I looked outside a little while ago, and observed that the sky was blue.

But when I asked POV-Ray to do some radiosity with a blue sky, the 
entire scene came out blue. Which, logically speaking, isn't surprising. 
If the sky is blue, clearly it's emitting blue light, which would result 
in everything being illuminated blue.

And yet, when I look outside, the world is /not/ tinged blue. So... WTH?

Post a reply to this message

Invisible <voi### [at] devnull> wrote:
> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?

Your visual cortex is good at correcting for it. Also, the sun casts a lot more
light than the sky, so in direct sunlight the effect is muted. And of course,
cloud cover will hide blue sky-light and replace it with diffuse sunlight.

On a cloudless day, I think the blue tinge to shadows is quite obvious.

Post a reply to this message

Le 15/06/2012 14:13, Invisible nous fit lire :
> I looked outside a little while ago, and observed that the sky was blue.
> 
> But when I asked POV-Ray to do some radiosity with a blue sky, the
> entire scene came out blue. Which, logically speaking, isn't surprising.
> If the sky is blue, clearly it's emitting blue light, which would result
> in everything being illuminated blue.
> 
> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?

If the sky is "blue", why do you see a yellow sun ?
What happened to the sky at night so that the moon is white ?

IIRC, the blue of the sky is due to the variant IOR (vs void) of the
atmosphere for the "white" light: the blue is turned more toward the
earth (well, sooner: it might explains a bit the redish-moon eclipse).

It's not like the sky is rgb(0,0,1) (or even rgb(0,0.8,1)) but rather
more alike hsl(240, ??, 1000000) (outside the traditional rgb cube)

or in povray term, more like blue with a very high transmit colour of
white, maybe with a subtle tinct of blue inside... (which cannot be
expressed so far in the current SDL)

Strictly speaking, the sky does not emit light. It's a dispersing media
at best. Faint media, of large dimensions.


Also, do not forget the capability of the eyes to accomodate to various
intensity of light. The white-balance done with camera is needed in
professional production: the light is not always the same.

Post a reply to this message

Am 15.06.2012 14:52, schrieb Le_Forgeron:

> IIRC, the blue of the sky is due to the variant IOR (vs void) of the
> atmosphere for the "white" light: the blue is turned more toward the
> earth (well, sooner: it might explains a bit the redish-moon eclipse).

Not really; the dispersion (wavelength-dependency of IOR) of the 
atmosphere in the visible light spectrum is rather low. But the gases in 
the atmosphere scatter a certain portion of the sunlight's photons, and 
at the blue end of the visible spectrum this portion is bigger than at 
the red end.

Post a reply to this message

> I looked outside a little while ago, and observed that the sky was blue.
>
> But when I asked POV-Ray to do some radiosity with a blue sky, the
> entire scene came out blue. Which, logically speaking, isn't surprising.
> If the sky is blue, clearly it's emitting blue light, which would result
> in everything being illuminated blue.
>
> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?

Because there's usually a light_source{ <0,0,0> color Yellow*6500 } in 
the scene.

Look at shadows on snow or on a beach, they'll be blue.

http://www.free-desktop-backgrounds.net/free-desktop-wallpapers-backgrounds/free-hd-desktop-wallpapers-backgrounds/122079702.jpg

http://www.amandapair.com/wp-content/uploads/2009/09/shadows-in-sand.jpg


-- 
/*Francois Labreque*/#local a=x+y;#local b=x+a;#local c=a+b;#macro P(F//
/*    flabreque    */L)polygon{5,F,F+z,L+z,L,F pigment{rgb 9}}#end union
/*        @        */{P(0,a)P(a,b)P(b,c)P(2*a,2*b)P(2*b,b+c)P(b+c,<2,3>)
/*   gmail.com     */}camera{orthographic location<6,1.25,-6>look_at a }

Post a reply to this message

Le_Forgeron <jgr### [at] freefr> wrote:
> If the sky is "blue", why do you see a yellow sun ?

  The sun is not yellow. It's white.

-- 
                                                          - Warp

Post a reply to this message

Invisible <voi### [at] devnull> wrote:
> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?

  Actually it is. Consider this rendering, for example:

http://www.ignorancia.org/uploads/images/rocky-desert/rocky-desert-09-sbtf.jpg

  Everything's tinted blue. Does it look unrealistic? No.

-- 
                                                          - Warp

Post a reply to this message

On 15/06/2012 04:14 PM, Warp wrote:

>    The sun is not yellow. It's white.

I had always assumed that the actual sunlight is white, and the Earth's 
atmosphere scatters the blue end of the spectrum more than the red, thus 
resulting in a sky that looks blue and a sun that looks comparatively 
yellow.

Then again, on some evenings, the sky actually appears to show a (very 
unsaturated) rainbow gradient, with a discernible green band...

Post a reply to this message

>> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?
>
> Because there's usually a light_source{ <0,0,0> color Yellow*6500 } in
> the scene.

So I just need to turn up the Sun a bit higher and make it more yellow?

> Look at shadows on snow or on a beach, they'll be blue.
>
>
http://www.free-desktop-backgrounds.net/free-desktop-wallpapers-backgrounds/free-hd-desktop-wallpapers-backgrounds/122079702.jpg
>
> http://www.amandapair.com/wp-content/uploads/2009/09/shadows-in-sand.jpg

Those are some neat images. They both look like the Sun is quite low in 
the sky though; and we all know that when that happens, you can get 
strange colours happening.

Post a reply to this message

On 15/06/2012 04:28 PM, Warp wrote:
> Invisible<voi### [at] devnull>  wrote:
>> And yet, when I look outside, the world is /not/ tinged blue. So... WTH?
>
>    Actually it is. Consider this rendering, for example:
>
> http://www.ignorancia.org/uploads/images/rocky-desert/rocky-desert-09-sbtf.jpg
>
>    Everything's tinted blue. Does it look unrealistic? No.

Uh... doesn't actually look all that blue to me.

Post a reply to this message