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2.1 Shadow Physics

In the real world, light and shadow behave in very specific ways: ways that can be predicted, measured, and quantified. In this lesson we dig into some of the real-world physics behind what forms a shadow.

2.1 Shadow Physics

Hello everybody, welcome to Light and Shadow in Photoshop. My name is Kirk Nelson, in this lesson we are gonna take a look at shadow physics. That is the real life physical properties of light and shadow. I set up this very simplistic scene in my studio for the purposes of this illustration. Now this is really nothing more than a blank white tabletop with a paper folded box and a single light source. This is about as simplistic as you can create a scene like this, but there's a surprising amount of physics going on with the light and the shadows. And that's really what I want to dig into in this lesson. I want to evaluate this scene, pick it apart, illustrate what's going on with the light, and analyze how we can best mimic that in our own digital artwork. First of all, this is a single light source, it's just a flood lamp. And when we think about light, we know it travels in a straight line. So we have this mental image of parallel light lines being broadcast out by a light source. Which isn't necessarily a bad way of thinking about it. It's a kind of a simplified means of understanding the way shadows actually work. Because this does illustrate how these lines would represent the light rays, so to speak, and they would just continue on out from the light until they hit something. Which would catch or obscure those light beams. And we can see with the box here, light beams that this box catches obscure those beams from hitting the table, therefore creating the shadow. That's a very simplistic way of understanding the way light and shadow works. I say simplistic because it's actually a lot more complicated than that. One thought is that, if we reduce this light to a single beam, right from the center of it, this is the way we would determine where the edge of our shadow is. So we can see that doesn't quite work because we have a very soft shadow here. What makes this shadow soft? Well, honestly, it has to do with the thickness or the depth of the light source. And every light source that we have will have a depth or thickness to it. About the closest thing that we have to a light source that better represents those exact parallel lines, would be the sun and that's only because the sun is so far away, that the lines coming from the top and bottom of it, do actually appear to be parallel to us. But even shadows, that are cast by bright sunshine, if you look closely at them, actually do have a very slight softness to it. So let's explore what's going on with those soft shadows and what causes those. If we look at the shadow, we can see a corner here that is rather obviously mapped to this top corner of this box, and so if we think of it in terms of the thickness of the light we can map the softness or least the range of that shadow. Cuz if we draw a line from the top of the light, so the highest point that this corner would be receiving light from, from this light source and we draw a straight line from that down to the corner and continue it on to meet the ground or the table top. We get one end of this soft shadow range. And likewise, if we trace out form the bottom of this light source, through that same point we get the other side of that range. Now what's going on here is that this whole area within the box, is receiving gradually more and more of that light. So if we think about it as this point over here as able to see the entire span of this light. So if we take this line, this point is gonna stay where it is on the tabletop. Notice that there is nothing obscuring, the light coming from the entire width of this light source, to hit that point. Therefore, that point is receiving light from the entire light source. But if we look at the other side, this other end here, if we take it from this point and leave it where it is, it's receiving light from this very top edge. But as soon as we start working away down the lamp, this area of that lamp is being obscured by the box. Therefore, it's only receiving a very slight portion of light from this light source, and that's from that very top rim. So if we think of a point right here in the middle, and we wanna think about what all it's receiving light from. Well, we would just trace it down. Clearly it can see the top of the light source and as we continue down, keep going until this line bumps into the corner of that box. So therefore, this point here is receiving light from about this point up to the top of this light source. And that explains what softens shadows. And truth be told we're really considering this on almost a two dimensional plane of our photo here. If we remembered that this light has a three dimensional depth to it. It creates an even more complex way of figuring out the softening of the shadows and that's what's going to lead to softening of the edges over here. Because if we think about the part of this lamp, that's maybe closest to us, cuz we know this is a round lamp. So we think about this edge, that's really close here. This is casting light as well. And if we see it going through this corner that's closest to us, it creates the other side of that soft shadow. So no longer do we have to evaluate the vertical extreme ends of this light source but also the horizontal edges of it as well because it is a circular source and it's casting light out from that entire surface. So you can see how just even a single simple light source isn't quite as simple as you might think it is. There's a lot going on within just the simple shadows and lighting in one very simplistic scene. And not only that, light doesn't tend to terminate when it hits a surface. It tends to bounce, and when it bounces it carries along with it color from that surface that bounced off. So that's why we see things like reflected light, of this green box onto the table here or this lighter color of green in the shadow areas because the light is actually bouncing off the table, back onto the surface here. In fact that's why we can even see any detail or color in this side of the box that's turned away from the lamp, is because light is bouncing around and revealing that side to us. So even this is a very simplified way of approaching the way light works with a scene in how it creates shadows. Not only can we measure and calculate the way a shadow falls according to its position to its light source, but we can also determine the relationship between the amount of lighting on various surfaces within an object. For example, in our cube here, this surface is the furthest away from the light source,so as expected, it will be the darkest. This surface is the closest to it. Which might be surprising because you would expect the top surface to possibly be a little bit brighter but because of the way the cube is turned, this surface actually catches more direct lighting than even the top does. So let's label these as surfaces A, B, and C and then let's bring back our Photoshop interface, and talk about the way Photoshop measures brightness. If we double-click the color chip, we get the eyedropper tool and we get the Color Picker window. The entry I want you to pay attention to, is this brightness percentage, here in HSB. Cuz if I click on the very dark, deep shadow we get very little brightness. That is ten, 13%. And the brighter the area is, the more brightness the value is. So with that in mind, we can actually develop a scale of brightness within Photoshop. If we see Photoshop's brightness 0% as being black, and 100% as being pure white. And then along this scale, we'll figure out where the base brightnesses of these different surfaces are. Now there is some variation within each surface because of the light bouncing that we talked about before. You're gonna get a little bit of a gradient, so you have to sample several times to sort of catch each one. Let's bring our color picker back up and let's start exploring this A surface, the dark ones. I'm seeing a brightness value of right about 25% which falls in right about there on our brightness spectrum. Now let's, begin exploring the brightest side of the cube. This is looking like it's right at about 95%. We see that there, which is right about here. If we consider that A is the dark areas of this cube, and C is the light areas. That means we now have a basic range. That we know all the brightness values within this cube will sit inside. And then this third side that we can see, should fall right in the middle of that. So the brightness of the B side should be the halfway point between A and C. Which if we count this up, this is zero, ten, 20, 30, 40, 50, 60. So let's bring up our color picker. And see how we did. Look at that, brightness is right at 60%, that works out pretty well. So how do we use this in our own illustrations? Well, there's actually a formula that we can use. I know that looks really scary like that, so let's just walk through that, make it a little bit easier to look at. So if we're trying to find the brightness of the B side. Essentially it is the brightness of C, minus the brightness of A divided by 2, to find the halfway point but then you have to add the A brightness for this offset so to speak. And that's what would give you the brightness level or at least the base brightness level of B. So let's plug in some values here. So, our B brightness would be C brightness, which is 95, minus the A brightness, which is 25, divided by 2 plus that 25. And if you plug that all into a calculator you will find that it comes right out at 60, which is what our graph here told us and we also verified by using the color picker tool. This seems very complicated and complex, but actually it's not that bad, and it's just a very fascinating way that light actually works. And when we understand these properties, we can create more convincing light and shadows within our own illustrations. So that's a basic understanding of the way the physics of lighting actually works, in our physical universe. Next lesson we start dealing with the different parts of a shadow, in a lesson that I like to call, shadow anatomy.

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