POV-Ray : Newsgroups : povray.binaries.images : Luna, Moon... desert satellite Server Time
7 Dec 2021 03:45:41 EST (-0500)
  Luna, Moon... desert satellite (Message 40 to 47 of 47)  
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From: Kenneth
Subject: Re: Luna, Moon... desert satellite
Date: 16 Sep 2021 14:55:00
Message: <web.614392dd756c3d5a4cef624e6e066e29@news.povray.org>
"Samuel B." <stb### [at] hotmailcom> wrote:
> "Kenneth" <kdw### [at] gmailcom> wrote:
> >
> > I'm still trying to spot those flaws, but I don't see them :-/
>
> The issue of the craters not casting shadows is just that: if the object was
> using actual displaced geometry you would see shadows cast across the basins on
> the craters. But all you get on a simple bump-mapped sphere is shading... No
> displacement of geometry, no shadows.

Ah yes, I see what you mean at the terminator (I wasn't even paying attention to
shadowing there, duh.) Other than typical(?) behavior of a bump_map re: lack of
real geometry shadows occurring, my guess is that the raking Sun-light itself is
naturally more dim there-- so maybe any shadows (or even simple shading) from
the normals is being swamped illumination-wise by the effect of the dim
'ambient' light at the terminator, and/or your bloom effect.

I've honestly never given much thought to the exact effect that occurs when
light shines on a bump_mapped surface; it sort of 'looks' like shadowing from
actual geometry... enough to fool me, ha... but seems to be just simple shading,
as you point out.


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From: Samuel B 
Subject: Re: Luna, Moon... desert satellite
Date: 16 Sep 2021 19:20:00
Message: <web.6143d008756c3d5acb705ca46e741498@news.povray.org>
Cousin Ricky <ric### [at] yahoocom> wrote:
> On 2021-09-15 6:22 PM (-4), Samuel B. wrote:
> >
> > "A" points to three features I suspect might have been produced by asteroids
> > scraping the surface. Notice how they seem to have occurred on the sides or
> > upper portions of a (really large) sloping hill? To me that indicates they were
> > perhaps caused by asteroids hitting the moon at glancing angles... But I can
> > definitely see how they could have been caused by collapsed lava tubes.
>
> I'm guessing rills; if not that, collapsed lava tubes.  It's not
> intuitive, but as far as I know, impact craters are always circular,
> regardless of impact angle.  If anything, a glancing impact might create
> a series of craters, rather than a linear gouge.

Hmm, interesting. Is it due to the high speeds at which the asteroids are
traveling, I wonder? Like, they're traveling so fast that any impact becomes an
explosion?

Sam


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From: Samuel B 
Subject: Re: Luna, Moon... desert satellite
Date: 16 Sep 2021 19:20:00
Message: <web.6143d0be756c3d5acb705ca46e741498@news.povray.org>
Thomas de Groot <tho### [at] degrootorg> wrote:
> Op 16/09/2021 om 00:10 schreef Samuel B.:
> > "Kenneth" <kdw### [at] gmailcom> wrote:
> >> I'm still trying to spot those flaws, but I don't see them :-/
> >
> > The issue of the craters not casting shadows is just that: if the object was
> > using actual displaced geometry you would see shadows cast across the basins on
> > the craters. But all you get on a simple bump-mapped sphere is shading... No
> > displacement of geometry, no shadows. It's not really a big issue at smaller
> > resolutions, but I think it is noticeable.
> >
> Ah, yes... that is indeed true. It may account also for the phenomenon I
> observe here, of the lunar light/shadow features optically switching to
> their opposites. I have to zoom out to get the correct aspect again.

I had that happen, too: suddenly all the craters inverted themselves and became
bumps. I'm sure it would not have happened so easily if the craters had proper
shadows.

Sam


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From: Cousin Ricky
Subject: Re: Luna, Moon... desert satellite
Date: 17 Sep 2021 00:38:10
Message: <61441bb2@news.povray.org>
On 2021-09-15 8:48 PM (-4), Cousin Ricky wrote:
> On 2021-09-15 6:22 PM (-4), Samuel B. wrote:
>>
>> "A" points to three features I suspect might have been produced by asteroids
>> scraping the surface. Notice how they seem to have occurred on the sides or
>> upper portions of a (really large) sloping hill? To me that indicates they were
>> perhaps caused by asteroids hitting the moon at glancing angles... But I can
>> definitely see how they could have been caused by collapsed lava tubes.
> 
> I'm guessing rills; if not that, collapsed lava tubes.  It's not
> intuitive, but as far as I know, impact craters are always circular,
> regardless of impact angle.  If anything, a glancing impact might create
> a series of craters, rather than a linear gouge.

