IBL in a Nutshell
Traditionally, a scene is lit by the sun and/or one or several additional lights. The Light Lab lets you configure the properties of those lights. The appearance of materials may change dramatically depending on the setup of the lights, their position, color, and intensities.
Image based lighting is a completely different approach to illuminating a scene. Here, a radiance picture - a picture with a very high dynamic range - is mapped on the inside of an imaginary sphere surrounding the whole "world.". It is a spherical panorama, 360° wide and 180° high, all around the scene, including sky and ground. The bright parts of the picture are so bright that they actually radiate light and thus illuminate the scene and can even throw shadows. It doesn’t matter whether that sphere is invisible in the render or the HDR image is rendered as a backdrop.
Image based light simulates the physics of light much more accurately than the light sources traditionally available in Bryce. When image based light is used exclusively to light a scene, the effect of some channels in the Materials Lab change. The Specularity channel in particular becomes almost ineffective because specularity is an artificial property used to simulate blurry reflection from a light source. In nature, there is no specularity as such - what we perceive as a quality of glossiness on a body is in fact a blurred, defocused reflection from a bumpy surface that disperses the light reflected on it. Consequently, you’ll have to substitute specularity with the Reflection channel.
Image based light can nicely simulate light with scientifically correct properties but there are moments when you want to pursue your own visions and such correctness is not an issue for you at all. You need that bright specularity on a body to make a point in your artwork and doing it with the Reflection channel might prove a bit of a challenge at times. You may have to resort to mixed light.
Just as a scene lit by no other means than the Bryce sun may give very pleasing results, the same is true when using image based light exclusively. However, this exclusivity also means limitation. Consider using image based light in conjunction with the traditional Bryce light sources. Then you have all the options you need to make your work look as extraordinary as you wish.
If you render the HDRI as background you might be disappointed by its blurriness. It is blurred because it is too small. If your document is 800 pixels high and the camera FOV set to 30°, you’ll need an Angular Map of 7,700 x 7,700 pixels to get it well focused. Even the most state of the art home computers of today will start to grunt under this load.
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