9.5.2

20 • Global Illumination
Global Illumination• 21
Each time a stochastic sample hits a surface, a new dome of stochastic samples is sent out from that point.
This second generation of stochastic samples may in turn trigger a third wave of stochastic samples and so on.
This chain reaction of events continues with further generations of stochastic samples created until a limiting
factor has been reached — as specied in the GI settings.
At this point, the various surfaces hit by the stochastic samples are taken into account to calculate the indirect
light for the shading points. (The shading points are those points hit by the camera rays — the points that
created the rst generation of stochastic samples.) Finally, the values for the shading points are interpolated
to calculate the indirect light for each image pixel.
HDRI
What exactly are HDRIs (High-Dynamic Range Images)?
HDRIs are ideal for use in combination with GI and to recreate very natural and realistic lighting situations.
HDRIs also have a big impact on the realism of reections because they can produce reections on objects
that are much brighter than reections caused by normal textures.
In short, HDRIs are images with a very high brightness range, much higher than normal RGB images, which
are limited to a brightness range of 8 bits (256 variations) per color. Standard RGB images have the limitation
of the brightest possible color being a 255/255/255 white. If you use such an image in combination with GI to
illuminate a scene, even the brightest white won’t be bright enough to simulate a real lighting situation. The
resulting renderings often look at and lack contrast. That’s because in reality the difference between a dark
light (e.g. candle ame) and a bright light (e.g. sun) is too big to be represented within an 8-bit RGB image.
However, using a HDRI, the sun could have a brightness value of, say, 6000% that is taken into account by the
GI algorithm and when rendering reections. This results in beautifully and realistically illuminated scenes.
The following pictures illustrate the tremendous difference that HDRIs can make:
Without HDRI.