all you need to know about mr fresnel

Transcription

1 all you need to know about mr fresnel Topics covered: Surface Textures, Gradients, Incidence Angle Difficulty:

2 Introduction welcome Augustin Jean Fresnel ( pronounced Frennel ) is one Frenchman that every 3D artist should be grateful to!his life's work was varied as an engineer, mathematician and physicist, but in 1815 he published his first paper on the Wave Theory of Light. Whilst this may not sound like the kind of literature you'd subscribe to Reader's Digest for, it began to fully explain, for the first time, some very important effects observed by scientists and artists alike for many hundreds of years. Because, in the computer age, 3D graphics software often makes use of the formulae Fresnel first described, the visual phenomena that they predict and explain have themselves become termed 'Fresnel Effects'. On the next page you can see a typical example of the 'Fresnel Effect' at work. This tutorial has been created to observe, explain and implement the effect within Lightwave. Enjoy! Augustin Jean Fresnel A very clever Frenchman! Notice: The portrait of Augustin Fresnel is believed to be in the public domain. If this is not the case and you believe it is being displayed iimproperly, please contact webmaster@the-worms-of-art.com The Worms of Art Ltd (15/12/05). All rights reserved 2

3 Fig 1.1 shows two photos of a glass-fronted building. In the first we are looking directly through a pane of glass. Although we can see there are some reflections, we can also clearly make out the people and objects inside. In the second photo we are still looking at the pane of glass, but this time from a much more acute angle. It has now become harder to see inside; we mostly just see the reflections from objects on our side of the glass. This is the first important observation to note regarding the 'Fresnel effect': Objects become more reflective the more acute the angle we look at them. In the case of a transparent medium like glass, this also means that the material becomes less transparent at acute angles. Fig1.1 - Two photos of a glass building. One front on, the other at an acute angle The Worms of Art Ltd (15/12/05). All rights reserved 3

4 Simple physics Transmission and reflectance The effect seen in the photos of Fig1.1 is a consequence of the way light behaves when it bumps into glass. Fig2.1 demonstrates graphically what's going on. Shockwave viewers can click and drag the eye. Fig2.1 - Light hits a transparent surface at different angles. Reflectance and transmission change depending on the viewing angle. When light hits glass, a percentage of it passes straight through (transmission) and a percentage is bounced back (reflectance). As the angle to the object increases, so less light is transmitted and more is reflected. From inside the building in Fig1.1 as the angle increases, less light is transmitted through the glass and so we find it harder to see what's inside. From our side of the glass, more of the exterior objects' light becomes reflected, further obscuring the inside from view. Reflection and transmission are therefore inversely related - as one goes up, the other goes down The Worms of Art Ltd (15/12/05). All rights reserved 4

5 Different materials, same effect Different materials have different reflective and transmissive properties, but the effect always works in the same general direction. Even totally non-transparent surfaces like stone and wood react according to the Fresnel principle and are in fact more reflective at acute angles (seefig 2.2). Although these materials are not transparent, light is still being absorbed and scattered within. This is the essence of the 'Fresnel effect': The Fresnel principle. All real-world materials react to light differently at different angles Go out there into the real-world and try to observe this for yourselves in everyday materials.try particularly hard to test the theory in ways that don't get you arrested for suspicious behaviour! Fig2.2 - In this photo, you can see two of the granite faces of this pillar. You can see how much more reflective one is than the other due to their different viewing angles. This is the Fresnel effect at work! The Worms of Art Ltd (15/12/05). All rights reserved 5

6 Glass v Metal Different materials, different effects How pronounced the differences in reflectance are at different angles depends, to a large degree, on how conductive the material is. Conductive materials include metals - non-conductive materials include plastics, ceramics and glass. Non-conductive materials are also known as dielectrics. Conductive materials exhibit a much more subtle Fresnel effect than non-conductive, dielectric materials. This means there is less difference in reflections at different viewing angles. For polished surfaces, our eyes use this as an important visual cue. We get the sense that something polished is metallic because the Fresnel effect is less pronounced than plastic or glass, which are dielectric in nature. Fig3.1 shows renders of two materials. Can you tell which is meant to be ceramic and which is metal? Fig3.1 - Apart from altering the Diffuse channels these surfaces are identical except for the differences in their Fresnel effects. Both surfaces are 85% reflective at their edges, but one is only 5% reflective when looking at it directly. Which looks like ceramic and which looks like metal? In Fig3.1 the non-conductive ceramic - or dielectric - is hardly reflective at all looking straight at it, but is very reflective at acute angles. The metal is slightly more reflective at the edges, but the effect is much more subtle. In the Pshelf metals collection we have actually exaggerated the Fresnel effect of metals slightly because, in the end, it looked nicer. Art is not a science!humanst512 It BT The Worms of Art Ltd (15/12/05). All rights reserved 6

