Christian Walther <cwa### [at] gmxch> wrote:

>
> Be careful with the word "linear" - what you mean is "linear to
> perceived brightness" (right?), whereas I (and again, I guess most other
> people with some background in the field) say "linear" when I mean
> "linear to physical light intensity". That could lead to
> misunderstandings -

I've tried to use the word "linear" solely as a way of describing the
relative differences between my initial grey-band test values. That is,
.....05, .10, .15, .20, etc. changing "linearly" by .05 each time. And
"non-linear" as being deviations from that. I thought I made this clear in
my initial post, but if I've muddled the word usages since then, my
apologies. So "linear to perceived brightness" would be correct, as to my
usage.
>
> ...you may change the sphere's reflectivity properties using the
> "brilliance" keyword. With "finish { ambient 0 diffuse 1 brilliance 2 }"
> (and assumed_gamma 1), a similar image to Ard's lower one is achieved.
> (I'm too lazy now to look up what brilliance does exactly, but it may be
> that  "brilliance <display_gamma>" exactly reproduces that image.)
>
Hey, that's a  brilliant use of brilliance! ;-) But upon re-reading the POV
docs explaining that keyword, no mention is made of your particular use of
it.  Quite an important, basic use!  If brilliance was actually intended
for correcting the lighting of an object, shouldn't that have been covered
in a major way in the POV docs? Such an omission makes me wonder if
"correcting" for lighting anomolies...when using assumed_gamma of 1.0...was
ever even considered!

I'm following your overall logic quite well. Yes, we ARE approaching this
topic from two different standpoints. Yours seems to be that of the
scientist/engineer...and that's fine, of course. Wheras, I think of
POV as an artist's tool, one where <.5,.5,.5> being
"half as perceptully bright as white" has real validity. Let me illustrate
my mind-set this way: I came to POV from having used Photoshop for years.
As a graphics toolset, PS allows me to pick color/brightness values either
visually or by choosing rgb values. Numerically, it's own range is from 0
to 255. But the important thing is that, if I choose 127,127,127...right in
the middle...I do indeed get an on-screen gray that is "half as perceptully
bright as white." (Which is what I thought POV's <.5,.5,.5> should give me
as well.) Photoshop is, in effect, insulating me from the need to worry
about gamma correction. A beautiful, intuitive way of working!!  Yes, I
assumed from the get-go that POV operated like Photoshop...leading me to
use simple and intuitive "linear" color values in all my scenes (and an
assumed_gamma of 2.0 rather than 1.0, to "visually correct" for
that in the POV render preview...against the wishes of the
POV docs.) Believe me, I do now understand
that POV does NOT insulate me from gamma worries!!

So which philosophy of use is the more correct? I think it depends on the
use to which POV is put, and the mind-set of the user.  Purists and others
may disagree. I'm not trying to be "stubborn" about this. From my own
standpoint, of simply wanting to produce nice, realistic,
perceptually-pleasing images on my own system, then using assumed_gamma of
2.0 is just...easier and more intuitive...while 1.0 creates hurdles and
difficulties that have to be constantly addressed: having to specify
"non-linear" color values; having to add and tweak other POV values (like
brilliance) because of lighting anomolies; having to deal with image_map
images (created in any typical graphics program) that don't render
correctly unless they are in the .png format with an embedded gamma of 1.0.
These are time-consuming and non-intuitive drawbacks...to me, anyway.

If my use of assumed_gamma of 2.0 is robbing POV of its intended way of
working, I honestly won't shed too many tears. Not at the present time,
anyway! ;-) (If doing so created odd PROBLEMS in my scenes, I would think
otherwise; but so far I haven't seen gross manifestations of
that...not even subtle ones, for that matter...or
perhaps I've just figured out my own way around them.) I'm not closing the
door on assumed_gamma of 1.0; but so far, the cons outweigh the pros. And I
guess I'll just have to deal with POV 3.7 when it arrives (in its mature
state.)

