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More Questions About Computer Generated Images


Mainer
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WARNING: .........   incoming .......  newbie question

 

In looking at the arrows image (the one with the most red) of the AGSL Computer Generated Light Performance Map  I can pretty much figure out the facets where  all the blue arrows and little blue chips are originating from. But some diamonds have a ring sometimes solid blue and sometimes made up of closely spaced blue triangles surrounding the center red or green facet.   What causes these blue contrast rings?  Why do some diamonds have them and others not?  They seem unrelated to anything I can figure out.  How do they affect the look of an unmagnified diamond?

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Yes although your picture shows very symmetrically placed blue marks around the center facet most of the ones I've seen will have triangles of blue half way around the center in a jumbled sort of way.  It may be the same phenomena.  I'll try to enlarge and post a sample picture.  

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If you click on the picture it will enlarge so that the colored image on the left is bigger and you can see the blue spots around the center facet.  

 

What causes the blue circle and why does it appear on some diamond images and not on others?

Edited by Mainer
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What causes the blue circle is the combination of lower girdle length and pavilion (main) angles - the centre of the stone is expected to look dark when observed straight on (and with the head of the observer obstructing light coming only from above).

 

Whether the effect is present in reality and is observable in "normal" conditions (diamond set in a ring, held at an angle from the observer and with more diffused lighting than a single, spot-like source right above the observer's head) is a completely different kettle of fish, and can only be determined with direct observation.

 

BTW - there is no "centre facet". The green (or red) octagonal shape at the centre is the 8 pavilion mains meeting at the culet/point.

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Thanks,  I knew center facet didn't sound right.   

 

Assuming that all images are taken in controlled lighting situations,  if it is  a phenomena of the way the facets and the angle react together why is it that some images have these somewhat disorganized blue triangles around the culet and others simply show the base of the arrows forming an 8 sided culet image with no blue triangles interspersed between the arrows?

 

 

The thing that confuses me  is that it is only on the Light Performance images that this shows up.  No other posted images show clusters around the culet even when the AGSL  images shows a lot of blue around the culet. 

 

Are the blue triangles maybe the result of the way light hits slightly uneven or bilaterally unsymmetrical  facets on the pavilion in AGSL imaging?   I'm out of my depth here!!!!!

Edited by Mainer
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Assuming that all images are taken in controlled lighting situations,  if it is  a phenomena of the way the facets and the angle react together why is it that some images have these somewhat disorganized blue triangles around the culet and others simply show the base of the arrows forming an 8 sided culet image with no blue triangles interspersed between the arrows?

Big assumption - and bear in mind that the computer-derived ASET images (e.g. those on AGS reports) use different "assumptions" for light source size and obstruction than what you will find in "real life" ASET photos. In any case, even assuming light source/environment characteristics are standardised, you will have certain combinations of angles and facet lengths that produce the blue ring, others that look red throughout and others still that look red and green, or red and blue.

 

The thing that confuses me  is that it is only on the Light Performance images that this shows up.  No other posted images show clusters around the culet even when the AGSL  images shows a lot of blue around the culet.

Ummm... I'm not sure what you mean. I can easily find images showing a blue circle that are not coming from AGS reports - for example the one I linked above: that one is not an AGSL image. Again, bear in mind that positioning and characteristics of the light source/environment will make a lot of difference; I have seen ASET photos that look nothing - and I mean nothing - like the computer-generated images of the same stone.

 

Are the blue triangles maybe the result of the way light hits slightly uneven or bilaterally unsymmetrical  facets on the pavilion in AGSL imaging?   I'm out of my depth here!!!!!

Whether you get a complete circle or a more "random" pattern of triangles depends on whether the facets are cut perfectly symmetrically or whether there are variations in the angles that result in some lower girdles reflecting light (red or green) and others not (blue) with the stone at 0° from the light source. Edited by davidelevi
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Even with my badly worded questions you have answered most of what I was curious about.  

 

In the images you posted can you compare what is going on in the two diamonds to give such different scope images and how that translates into what the diamond looks like if you are looking at the actual diamond.  

 

Are there diamond fairs where a novice  can  see diamonds and talk to diamond people? 

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What is going on - from a "physics" point of view I don't know how to describe it other than as I have done above: some angles result in light being reflected back to the observer i.e. red/green, other angles result in light not reaching the stone ("head obstruction") i.e. blue. From a "practical" point of view, in stones that show evident obstruction you may see a dark centre; literally the stone's "heart" is dark:

 

Fig09.gif

Left image: under diffused light without "head"

Right image: same lighting, but with a "head" between the light and the stone. The dark areas would most likely show blue in an ASET.

 

The two stones on the left are extreme cases, and would almost certainly not qualify as GIA excellent (never mind AGS ideal). The stone on the right is well cut yet still shows a certain amount of darkening.

 

Note that this effect depends on:

 

1. The light source being directly above the observer (or in any case there is an axis stone table-observer-light, with the observer in the middle)

2. The distance between stone and observer and observer and light being within a relatively narrow range (theoretically "typical" of average viewing conditions, but practically not)

3. The observer's "head" (or other obstructing body part with eyes on) having a certain size relative to the stone

 

So you may not see this at all when seeing the stone in real life (and this is why I and others tend to take reflector images with a big pinch of salt: they don't really tell you everything that their staunchest advocates say they do).

 

Diamond - or at least gem - fairs: yes, they exist, but they tend to be addressed to professionals (not that getting in is impossible or even difficult for consumers; it's just the type of conversation tends to be different). Rather than focusing on that, I would try to find a few good vendors relatively near to each other: for example New York has a huge concentration of diamond resellers - some are sharks, but some are very very good.

 

Edit: clarified what is in the image linked.

Edited by davidelevi
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Irrationally pursuing meaning in various diamond images I took 14 AGS lab reports and sorted them into three groups according to the amount of blue dots around the culet.  The results were interesting.   I don't know what they mean but they were interesting.

 

Group 1 a solid and wide ring of blue dots around the culet:

HCA rating   .8 to .9 

Table    55.4  to 56. 3

Length  61.0 to 61.6

Crown Angle 34.2 to 34.4

Pav. Angle    all were  40.7

Star length 50-53

Pav facet length 76 to 77

Crown %  15.0  to 15.2

Pav %  42.9 to 43.1

 

Group 2 some blue dots but not a complete ring around the culet

HCA rating  .9 to 1.1

Table   55.2  to 56.4

Length 60.9  to 61.7

Crown Angle  34.2 to 34.3

Pav. Angle   40.7 to 40.8   (those with angles of 40.7 had more blue dots around the culet than those at 40.8)

Star length 50 to 53  (only one star at 53)

Pav facet length  76 to 79

Crown % 14.8  to 15.3

Pav. %   42.9 to 43.1 

 

Group 3  only one or two blue dots around the culet.  This was the smallest group and the most erratic

HCA  .8  to 1.4 (only one was .8  the rest were 1.3 or above)

Table  55.6 to 57  (most were larger tables only one was 55.6)

Length 61.1  to 61.6

Crown  Angle  34.2 to 34.6

Pav Angle   40.7  to 40.8  (only one at 40.7 and that is the one with HCA of .8)

Star length 47 to 52

Pav. facet length 76 to 78

Crown % 14.8 to 15.1

Pav %  43 to 43.1

 

I have no idea what all this means but as crown angle, pav angle and table size increase the number of blue dots around the culet decreases and the HCA rating goes down.     

 

 

I should probably go back to weaving and leave diamonds alone.  :-)  

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