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"One of the easiest ways to create a light probe image is to acquire a high dynamic range image of a mirrored ball; other techniques involve stitching multiple images together (such as with RealViz's Stitcher), or using a scanning panoramic camera such as the ones available from Panoscan or Spheron.
.....by taking two pictures of a mirrored ball at ninety degrees of separation and assembling the two radiance maps into this registered dataset. The coordinate mapping of these images is such that the center of the image is straight forward, the circumference of the image is straight backwards, and the horizontal line through the center linearly maps azimuthal angle to pixel coordinate.
Thus, if we consider the images to be normalized to have coordinates u=[-1,1], v=[-1,1], we have theta=atan2(v,u), phi=pi*sqrt(u*u+v*v). The unit vector pointing in the corresponding direction is obtained by rotating (0,0,-1) by phi degrees around the y (up) axis and then theta degrees around the -z (forward) axis. If for a direction vector in the world (Dx, Dy, Dz), the corresponding (u,v) coordinate in the light probe image is (Dx*r,Dy*r) where r=(1/pi)*acos(Dz)/sqrt(Dx^2 + Dy^2).
Note that each light probe image represents a full 360 × 360 degrees, or 4pi steridians. Also note that the mapping being used is different than the mapping one observes in a mirrored ball -- the mapping we're using avoids the problems of poor sampling near the backwards-facing directions around the circumference of the image."
Es geht also um die "Spannbreite" der Helligkeitsmöglichkeiten. HDR "kennt" nicht nur hell (255/255/255), sondern auch "viel heller" (als ein "Monitor" darzustellen bereit wäre). Für die Berechnungen in einem 3D-Programm macht dies aber eben Sinn.The difference between HDR (High Dynamic Range) images and normal RGB images is basically the amount of information the format can hold. Since HDR images can hold so much more information you are able to get extreme contrast within the image and use that contrast to provide your scene with true reflective specularity and accurate illumination.
wookenny hat gesagt.:Der Vorteil bei den HDR Images ist ja, das die Helligkeit unabhängig von der Farbe zusätzlich gespeichert wird. Und das ist bei nem jpeg eben nicht so.