads do not have binocular disparity cues, so they do pushups in order to obtain depth cues from motion parallax
What is the correspondence problem for binocular vision?
An external object generates a left retinal image and a right retinal image. The
correspondence problem is the problem of deciding which features in the left retinal image correspond to (i.e., were generated by the same external object as) which features in the right retinal image
Your sensitivity to colours is different in the fovea, the parafovea (the region just
surrounding the fovea), and the periphery. Explain why.
The sensitivity to colors in different parts of your eye varies because of the different types and densities of cells called cones and rods.
In the fovea, there no S cones so you are a tritanope (i.e., blue-yellow colourblind). Because of the absence of S cones in the fovea, your sensitivity to blue and yellow colors is lower and this makes it harder to distinguish between these colors.
In the parafovea, which is the area surrounding the fovea, there are densely packed L, M, and S cones. These cones work together to provide good color vision in this area.
In the periphery, which is the outer edges of your vision, there are more rods than cones. Rods are more sensitive to light but are not as good at distinguishing colors as cones. Therefore, your sensitivity to colors is lower in the periphery.
How to know whether a dot will appear nearer or farther than the object being fixated?
We need to consider the type of disparity that is present.
1) If the dot has crossed disparity, meaning it falls to the left of the left fovea and to the right of the right fovea, it will appear to be nearer than the object being fixated. This is because crossed disparity occurs when the eyes must converge or rotate inward more to focus on an object that is closer, leading to a greater degree of crossed disparity.
2) Uncrossed disparity occurs when the image of an object is shifted to the right in the right eye’s image and to the left in the left eye’s image. This type of disparity indicates that the object is farther away from the observer than the fixated object.
. Explain Geisler et al.’s evidence that the
Gestalt principle of good continuation is related
to the distribution of edges in images of realworld scenes. The figure to the right may jog your memory.
Geisler et al. examined edges in photographs of real-world scenes. They measured how probable it is that two edges belong to the same contour in the image, if they are some distance apart and at some angle relative to one another. This is what the figure to the right shows. Briefly, two edges are more likely to belong to the same contour if they are aligned towards one another. Then, Geisler et al. measured human observers’ performance at detecting contours in stimuli like the one at the right, under several conditions, e.g., different contour lengths. They found that human
observers detected the contours best if their edges were approximately lined up.
