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A girl’s best friend

13 Apr , 2016   Video

Diamonds are not easy in VFX

Contents

Introduction
Fire
The VFX problems
Solving…with modelling
Solving…with rendering
Finished Diamonds
Links

Introduction

Rendering images of diamonds in VFX is not easy. This is true because of what gives a diamond, a strongly bonded covalent lattice of carbon atoms, its key characteristics. When a diamond is found in rock it normally shows little sign of its desirable characteristics. This is because while one of them is naturally occurring and an intrinsic property of carbon in this state, the other, is man made.

artificial diamond crystal

Fire

The “fire” we see in a diamond is the result of the naturally occurring property of dispersion. Dispersion is when different wavelengths (colours) of light are ‘bent’ by different amounts by a physical media boundary. That is, when light travels from say, air, to diamond. Blue light wavelengths have higher energies than green light wavelengths, that in turn have higher energies than yellow, and red wavelengths. The angle that these colours bend when they pass through a boundary between materials is dependent on their energy levels, with higher blue wavelengths bending more than green wavelengths and green more than red.


dispersion
This has the effect of white light being split into its component wavelengths, just as we see in a rainbow. In the case of a diamond, this effect is usually present but not visible to us when it comes out of the ground. This is because the effect is artificially increased by gemstone cutting. This is the process that gives a diamond the characteristic shape we see in pieces of jewellery. The angles chosen for those cuts aren’t just a pretty collection of triangles and octahedrons, but a cleverly engineered light-trap, designed to enhance the effect of fire in a diamond, by increasing the number of times light bounces around inside the stone before emerging to our eyes. The more bounces: the more fire the diamond exhibits.

synthetic diamond

Even with all this careful cutting, polishing and geometric design, the effect can still only be seen under ideal lighting, when the diamond is held at certain angles.

The VFX problems

Since diamonds are used in a luxury market, saturated with brand image and controlled look and feel to the majority of jewellery designers marketing content, we want to be able to produce realistic looking diamonds in 3D using visual effects. This will allow us to “film” pieces of jewellery with perfect image resolution, detail, and clarity because we aren’t dealing with the limitations of real-world lenses, cameras, lighting or photography studios. Great, but there’s a problem.

The most common type of diamond cut used in jewellery is the round brilliant cut. There are several different types, defined by different companies and people. One of these, the Eulitz Brilliant, is mathematically derived and specifies all of the geometric properties of the diamond precisely, to result in maximum “fire”.


diamond brilliant round cut geometry
Okay, but we’re pros. Let’s assume we can model an exact replica of this ideal diamond cut. There is still another problem

In computer generated VFX, there comes a part in the process where we generate the images from our 3D environment. This process is known as rendering, and the software used to carry it out, are known as renderers. Almost all renderers work on the same principle as all colour on almost every computer. RGB colour. Red, green and blue pixel colours are displayed on monitors because this loosely approximates how our eyes and brains interpret colour. You can read more about how we perceive colour in our article: Your eyes and RGB colour (coming soon).


RGB colour
Some of you might already be guessing the problem we are going to have here. We have three colour channels to play with, which is fine as far as our eyes are concerned, but what about all the colours in between red, green and blue? Well, our brains can analyse the weighted proportions of the wavelengths that we do see, and very loosely, interpret what comes in between. If you think about it, there is a quirk here, in that blue and red are at opposite ends of the colour spectrum. Directly between them is green, but when we mix blue and red, our brains know it is not green we are seeing, so we “make up” a colour. Magenta.

To get this right, our renderer needs to be able to bounce each of the main colours of light up inside our diamond model, but only deals in RGB colour, and there are more than 3 colours in the spectrum. There is no individual consideration in the maths for cyan or orange, let along magenta, which doesn’t even exist outside of our brains.

Solving…with modelling

We very carefully built a model of a 96 faceted Eulitz Brilliant Round cut diamond. The model had to adhere to the following proportions, taken as percentages of the total girdle diameter.

Crown height Pavilion depth Table diameter Girdle thickness Crown angle Pavilion angle Brilliance grade
14.45% 43.15% 56.5% 1.5% 33.36° 40.48° 100%

And here it is…

diamond-wireframe

 

Solving…with rendering

Finally, it was time to generate some imagery of our new model. The problem of colour was addressed by rendering several separate images of the diamonds, one for each of the different wavelengths of light we would calculate, and then to stick them all together after rendering in an image compositing tool.

This might seem like a lot of effort to go to for one image, but take it from us, this little node noodle is a baby in the world of compositing.

diamond-blender-composite

Finished Diamonds

Here is our final diamond image, and a quick turntable video of some we made earlier.

diamond-dispersion-greybg

Lastly, we designed an engagement ring model to show off our shiny new diamond renders.

Links

bit.ly: http://bit.ly/262cid6
permalink: http://www.superframevfx.com/blog/a-girls-best-friend/
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