7 Fig 3.2 shows the same surfaces as before but as two diagrams, showing the reflectance amounts as the viewing angle changes. It's hard to consciously perceive these differences, because our brains cleverly process the information to produce the idea of "metal" or "plastic" or "ceramic". But as a 3D artist it's important to realise that, subconsciously, it's these reflectance changes that are influencing our judgement. Fig3.2 - Hopefully, you're thinking that the sphere on the left looks metallic and the sphere on the right looks plastic! The reflection amounts change as the viewing angle changes. Our eyes use this as a visual cue, telling us what a material is made from. The bigger the difference, the more 'dielectric' the material appears. folders v saucepans Fig 3.3 shows two photos. We'd like you to answer a simple question about them: Humanst512 It BT Fig3.3 - Which of these two objects is reflective? The Worms of Art Ltd (15/12/05). All rights reserved 7

8 The natural answer to give is that the metal saucepan is reflective and the blue folder is not. For everyday life this conclusion is sufficient. We are all happy to observe that a mirror is reflective but a wall isn't. However as an artist this isn't good enough. You must ask yourself more questions, such as the following: Humanst512 It BT This second question requires a much deeper understanding of how light works in order to answer. Discussions about this very subject raged throughout Augustin Fresnel's own lifetime. A commonly held belief was that people's eyes actually emitted light and illuminated the objects all around them! The true answer to the second question is therefore not as obvious as we might like to think. What is reflective? OK. Let's eliminate the possibility that our eyes are sending out light particles. This is not true! A more helpful way to think of light bouncing around our environment is as follows: When light hits a surface, that surface can deal with the light in several ways: It can absorb the light and convert it into something else - usually heat. It can ignore certain light frequencies and let them pass straight through - known as transmission or transparency (just as glass or water lets through most visible light frequencies). It can absorb the light but immediately reflect it back out in the general direction of where it came from. This is known as reflection. If, on a microscopic level, the surface is very flat and polished, all the reflected light will come back at uniform and predictable angles. This gives the surface a mirrored appearance, as with the metal saucepan. This is known as SPECULAR reflection. If the surface is random and rough on the microscopic level, the reflected light will be scattered outwards with a degree of randomness. This is a little like pool balls scattering after a break-off shot. This kind of reflection is known as DIFFUSE reflection and is what's happening to the blue folder. The concept of two reflection types - SPECULAR and DIFFUSE - explains the differences between the appearance of the blue folder and the metal saucepan The Worms of Art Ltd (15/12/05). All rights reserved 8

9 When we look at the blue folder, the surface is absorbing most of the light frequencies sent into it, converting them into small amounts of heat. The exception is visible blue light, which causes the material's molecules to vibrate and bounce that frequency of light outwards. This is why we perceive the surface as 'blue'. However, because the surface is also rough, the light is scattered randomly, meaning that we can't perceive any mirror-like reflections. This is DIFFUSE reflection at work. Therefore, the complete answer to our original question is as follows: Q. Which of these two objects is reflective? A. They both are. The folder exhibits DIFFUSE reflection, the saucepan exhibits SPECULAR reflection. Fig3.4 - There are two basic types of reflection - DIFFUSE and SPECULAR. Many surfaces can show various degrees of both types of reflection; the real world is a complicated place. However, the sum of diffuse and specular reflection can never be greater than the amount of light originally hitting the surface. In 3D raytracing there is no equivalent of heat conversion, so we can simplify the sum of reflections into a neat formula: The amount of light shining on a surface (100%) = Diffuse % + Reflection % For transparent objects we need to add to this formula: The amount of light shining on a surface (100%) = Diffuse % + Reflection % + Transparency % This is simply a fancy way of saying that what goes in must come out! The sum of these settings should always be 100% The Worms of Art Ltd (15/12/05). All rights reserved 9

10 Specular and the specularity channel Within most 3D software programs, the term 'specularity' is used in a specific context. It's important not to get confused here. The factual term 'Specular Reflection' refers to all mirror-like reflections. It's only in 3D software that the word represents something more specific (See Fig3.6). The specular setting in 3D software is a kind of cheat! It has evolved to deal with the problem of representing very bright light sources, reflecting off surfaces, in low dynamic range renderings it basically creates the impression of lightbulbs on a surface, which is another important visual cue. The notion of a separate specularity setting has no basis in real life. As a 3D artist, you need to know that whatever principle you apply to the 'Reflection' channel, you should also apply to the 'specularity' channel. They do, in fact, both represent specular reflections in the scientific sense. This doesn't mean that Specularity and Reflection should be set to the same number - specularity needs to be tailored to best suit each scene and surface - but if Reflection is high then Specularity should be high also and if one is set low, then the other should be as well. The next section uses the ideas we've learned here and implements them within a simple Lightwave project. Make sure you download the project files for this tutorial now from our website. Fig3.6 - In Lightwave's Surface Editor, there are separate controls for Specularity and Reflection. In real life, there's no such distinction. These settings both represent Specular reflection in the scientific sense. The settings should therefore always rise and fall in the same general direction The Worms of Art Ltd (15/12/05). All rights reserved 10

11 Practical polished gold In this practical, we're going to create a simple but gorgeous polished gold! Run Lightwave and load up scene [Taps_Working.lws] from the project files for this tutorial. Hit 'F9' to make a test render. You should see something like Fig4.1 These taps have been given a basic, muted yellow color, but no other surface properties. You can see that they look a bit ordinary at the moment and certainly not particularly gold-like! We're going to rectify this by creating a surface based on the two new principles we've learned in previous sections: 1. Objects react to light differently at different viewing angles 2. Diffuse% and Reflection% should always add up to 100% on non-transparent objects. Fig4.1 - A first render of scene [Taps_Working.lws] The Worms of Art Ltd (15/12/05). All rights reserved 11

12 Objects react to light differently from different viewing angles In Lightwave speak, the angle the camera presents to each point on a surface is known as the 'incidence angle'. See Fig4.2. This is the equivalent of the angle that our eyes see surfaces from in real life. Lightwave allows us to vary the properties of a surface according to its 'incidence angle' to the camera. This is achieved through the use of Gradient Layers within the Surface Editor. We're going to put gradient layers into the Reflection, Diffuse and Specularity channels in order to control those properties of our gold surface depending on the angle each point presents to the camera. The changes will be continuous as the objects the gold is applied to curve and flow. Fig4.2 - The incidence angle is what we're going to use to mimic surface changes at different viewing angles. In Lightwave, an incidence angle of 90º means the camera is looking directly at a point on the surface. An angle of 0º means the camera is looking straight along the edge, as with the edge of a sphere. The gold surface needs to be applied to the faucet object (we call them 'taps' in the UK). Open up the Surface Editor (Cntl + F3) and select the surface 'Taps_Surface' which belongs to the object 'bath_taps' The Worms of Art Ltd (15/12/05). All rights reserved 12

13 The default surface properties for 'Taps_Surface' are very basic, so we're going to start adding to these by creating a Gradient Layer in the Reflection channel. This will control how reflective the surface is dependent on the 'incidence angle'. Click the 'T' symbol next to the Reflection heading. The texture window will open with a default texture layer already created. We need to change the type of this layer to a gradient. Do this by selecting the 'Gradient' option from the 'Layer Type' dropdown, located in the top right corner of the texture window. A gradient layer with default settings will appear. Fig4.3 shows an example gradient layer. By using 'incidence angle' as the Input Parameter, we'll be able to vary the Reflection amounts across this surface. Fig4.3 - This gradient has been assigned 'incidence angle' as the Input Parameter. We're going to do this also, in order to control the Reflections across our gold surface The Worms of Art Ltd (15/12/05). All rights reserved 13

14 Change the Input Parameter of your newly created gradient layer to 'Incidence Angle'. Look at the Start and End range of this gradient, located at the top and bottom of the gradient bar. You'll see that at the top is 0º, which represents totally oblique angles to the camera; at the bottom is 90º, which represents the surface when the camera is directly facing it. Fig4.4 shows a graph which represents a good approximation to the way gold reflects dependent on the viewing angle. These figures are slightly altered from reality, but have been arrived at through many boring hours of experimentation in Lightwave! Fig4.4 - This graph represents the figures for Reflection we're going to recreate using a gradient layer. You may remember from earlier sections that because gold is very conductive, the differences in reflection across its surface are quite subtle. Using the Gradient Layer you've created in the Reflection channel, recreate the values shown in the graph on the previous page. Fig4.5 shows you the equivalent values. New arrow Keys can be created by simply clicking somewhere on the main gradient bar. The currently selected Key is yellow. When you've created your Key arrows as shown in Fig4.5 on the next page, hit 'F9' to make another test render. It should look quite different, but not quite right! The Worms of Art Ltd (15/12/05). All rights reserved 14

15 Fig4.5 - This is the gradient for the Reflection channel of our gold surface. It shows the settings for the Key at 0º. Roll over the other Key arrows to view their settings. 'Parameter' means the incidence angle and 'Value' means the Reflection% at that point. A test render of the scene will show that our taps (faucets) are now very reflective, but also way too yellow and bright! This is because we haven't yet conformed to our second important rule. The amount of light shining on a surface (100%) = Diffuse % + Reflection % At the moment, we have a flat value of 100% Diffuse adding to our already high Reflection values. This is creating a sum value of way above 100% right across the surface. We can rectify this problem by creating a similar Gradient Layer in the Diffuse channel but with each value just high enough, so that Diffuse% + Reflection% for each incidence angle is equal to 100%. On the next page, Fig4.6 shows this process graphically. But first, take the following steps to create a gradient layer in the Diffuse channel The Worms of Art Ltd (15/12/05). All rights reserved 15

16 With your Reflection gradient still open, copy the gradient layer to the clipboard by selecting Copy > Current Layer from the top left of the texture window. Close the Reflection texture window by clicking 'Use Texture' from the bottom right of the window. Create a texture for the Diffuse channel by clicking the 'T' next to the Diffuse heading. Paste > Replace Current Layer from the top left of the texture window. Your Reflection gradient will be copied in, ready for editing. Change the values for each Key arrow according to Fig4.6. The Diffuse and Reflection amounts for each Key s 'parameter' (incidence angle) should finally add up to 100%. Hit F9 to make another test render when you've finished the Diffuse gradient. Fig4.6 - Alter the Diffuse gradient layer so that its Key arrows reflect the values shown here. The sum of Reflection and Diffuse will now always add up to 100%, ensuring that our surface is not too bright or too dark, just like real life! The Worms of Art Ltd (15/12/05). All rights reserved 16

17 The first image of Fig4.7 shows a render of the scene after we've created our Diffuse and Reflection gradients. It looks much more metallic, but now our lovely yellow color has disappeared! This is because we've lowered our diffuse amount which determines how much of the base color influences the surface. Metals are unusual in that they can be highly SPECULAR but still retain color, which is also related to their conductive properties. Lightwave has a special setting to deal with this issue. It's called 'Color Highlights' and can be found within the surface editor, under the 'Advanced' tab. Locate the 'Color Highlights' setting within the 'Advanced' tab and change it from 0% to 80%. This is a figure that, again, we've found to work well through trial and error. Hit 'F9' to make another test render. It should now look like the second image of Fig4.7. This looks much better! Fig4.7 - two more renders of the scene [Taps_Working.lws] The first image is without Color Hilights, the second one uses a setting of 80% to restore our gorgeous gold color! We're nearly done. The final step we need to undertake is to put some Specularity onto the surface. This needs to be done because of something covered in previous sections: The Specularity and Reflection settings both represent elements of the same thing - SPECULAR reflection. Therefore, when one is high so the other should be as well. This is most easily achieved by copying the gradient layer from the Reflection channel, pasting it into the Specularity channel and tweaking the values for each Key arrow The Worms of Art Ltd (15/12/05). All rights reserved 17

18 Go into the Reflection channel and Copy the gradient layer located there (Copy -> Current Layer) From the main Surface Editor, click the 'T' located next to the Specularity heading. From within the texture window that appears - Paste -> Replace Current Layer to insert the gradient layer. The Key arrows are now located at the correct angles. Change the Key values to those shown in Fig4.8. Once again, we've found these figures to work through trial and error. However, you can see for yourselves that the increase in Specularity flows in the same general direction as the increase in Reflection. Fig4.8 - Copy these settings for your gradient layer in the Specularity Channel. These figures tend to look good for polished metals in Lightwave. We know this through trial and error! The Worms of Art Ltd (15/12/05). All rights reserved 18

19 conclusion Congratulations, your final render of [Taps_Working.lws] should look pretty spectacular! See Fig4.9 We hope you've found this tutorial useful. The ideas you've learned here are just some of the tricks we've used to create the Pshelf metals collection. Check out the Products section of our website if you're interested in learning more about metals and surfacing in general. Fig4.9 - The final render! The Worms of Art Ltd (15/12/05). All rights reserved 19