#Terragen skies 32 bit

That's very close to the default sunlight which does indeed give us a nicely exposed clear blue sky at noon. If I divide that by 92525 to get the sunlight strength parameter, I get 5.783. If I wanted to expose the sky so it has a value of 0.5 in the image, the sun luminance should be 303,750 * 0.5. The ratio of sun luminance to blue sky luminance should be 303,750. So I wanted to do a sanity check against the other number you gave, the 8000 nits for a blue sky on a clear day at noon. This might be the first time I've derived these figures, or it was so long that I don't remember doing it. But I would try to standardize on either 1 or 5 for the sun intensity and then you have a constant scaling factor. Unfortunately because Terragen outputs only 16-bit EXR at the moment, you can't create an EXR with actual values in nits, if that's what you wanted. If the pre-atmosphere sun needs to be 2.43e9 nits, and you rendered with sun intensity 5 and exposure 1, you should scale the EXR output by 2.43e9 / (26263 * 5) ~= 18505. If the pre-atmosphere sun needs to be 2.43e9 nits, and you rendered with sun intensity 1 and exposure 1, you should scale the EXR output by 2.43e9 / 26263 ~= 92525. Incidentally that's beyond what 16-bit EXR can store, but if you increase the diameter 0.75 that brings the luminance in range. In the default scene the sunlight intensity is actually 5 so this creates pixels with 5 times that value. That's for sun strength and exposure of 1. That's because luminance is inversely proportional to the square of the angular diameter (for relatively small angles) so that the total energy from the sun remains the same. If you change the sun's angular diameter you get a different value. That means if you use a sunlight strength 1 and camera exposure 1, pre-atmosphere the sun will appear with values of about 26263 in the EXR. With the sun's angular diameter at 0.5 degrees (approximately correct for Earth), the math I use in Terragen creates a pixel luminance ratio of approximately 26263. That's the scale that everything else derives from. In the absence of atmosphere, a Terragen sunlight strength of 1 falling directly (perpendicularly) onto a diffuse surface with albedo 1 seen with camera exposure 1 produces a pixel of value 1 in the EXR. We just need to figure out the constant scalar to match the target of 2.43e9 nits. I've aimed to make all the ratios in Terragen physically plausible with default atmosphere settings, so the ratio of sun to sky luminance should be about right, and the surface of the sun should be the physically correct ratio to the illumination it provides. But it should be possible to work out the scaling factor with the information we have. QuoteLuminance values in Terragen are based on pixel values without any real-world units, because the camera exposure is also without units. Here is some info on real-world lighting equivalence for Terragen, quoted from Matt Fairclough, Lead Developer/Software Architect:

Those HDRIs at SAStudios are created in Terragen 4. I don't really know how this can be measured accurately, without real world values.

It claims that it has a dynamic range of 25-30 stops. Can you import a physical camera that measures shutter speed, aperture, and exposure? How can you render a cg sky in Terragen and mimic desired real-world measurements like lux values and sun to shadow ratios? At least to gauge the accuracy of the the cg sky. However, Terragen's rendering camera doesn't have a physical camera. This is better than most commercially available panoramic hdris because of the the technical limitations of phyiscal cameras (except if you use multiple ND filters).

#Terragen skies 32 bit

From looking at Terragen 4, it can render very dynamic hdris with a sun intensity that rendered unclamped and spans the whole range of a 32 bit float texture. I am researching a good way to create physically accurate hdri panoramas for Realtime rendering game engines.