scott wrote:

> For my sun radius calculation I used the values from wikipeda, distance 
> to sun is 1.5e8 and radius of the sun is 6.96e5, dividing those two 
> gives the 214.8.

Yes, that is the correct value known since antiquity (the ratio is
simply the sine of the apparent angle that is readily observable). It
was the figuring out the actual distances/ radii that took longer ;)

http://en.wikipedia.org/wiki/Aristarchus_of_Samos

Regarding the Sun at least we rarely see the actual disk as it is lost
in the glare, and this has a larger size. And at sunset when we see it
we perceive it as larger than we would overhead:

Overhead, we tend to assume an object is not so far away (maybe typical
cloud distance). On the horizon an object is perceived to be very far
away. So the same angular size is interpreted as a larger object. This
effect persists even when no objects are available for comparison, so
that often heard explanation is an urban myth.

> IMO it also depends heavily on the angle/focal length of the camera you 
> are using, you can easily make the sun or moon look way too big or way 
> too small (IRL and in POV). 

The thing here is that the angular size of the sun is fixed, but the
angular size of other objects in the scene depends on the distance. And
in a photo or render the distance is not readily apparent, we need to
estimate it from the scale of objects.

As an extreme example suppose you simulate a telescopic view with angle
1 degree on a house on the horizon. Now the image contains a house of
reasonable size with a sun filling half the image behind it, so it looks
way to big. But if we were smarter we would judge scale from the Sun and
conclude the house is either a tiny model or very far away).

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scott <sco### [at] scottcom> wrote:
> I've created a macro called SkySim (I'll post it in p.b.s-f once I've
> tidied it up and added comments) that creates a realistic looking sky
> pigment in a sky_sphere based on the sun position and the "haziness" of
> the sky.

These look great, I am struggling with creating a realistic sky colour at the
moment so would look forward to giving this a try.

Sean

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Thanks Christian. Very comprehensive.

Thomas

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scott <sco### [at] scottcom> wrote:
> I've created a macro called SkySim (I'll post it in p.b.s-f once I've
> tidied it up and added comments) that creates a realistic looking sky
> pigment in a sky_sphere based on the sun position and the "haziness" of
> the sky.

Very nice. I'll definitely be using this!

Bill

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Thomas de Groot <tho### [at] degrootorg> wrote:
> Question:
>
> For the visible Sun, you give: #local SunRadius = vlength(sp)/214.8;
>
> I seem to remember that Cousin Ricky evaluated the apparent size of the
> Sun as to be twice as much, i.e.: #local SunRadius = vlength(sp)*2/214.8;

He is correct.  The formula you attribute to me is for the /diameter/ of the
Sun, not the radius.  (References to celestial objects tend to give diameters
instead of radii.  Another possibility is that the original conversation was
about area_lights.)

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On 9-6-2013 19:26, Cousin Ricky wrote:
> He is correct.  The formula you attribute to me is for the /diameter/ of the
> Sun, not the radius.  (References to celestial objects tend to give diameters
> instead of radii.  Another possibility is that the original conversation was
> about area_lights.)
>

I stand corrected! The original discussion was /indeed/ about area lights.

I mixed up radius and diameter subsequently.

How little things may wreak havoc in the celestial clockwork ;-)

Thomas

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I wonder at your scene settings.

With you test scene, I get the following image. Sun at 12:00 hours. Way 
too dark overall.

Using for all my scenes version 3.7RC7, with assumed_gamma=1 and 
Display_Gamma=sRGB.

Thomas

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Attachments:
Download 'skysimtestground.png' (148 KB)

Preview of image 'skysimtestground.png'

> I wonder at your scene settings.
>
> With you test scene, I get the following image. Sun at 12:00 hours. Way
> too dark overall.

You can adjust the EXP variable in the test scene to control the 
brightness of the sky (it controls how the physical brightness values 
calculated are converted to POV units). Try increasing it from 4e-5 to 
6e-5 or even higher.

I have no idea what the bright yellow patches on the ground are - I 
didn't get those on mine???

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On 17-6-2013 11:34, scott wrote:
> You can adjust the EXP variable in the test scene to control the
> brightness of the sky (it controls how the physical brightness values
> calculated are converted to POV units). Try increasing it from 4e-5 to
> 6e-5 or even higher.

Yes I did that indeed, but somehow the scene never comes close to the 
aspect of a regular light and sky_sphere.


> I have no idea what the bright yellow patches on the ground are - I
> didn't get those on mine???

I added radiosity, and they seem to come from that. Without radiosity 
the landscape remains totally dark whatever the value for EXP and the 
hour of the day.

Thomas

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I think I found the culprit: Your original SunPos() is divided by 1000!

Thomas

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