Rendering Using Sky Images as Light Sources

I rendered the following bunny images in Photorealizer using equirectangular HDR sky images as light sources.

Bunny at sunrise / sunset.

Bunny during twilight.

Below are LDR versions of the sky images that I used for the bunny renders above. These are the same images that I posted and described in the previous post.

Sunrise / sunset.

Twilight.

Note the realistic deep blue tone of the twilight images. This blue is a result of ozone absorption. Without ozone, these images would be a dull gray.

Converting Between Fisheye and Equirectangular

I used Photorealizer to convert some of my recent fisheye sky renders to equirectangular (latitude–longitude) format. To give Photorealizer the capability to do this, I copied the panoramic cameras from the sky renderer to Photorealizer, and gave Photorealizer the ability to use a high dynamic range fisheye image as an environment map. Then, to do the actual conversion, I simply set up an environment map using a sky renderer render, and rendered a picture of it using the equirectangular camera (an analogous process could be used to convert from equirectangular to fisheye).

I wanted to convert the fisheye images to equirectangular format because Photorealizer already supports importance sampling for equirectangular environment maps. I plan to use these images as light sources for Photorealizer renders, so being able to sample them efficiently—even when they contain the sun—is very important.

Alternatively, I could have used existing software to do the conversion, but I prefer to do things myself when possible. Or I could have re-rendered the sky images in equirectangular format, but that would have taken a long time. The way I did the conversion worked very well, and it let me add features to and make improvements to Photorealizer in the process.

Here are two sets of before and after images:

Before: sunrise / sunset fisheye render.

After: converted to equirectangular format using Photorealizer.

Before: twilight fisheye render.

After: converted to equirectangular format using Photorealizer.

I recently made some other relevant improvements to Photorealizer as well, which you can read about on my Photorealizer blog:
http://photorealizer.blogspot.com/2012/12/environment-map-improvements.html
http://photorealizer.blogspot.com/2012/12/photon-mapping-with-hdr-environment-maps.html

Twilight with Aerosols

Here are two new twilight images that I rendered with aerosols. I haven't tone-mapped either of these images—they are display-linear. In the second image, the sun is lower, there is a lower concentration of aerosols in the atmosphere, and the ground has a desert-like reflectivity. Because the aerosol concentrations and ground reflectivities differ, the images are not directly comparable, but I believe that a few of the differences are due to factors besides the aerosols. Based on my research (including Color and Light in Nature and The Influence of Ozone and Aerosols on the Brightness and Color of the Twilight Sky) and observations, at −4.8° I would expect a more defined twilight arch, an area of low saturation purple above the twilight arch, and more saturated blue in the zenith direction. All of these phenomena seem to be present in the −4.8° render.

−3° solar elevation angle.

−4.8° solar elevation angle. Half the amount of aerosols as in the
image above.