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These are the user uploaded subtitles that are being translated: 1 00:00:05,240 --> 00:00:08,240 Let's take a basic look at how a scene is rendered. 2 00:00:08,240 --> 00:00:11,320 This explanation describes path tracers, like Cycles. 3 00:00:11,320 --> 00:00:15,080 Meanwhile, rasterizers like Eevee use very different methods. 4 00:00:15,080 --> 00:00:19,210 But this is still relevant when working with Eevee, because it tries to emulate the same 5 00:00:19,210 --> 00:00:24,199 behavior as much as possible, so to the user a lot of the same rationale applies, even 6 00:00:24,199 --> 00:00:27,000 though it works differently behind the scenes. 7 00:00:27,000 --> 00:00:31,300 Also keep in mind that this is extremely simplified, and doesn't accurately describe what's actually 8 00:00:31,300 --> 00:00:36,020 going on in Cycles or in the real world, but it's enough as a foundation to start making 9 00:00:36,020 --> 00:00:37,020 shaders. 10 00:00:37,020 --> 00:00:38,640 Here we have a very simple scene. 11 00:00:38,640 --> 00:00:42,660 Let's take note that the cone which has a very matt surface, and the sphere has a polished 12 00:00:42,660 --> 00:00:46,550 mirror-like surface, perfectly reflecting almost all the light that hits it. 13 00:00:46,550 --> 00:00:50,630 The scene is lit with a single lamp, which we can see as a little white dot reflected 14 00:00:50,630 --> 00:00:51,980 on the sphere. 15 00:00:51,980 --> 00:00:56,230 There is also the ground, and some environment light, but we can ignore those for now. 16 00:00:56,230 --> 00:00:59,750 Let's keep the rendered scene in the corner as a reference, and let's look at it from 17 00:00:59,750 --> 00:01:01,059 outside the camera. 18 00:01:01,059 --> 00:01:05,700 If this were a real scene, the lamp would emit photons in all sorts of directions, some 19 00:01:05,700 --> 00:01:08,690 of which are represented here by the yellow dots. 20 00:01:08,690 --> 00:01:12,939 These photons would mostly travel in a straight path, until they hit an object. 21 00:01:12,939 --> 00:01:17,299 This is a lot of paths, so let's just focus on one of them, to see what happens. 22 00:01:17,299 --> 00:01:20,380 Here, the path hit the cone, which has a matt surface. 23 00:01:20,380 --> 00:01:24,649 Basically, this means that when light hits it, it bounces in a random direction. 24 00:01:24,649 --> 00:01:29,229 The cone also has a color, which means that some of the light gets absorbed, and the light 25 00:01:29,229 --> 00:01:33,609 that gets reflected determines the surface color, in this case red. 26 00:01:33,609 --> 00:01:37,850 So now our light that was previously white, gets colored red, and continues in a random 27 00:01:37,850 --> 00:01:39,180 direction. 28 00:01:39,180 --> 00:01:43,579 It happened to hit the sphere, which has a sharp reflective surface, so that means that 29 00:01:43,579 --> 00:01:48,329 the ray can only bounce in a specific direction, in which the outgoing angle is a perfect mirror 30 00:01:48,329 --> 00:01:50,350 of the incoming angle. 31 00:01:50,350 --> 00:01:54,539 Also note that the sphere is not completely reflective, so it absorbs a bit of light, 32 00:01:54,539 --> 00:01:57,350 but being gray, it absorbs all colors equally. 33 00:01:57,350 --> 00:02:02,829 Finally, our reflected ray hits the camera, contributing to the color of a specific pixel. 34 00:02:02,829 --> 00:02:07,609 Now, the thing is that this was a very specific ray, which happened to hit the camera, but 35 00:02:07,609 --> 00:02:11,900 there would have been many more rays that didn't hit the camera, and thus wouldn't contribute 36 00:02:11,900 --> 00:02:13,750 to the resulting image. 37 00:02:13,750 --> 00:02:17,930 But luckily, this whole process is mostly reversible, so render engines usually do the 38 00:02:17,930 --> 00:02:19,590 whole thing backwards. 39 00:02:19,590 --> 00:02:24,030 We start by shooting many rays from the camera, and trace them until they hit an object. 40 00:02:24,030 --> 00:02:26,710 Again, let's focus on a single ray. 41 00:02:26,710 --> 00:02:29,090 At this point, the shader is evaluated. 42 00:02:29,090 --> 00:02:32,911 This happens each time a ray hits an object, and information about the object and the ray 43 00:02:32,911 --> 00:02:37,570 is passed in as input to the shader, which we can retrieve with a number of nodes. 44 00:02:37,570 --> 00:02:40,180 We'll look more at this in a later chapter. 45 00:02:40,180 --> 00:02:44,860 Since this object has a fully sharp glossy surface, the ray can only do one thing from 46 00:02:44,860 --> 00:02:47,060 here, a perfect reflection. 47 00:02:47,060 --> 00:02:50,650 Now the ray hits the cone, which having a diffuse surface, would cause the ray to bounce 48 00:02:50,650 --> 00:02:52,520 off in a random direction. 49 00:02:52,520 --> 00:02:56,380 This would be very inefficient, as lots of rays would never hit a light source, and be 50 00:02:56,380 --> 00:02:57,380 wasted. 51 00:02:57,380 --> 00:03:01,840 So instead, we check if this ray could reach a light source without hitting any obstacles. 52 00:03:01,840 --> 00:03:05,940 For all we care, this has the same result as if many rays were actually randomly bounced 53 00:03:05,940 --> 00:03:07,540 off the surface. 54 00:03:07,540 --> 00:03:12,030 Now that we reached the light, we can take it's color and intensity, and trace it back, 55 00:03:12,030 --> 00:03:16,800 basically multiplying the light with the object colors along the way, to get the final color. 56 00:03:16,800 --> 00:03:21,480 In reality it's a bit more complicated, but that's the general gist. 57 00:03:21,480 --> 00:03:25,261 Now that we traced this path, let's check it by just keeping that one dot from the ray 58 00:03:25,261 --> 00:03:30,170 that comes directly from the camera, and let's get back into the camera view. 59 00:03:30,170 --> 00:03:34,110 Now if we fade in the actual render, we see that the dot is very close in color to that 60 00:03:34,110 --> 00:03:35,590 point in the image. 61 00:03:35,590 --> 00:03:39,590 The only differences are because we ignored world lighting and because we got the color 62 00:03:39,590 --> 00:03:43,690 by tracing a single ray, which kinda works in such a simple scene, but generally you'd 63 00:03:43,690 --> 00:03:45,850 need many more rays. 64 00:03:45,850 --> 00:03:50,290 Once this whole process of shooting rays from the camera is done many times for each pixel, 65 00:03:50,290 --> 00:03:54,730 their individual results get integrated into the pixel color, which in rough terms is kinda 66 00:03:54,730 --> 00:03:57,790 like averaging the results of all the rays. 67 00:03:57,790 --> 00:04:00,180 And that's how we get the final image. 68 00:04:00,180 --> 00:04:04,540 The key point to remember about path tracing in Cycles, is that the rays always start at 69 00:04:04,540 --> 00:04:07,980 the camera, and bounce around the scene until they hit some light source. 7246

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