Wednesday, 21 October 2009

Z Line Comment Comment

Hi Nikos!

There is also the stunningly obvious issue that small objects are perceived to be further away!

I can't quite make out whether this paper addresses that.

It might seem stunningly obvious - but it's not that simple.

The psychological 'process' involved is called 'looming' - simply that as things get closer they get bigger. But in a weird way it's not linear.

Have a read of this:
Committee Report: Conspicuity Enhancement for Police Interceptor Rear-end Crash Mitigation

How Do I Know Closing or Relative Speed?
Drivers sometimes drive into the back of slow-moving vehicles, vehicles stopped in the travel lane, or parked vehicles on the side of the road. These crashes occur when visibility is not a problem and intoxication or health factors are not at play. These types of crashes are often attributed to driver inattention but perceptual deficiencies may be intermingled as well.
Olson (1993) and others have analyzed the difficulties people have in judging closing speed. It was mentioned above that a main cue to distance is the image size an object subtends at the driver's point of view. If the image grows larger, we know the object is coming toward us (if we judge we are at a standstill as indicated by other information in the visual flow field) or we are moving toward it (if so indicated by the information in the visual flow field). The rate of change of object size, looming, has been discussed as a primary cue to control braking.
Perceptual deficiencies in detecting change in object size will contribute to crash risk under certain conditions, e.g., at night.
The rate of change in image size depends on both speed of approach and viewing distance (Olson, 1993). Consider Figure 1, which shows how the visual angle an object subtends at the driver's vantage point changes with viewing distance (See Appendix for explanation). This figure assumes a 6-ft wide vehicle viewed from an initial separation distance of about 1000 ft. The first point to note is that the relationship between object size and distance is highly non-linear. The second point to note is that the image size of the object does not change much for most of the approach, even though it doubles with every halving of the viewing distance. At 1000 ft, the object subtends about 0.006 radians or one-third of one degree of visual angle. At 500 ft, the image size doubles to about 0.012 radians or two-thirds of one degree of visual angle. At 250 ft it doubles again. Because of this nonlinear relationship, drivers may not realize they are closing in at high speed until quite close to collision.

This non-linear effect is also known as the 'time to arrival' illusion, and was the reasoning behind a TfL safety advert:

So, Nikos, now addressed - hope that helps! If not, ask away.


Nikos said...

Ah ha...but does it explain why the sun appears to look bigger when it is closer to the horizon?

PS Thanks for posting this - I will try and understand it later on tonight!

Nikos said...

Yes but...rate of change of angle subtended - There must be a stereo image "angle" to this too (ie. depth/distance perception much more difficult for one eyed Gordon Brown compared to Mr Cameron?