"JND" notation

In various areas (such as metrics for various aspects of image quality) we run into "JND" notation.

As with many such matters, this flows from a very clear concept, but the "editorial" way in which it is presented has been screwed up in a way that can lead to confusion (as when we call the circle of confusion diameter limit the "circle of confusion".)

In the subjective assessment of various things, it can be useful to adopt a score in which the unit is the smallest amount of change in the actual technical property that can be noticed by a "standard observer". The unit is called the "just noticeable difference (JND). (Because of the logarithmic nature of many human perceptual properties, this is often a ratio.)


Now if we compare "case A" with "case B", and the typical human observer will find the perceived property for "case B" less (maybe worse) than "case A" by twice the amount one could just notice, we can say that the property for "case B", compared to "case A", scores as -2 JND.

Again, "JND" designates the unit.

But of course technical authors could not hold to that notion. So it gets a bit twisted, linguistically.

Today, we might read of our "case A, case B" example that:


In other words, "JND", which originally meant a unit of perceived difference, has come to mean "a perceived difference, expressed in the unit JND."

It is as if we called the length of a board, to be expressed expressed in the unit foot, its "foot". "What foot do we need for the shelves at the back of the shed?" "Oh, about 6."

"The sharpness JND for that system is -3". "Oh, what units is that in?"

Best regards,

Doug

It remind me of my brother's dentistry practice in determining the prices he charged patients for his precise goldwork.

"What are your prices?"

They're based on very exact standards and very accpetable calculations. I'll give you the bill and you'll see it's reasonable.

Then he looked at the fellow's shoes and had his wife report what car he came in and came up with a price that wouldn't force them to sell the car or give up buying such shoes.

A gold crown to save a tooth, could run from ten pounds for a guy who came by bus, 400 pounds for a fellow in an Austin Princess and 1200 pounds to the lady sending her driver to give her groomed poodle by a tour of the neighborhood in their lilac colored Rolls Bentley!

His standards were moralistic! He made them and they were "exact", all done in multiplying his "want" price by factors based on observations. However, when he landed a job as the director of dental services in Israel, he had a rude awakening as his exact standards, saying prices have to be dropped! His authoritarian guides were very harshly received. Both his negative views towards current pricing .......... and the dismal training of some Russian dentists, resulted in several hits being ordered and he had to carry a firearm for years until tempers cooled.

"Just right" and other judgements we make can be pretty accurate in practice for our personal use, such as the pull back travel and angle on a hunters bow really can be so exact in hunting. Or rather, it's "exact enough", to bring the quarry down, before the 600 lb charging moose tramples him!

However, one can't pass on one's personal "adjustment factors" to others faced with the same angry beast!

Asher