More on Saliency

Visual saliency provides a relatively efficient way to quickly eliminate items of disinterest from the field of view.  This is largely due to the concepts behind the "bottom-up" approach mentioned in our introductory blog post.

The key objective of visual saliency is to quickly identify objects of interest.  When observing a picture such as the one below:

the object that stands out is clearly the red bar amongst the green.  This of course depends on the capabilities of the observer to distinguish colors and intensities.  Consider that without the capability to distinguish red from green, there would be no obvious item of interest in the image.

In the next image, it is difficult to spot anything of interest without searching through the image.  This is because there is little salience to guide you to the unique bar (can you find it?).  The orientation and color of most of the objects in the image are very similar and as such nothing seems too important.

Looking at the same image again, it is interesting to note what happens when the size of the viewable area is decreased:

Can you spot the unique bar now?  While it still may not be immediately obvious as in the first image, looking at a subset of the image can make determining which objects are significant easier.  This approach can be taken further:

Now it should be clear that the vertical red bar is our object of interest.  Consider what happens if the view is further restricted:

In the above image, it now seems as if the horizontal red bar is our object of interest because all other bars are vertical.  Taken one final step:

Now we cannot tell which of the two bars is significant because they are the only two items in the field of view.  This presents some interesting facts about saliency:

Saliency is not an inherent attribute of an object; saliency depends upon the combined effects of many stimuli to make a given stimulus more interesting.

Changing the scale of search can improve our ability to find more salient locations in an image though it may cause objects which are of no interest on the broad scale to suddenly seem interesting.

The Introduction

Our CSE 190 project will deal with the topic of saliency, which is the state or quality of standing out relative to neighboring items.  We intend to focus on the bottom-up approach, which is based upon stimulus-driven signals that announce that a location is sufficiently different from its surroundings to be worthy of attention.

In contrast the top-down approach focuses on object recognition using discriminant analysis.

Saliency is a plausible model for how biological systems and is considered to be a key attentional mechanism that facilitates learning and survival by enabling organisms to focus their limited perceptual and cognitive resources on the most pertinent subset of the available sensory data.

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As of now our plan is to spend the next two weeks researching and learning more about saliency and the relevent algorithms associated with the bottom-up approach.  The goal of our project is to create a bottom-up saliency detector and apply it specifically to crowds, though the detector should be general enough for use in other applications.

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Video of bottom-up saliency mapping on a busy freeway: