# TU Wien Rendering #14 - Global Illumination Benefits ## Метаданные - **Канал:** Two Minute Papers - **YouTube:** https://www.youtube.com/watch?v=vS0g9SVHRFc - **Дата:** 29.04.2015 - **Длительность:** 11:55 - **Просмотры:** 6,872 - **Источник:** https://ekstraktznaniy.ru/video/14998 ## Описание Global illumination programs, unlike recursive ray tracers, are able to compute beautiful effects like indirect illumination and caustics. We take a closer look on how this is possible, and why the definition of shadows is fundamentally different in global illumination - this alternative definition allows us to get perfect soft shadows without explicitly computing many shadow rays against light sources. About the course: This course aims to give an overview of basic and state-of-the-art methods of rendering. Offline methods such as ray and path tracing, photon mapping and many other algorithms are introduced and various refinement are explained. The basics of the involved physics, such as geometric optics, surface and media interaction with light and camera models are outlined. The apparatus of Monte Carlo methods is introduced which is heavily used in several algorithms and its refinement in the form of stratified sampling and the Metropolis-Hastings method is explained. At the ## Транскрипт ### [] okay so we have these two guys in the ring and we know already how to solve the illumination equation equation we don't measure radians we measure intensity it's not really a unit in physics it's just some hacked up thing that happens to work in the rendering equation we measure radians and we have to do some kind of integration and this is if you the more you think about it the more impossible it will sound to even the thought of solving this problem so the first question is what can i earn by solving this equation because i have to be motivated to do it so obviously the result better be better look really good in order to give me the motivation and the resources to solve it so this is an image from the first assignment and this we have computed with recursive ray tracing so you can see for instance heart shadows and you can see that this is a reasonably boring image i mean it's great compared to the simplicity of the model that we have but it's not really the greatest well what is missing let's take a look and look very closely at the very same scene but not with recursive ray tracing but the global illumination algorithm so not the illumination equation but the full rendering equation take a look at the difference look closely this is full global illumination ah finally absolutely beautiful let's take another look this is recursive rate racing and global illumination so apparently there are some effects that recursive ray tracers cannot account for what are these effects well we have talked about indirect illumination or color bleeding this is the very same thing this means that i am hitting two diffuse objects one after each other is this visible enough okay i'll just pull a bit on this curtain so you guys can see better okay perhaps a bit better right yes yeah okay cool so these are in this case ldde paths what's that what does it mean everyone knows you start out from the light source you hit two diffuse objects and you hit the i excellent now indirect illumination is ### Indirect Illumination [2:36] all around us everywhere both in the real world and both in the better computer games out there which have approximations of indirect illumination and you can see that on the left image it almost looks like photoshop it is completely alien from its surroundings it is almost as if it didn't take into consideration its surroundings so you're standing in the fro in the middle of the desert not just somewhere you would have to have some color bleeding effect that you get from yourself and this is what usually the problem is with many of the photoshopped images you just rip out the person from somewhere and you put it in another photograph and it looks super fake and yes mostly because of the illumination conditions but even if you try to account for that recolor it to have more or less the same color scheme than the rest of the photograph you're still missing the indirect illumination effect and the human eye is very keen in recognizing that so you recognize that something is wrong but you don't know what exactly is missing and it's usually indirect illumination but there's something else let's take a look at this scene with ### Recursive Ray Tracing [3:57] recursive ray tracing so we have refractive materials for instance this flask sphere on the left the mirror sphere in the middle and the completely diffuse sphere on the right let's take a look at how the very same scene looks like with global illumination this is the difference one more time ### Recursive Retracing [4:20] recursive retracing and global illumination so like we have talked about this before i can see the differences in indirect illumination so on the upper left i can see that some of the red color is bleeding onto the other wall and the very same with the green wall in the background also with this diffuse ball so even a simple diffuse sphere looks much more interesting and much more beautiful with global illumination don't say anything but i say something else i see something else as well not only indirect illumination i see some other effect on this image that i couldn't compute with raytracers before don't say anything raise your hand if you know what i'm talking about excellent almost everyone and what don't say anything okay i'm talking about this and this so this interesting light effect on the wall and below this the glass sphere so raise your hand again if you know what what this is exactly okay don't say anything because so many people know you will have to say all of you at the same time after three got it okay so everyone one two three what is this the solution okay what are the other guesses that's technically a fraction yes but that's not how we call the effect anyone else okay this is what we call ### Caustics [6:09] caustics so what kind of light path is this is an interesting light path in this case this is l ssde why because we start out from the light source we hit the glass sphere from the outside then we have refraction we hit it from the inside and then we hit some diffuse object that is either this checkerboard down there or the red wall on the left and then to the eye and if we have this effect then we are going to have caustics it's a beautiful phenomenon in nature that we can finally account for and it's and you can see this at many places now let's take a look at another example this is the famous school corridor example ### School Corridor Example [7:00] from black sander again with recursive ray tracing and global illumination so you can see lots of indirect illumination this reddish light on the floor and perhaps some caustics or at least caustic looking thing in front of the lockers okay so next question ### What Is the Definition of Shadows [7:25] question what is the definition of shadows again so what we have said before that shadows are regions that are not visible from the light source now an alternative definition of shadows is the absence of light this is what definition we will use in global illumination so there's you could say that there's no such thing as shadows there's no uh that's not something that's just the absence of something else if there is less light somewhere then there's gonna be shadows so this is the definition of shadows in global illumination and in zen culture and take a look at this image we can see some beautiful soft shadows and the thing is that you don't even need to do anything to compute these in global illumination so if i have a ray tracer what do i shoot out shadow rays from these regions and i try to approximate what regions of the light source are visible from this point in global illumination you don't need to do anything you just solve this equation and out comes physical reality and shadows are parts of physical reality you don't need to do anything in order to obtain shadows it's not like a bottom-up approach like ray tracing so you start from the baseline and you add more and more hacks to account for more and more effects and for global illumination you will see that we will have a simple algorithm that can give you all of this and you don't need to account for shadows and caustics and all of these things another beautiful example of caustics ### Caustics from the Point Light Source [9:04] this is caustics from the point light source because for instance you can take a look at the shadows are hard so it's likely to be a small or a point light source and the caustics are very sharp so they have the same behavior to large light sources as shadows and another beauty with caustics okay so let's assess what these recursive ray tracers are capable of doing and what they cannot well obviously they cannot compute indirect illumination means two diffuse bounces or possibly more this you cannot compute correctly we will talk about why and you cannot compute caustics well caustics i have written in a few scenes ago that it was lssde so two specular bounces until the diffuse because you have to go through the glass ball and here i'm writing something completely different i just say one specular bounce is necessary the rest are optional is this true or how can we verify this is true in order to find out if this is true or not i don't even need to say a word i can just do this do you see the caustics inside my fiance is gonna kill me okay you two have seen it got it yeah thanks okay so apparently rings have caustics well i start out from the light source i hit one specular object one mirror-like object and then a diffuse which is the table and then the i and i have caustic so l s d e is enough for constants there's no need to prove it in any other way just take a look at physical reality and let it be your question always