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The hemispherical fisheye projects the hemisphere in front of the viewpoint on a circular image plane. This type of view may be useful for visibility checks, and to evaluate light impact on a given surface.
![[Hemispherical Fisheye Graphic]](images/viewedit_h.gif)
A hemispherical projection treats each ray as if it was stopped on a unit radius hemisphere, and then projected from that point onto the back plane of that hemisphere. This maps every direction in front of that plane into a circle, at the price of very strong distortion for peripheral angles. Each solid angle of the space in front of the viewpoint is mapped to an area on the image plane that relates to the cosine of the direction of that angle to the view direction.
The maximum horizontal and vertical opening angles for this projection are 180 degrees. The virtual diagonal opening angle of the full image may get greater, while of course the part of it that actually displays any image data is still limited to 180 degrees. The focal length for sizes where the theoretical diagonal angle is greater than 180 degrees is always zero. The area outside of the actual circular image is rendered black by Radiance.
![[Hemispherical Fisheye Diagram]](images/vth_ang.gif)
| The diagram to the left illustrates the mapping of angles onto the image plane with a hemispherical projection. |
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