The depth map is the depth texture as rendered from the light's perspective that we'll be using for testing for shadows. The first pass requires us to generate a depth map. It may sound a bit complicated, but as soon as we walk through the technique step-by-step it'll likely start to make sense. Shadow mapping therefore consists of two passes: first we render the depth map, and in the second pass we render the scene as normal and use the generated depth map to calculate whether fragments are in shadow. We render a fragment at point \(\bar\) is occluded and thus in shadow. In the right image we see the same directional light and the viewer. However, for the sake of shadow mapping we need to render the scene from a light's perspective and thus render the scene from a position somewhere along the lines of the light direction. This projection and view matrix together form a transformation \(T\) that transforms any 3D position to the light's (visible) coordinate space.Ī directional light doesn't have a position as it's modelled to be infinitely far away. We create the depth map by rendering the scene (from the light's perspective) using a view and projection matrix specific to that light source. Using the depth values stored in the depth map we find the closest point and use that to determine whether fragments are in shadow. The left image shows a directional light source (all light rays are parallel) casting a shadow on the surface below the cube. We store all these depth values in a texture that we call a depth map or shadow map. After all, the depth values show the first fragment visible from the light's perspective. What if we were to render the scene from the light's perspective and store the resulting depth values in a texture? This way, we can sample the closest depth values as seen from the light's perspective. You may remember from the depth testing chapter that a value in the depth buffer corresponds to the depth of a fragment clamped to from the camera's point of view. Instead, we use something we're quite familiar with: the depth buffer. We can do something similar, but without casting light rays. Iterating through possibly thousands of light rays from such a light source is an extremely inefficient approach and doesn't lend itself too well for real-time rendering. We then do a basic test to see if a test point's ray position is further down the ray than the closest point and if so, the test point must be in shadow. We want to get the point on the ray where it first hit an object and compare this closest point to other points on this ray. As a result, the floating container's fragment is lit and the right-most container's fragment is not lit and thus in shadow. If we were to draw a line or ray from the light source to a fragment on the right-most box we can see the ray first hits the floating container before hitting the right-most container. The occluded fragments are shown as black lines: these are rendered as being shadowed. Here all the blue lines represent the fragments that the light source can see. Since the light source will see this box and not the floor section when looking in its direction that specific floor section should be in shadow. Imagine a floor section with a large box between itself and a light source. The idea behind shadow mapping is quite simple: we render the scene from the light's point of view and everything we see from the light's perspective is lit and everything we can't see must be in shadow. Shadow mapping is not too difficult to understand, doesn't cost too much in performance and quite easily extends into more advanced algorithms (like Omnidirectional Shadow Maps and Cascaded Shadow Maps). One technique used by most videogames that gives decent results and is relatively easy to implement is shadow mapping. There are several good shadow approximation techniques, but they all have their little quirks and annoyances which we have to take into account. Shadows are a bit tricky to implement though, specifically because in current real-time (rasterized graphics) research a perfect shadow algorithm hasn't been developed yet. For instance, the fact that one of the cubes is floating above the others is only really noticeable when we have shadows. You can see that with shadows it becomes much more obvious how the objects relate to each other. For example, take a look at the following image of a scene with and without shadows: They give a greater sense of depth to our scene and objects. Shadows add a great deal of realism to a lit scene and make it easier for a viewer to observe spatial relationships between objects. When a light source's light rays do not hit an object because it gets occluded by some other object, the object is in shadow. Shadows are a result of the absence of light due to occlusion. Shadow Mapping Advanced-Lighting/Shadows/Shadow-Mapping
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