@Thomas de Groot - Are you aware that your reply to this post went to my
personal email, rather than the newsgroup?


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From: Thomas de Groot
Subject: Re: Luna, Moon... desert satellite
Date: 17 Sep 2021 02:07:38
Message: <614430aa$1@news.povray.org>
Op 17/09/2021 om 06:38 schreef Cousin Ricky:
> @Thomas de Groot - Are you aware that your reply to this post went to my
> personal email, rather than the newsgroup?
> 

Oh! I am truly sorry :-(

Wrong button on Thunderbird obviously. I guessed something was wrong 
when I didn't see my message appear here.

-- 
Thomas


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From: Thomas de Groot
Subject: Re: Luna, Moon... desert satellite
Date: 17 Sep 2021 03:02:38
Message: <61443d8e$1@news.povray.org>
Op 16/09/2021 om 18:02 schreef Alain Martel:

>> On 2021-09-15 6:22 PM (-4), Samuel B. wrote:
>>>
>>> "A" points to three features I suspect might have been produced by 
>>> asteroids
>>> scraping the surface. Notice how they seem to have occurred on the 
>>> sides or
>>> upper portions of a (really large) sloping hill? To me that indicates 
>>> they were
>>> perhaps caused by asteroids hitting the moon at glancing angles... 
>>> But I can
>>> definitely see how they could have been caused by collapsed lava tubes.
>>

>> intuitive, but as far as I know, impact craters are always circular,

>> a series of craters, rather than a linear gouge.
>>
> 
> There are a very few craters that are not circular. You can count them 
> on your fingers. When the impact angle is extremely shallow, the crater 
> can get elongated.
> I've seen photos of three of those.

In my answer to the ng which went to the wrong destination, I commented 
(more or less) as follows:

A: those are colapsed rills indeed.

B: those are my "lava flows". Possibly, they may be also or partly, 
something else, like almost completely buried impact crater rims. The 
curvature of some parts may hint at this. However, my first gut feeling 
is "lava flows" (I might still be wrong). ;-)


@Alain Martel: you are right. See for instance: 
https://www.nature.com/articles/s41467-020-15269-x


-- 
Thomas


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From: Alain Martel
Subject: Re: Luna, Moon... desert satellite
Date: 17 Sep 2021 10:12:08
Message: <6144a238$1@news.povray.org>

> Cousin Ricky <ric### [at] yahoocom> wrote:
>> On 2021-09-15 6:22 PM (-4), Samuel B. wrote:
>>>
>>> "A" points to three features I suspect might have been produced by asteroids
>>> scraping the surface. Notice how they seem to have occurred on the sides or
>>> upper portions of a (really large) sloping hill? To me that indicates they were
>>> perhaps caused by asteroids hitting the moon at glancing angles... But I can
>>> definitely see how they could have been caused by collapsed lava tubes.
>>
>> I'm guessing rills; if not that, collapsed lava tubes.  It's not
>> intuitive, but as far as I know, impact craters are always circular,
>> regardless of impact angle.  If anything, a glancing impact might create
>> a series of craters, rather than a linear gouge.
> 
> Hmm, interesting. Is it due to the high speeds at which the asteroids are
> travelling, I wonder? Like, they're travelling so fast that any impact becomes an
> explosion?
> 
> Sam
> 



They ARE travelling so fast that the impact cause an explosion.
Supersonic projectiles, velocity in the 1 to 2 km/s range, tend to cause 
circular impact craters within a broad range of incident angles. For the 
asteroids, that's about 8 to 27 Km/s.


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From: Bald Eagle
Subject: Re: Luna, Moon... desert satellite
Date: 17 Sep 2021 14:15:00
Message: <web.6144da80756c3d5a1f9dae3025979125@news.povray.org>
Alain Martel <kua### [at] videotronca> wrote:

> They ARE travelling so fast that the impact cause an explosion.


https://www.wired.com/2013/08/a-scientist-and-his-gun/


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