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Image-based lighting, or IBL, is a technique for lighting a 3D scene using a special type of image file. This is known as a high dynamic range or HDR file. Instead of conventional 3D lights, the scene is illuminated by the brightness of pixels in an environment map. The image is usually captured from a real-world location, and because of this, the 3D rendered objects can be made to look as if they're actually in that real environment. Image based lighting requires that the environment map have a greater range of brightness values than a standard image. A high dynamic range image can capture all the possible brightness values that the human eye can see.
The dynamic range of a signal is the ratio between its highest and lowest values. And for light, it's also expressed as contrast ratio. The human eye has a contrast ratio of about a million to one. But a standard camera or an ordinary image file has a contrast ratio of about a thousand to one. Contrast ratio can also be expressed in terms of F-stops. The F-number is a setting on a real camera that corresponds to how much light is entering through the iris. Each F-stop lets in about twice as much light. The human eye can perceive about 20 stops.
However, most cameras can only capture about ten stops. But remember that each F-stop lets in twice as much light. The ten-stop difference between the camera and the eye corresponds to a value of 2 to the 10th power, or 1024. So the dynamic range of the eye is about a thousand times greater than that of a standard camera. To capture all of that dynamic range, the camera will have to take multiple photos at different exposure settings. This is called bracketing. The bracketed images are combined in post-processing to synthesize a single image with all of the possible brightness values.
The conditional method for capturing the lighting of a location is to take bracketed photos of a mirrored sphere. This is called a light probe. These images can be purchased over the Internet for use in your productions. In this course, we'll use a slightly different method of image capture, using a fisheye lens, but the IBL and HDR concepts are the same. In order to store a thousand times more information, special high dynamic range file formats have been developed, such as HDR and EXR. Instead of just 8 bits per pixel per channel, a high dynamic range image can store 16 or 32 bits.
A standard image such as a JPEG can store about 16 million colors. But a 32-bit HDR file can store 79 octillion colors. That's 79 followed by 27 zeros. But it needs all of that data in order to store the full dynamic range that the human eye can see.
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