My wish, from an artist's standpoint:  WOULDN'T IT BE NICE IF...POV allowed
an alternate way of working, one that allowed the use of "linear" color
values (as in Photoshop, and as in my initial POV gray-band test scene),
CHANGED those values internally to work in its own ideal color space of
assumed_gamma 1.0, and then re-massaged THOSE values before spitting them
back to the monitor, so that they appeared perceptually correct in the
user's chosen gamma environment? This would allow us graphics-oriented
folks to work in a way, and in an environment, that we're used to. It seems
that all the ingredients are there to do so..by a different internal use of
display_gamma and assumed_gamma, perhaps.  (Of course, my own use of
assumed_gamma of 2.0 mimics that behavior! But as has been pointed out,
that's not the correct way of working. A real conundrum.)

Ken

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Christian Walther <cwa### [at] gmxch> wrote:

> If the lower image looks more aesthetically pleasing to you, that's
> probably because ideal diffuse reflectors aren't that common in the real
> world, so...
> ...you may change the sphere's reflectivity properties using the
> "brilliance" keyword. With "finish { ambient 0 diffuse 1 brilliance 2 }"
> (and assumed_gamma 1), a similar image to Ard's lower one is achieved.

I'm curious; would you yourself use the
brilliance trick to visually correct the
rendered image (as seen in the POV preview render),or would you
leave it as-is because it's physically correct?
I'd genuinely like to know, as it will help me to further understand how you
(and others) use and interact with POV.

Ken

Post a reply to this message

Kenneth wrote:

>>...you may change the sphere's reflectivity properties using the
>>"brilliance" keyword. With "finish { ambient 0 diffuse 1 brilliance 2 }"
>>(and assumed_gamma 1), a similar image to Ard's lower one is achieved.
>>(I'm too lazy now to look up what brilliance does exactly, but it may be
>>that  "brilliance <display_gamma>" exactly reproduces that image.)
>>
> Hey, that's a  brilliant use of brilliance! ;-) But upon re-reading the POV
> docs explaining that keyword, no mention is made of your particular use of
> it.  Quite an important, basic use!  If brilliance was actually intended
> for correcting the lighting of an object, shouldn't that have been covered
> in a major way in the POV docs? Such an omission makes me wonder if
> "correcting" for lighting anomolies...when using assumed_gamma of 1.0...was
> ever even considered!

I don't know - reading 
<http://www.povray.org/documentation/view/3.6.1/346/> in my opinion 
describes quite well what brilliance does. Isn't what's described there 
exactly "correcting the lighting"? I don't see a fundamental difference 
between those two uses, I rather see them as one and the same and the 
only use of brilliance. The only thing that I'm missing there is a 
formula that says what it does *exactly*.

I guess you're just the first one to see physically correct rendering of 
an ideal diffuse reflector (which is just an approximation to real-world 
surfaces, admittedly, but not that bad for many types of materials, if 
combined with highlights) as an "anomaly".

> I came to POV from having used Photoshop for years.
> As a graphics toolset, PS allows me to pick color/brightness values either
> visually or by choosing rgb values. Numerically, it's own range is from 0
> to 255. But the important thing is that, if I choose 127,127,127...right in
> the middle...I do indeed get an on-screen gray that is "half as perceptully
> bright as white." (Which is what I thought POV's <.5,.5,.5> should give me
> as well.) Photoshop is, in effect, insulating me from the need to worry
> about gamma correction. A beautiful, intuitive way of working!!

Right, but I suspect this is more by accident than by design. 127 is 
converted by the display's response to approximately (127/255)^2.2 = 21% 
light intensity, which by the response of the human visual system is 
converted to approximately "half the perceptual brightness". It's not 
that Photoshop is doing any extra work to insulate you from the physical 
intensity values. And encoding brightness as integers from 0 to 255 is 
not a decision made by Photoshop's designers to insulate you from 
anything either, to the contrary it is just what is used internally in 
the video memory of computer displays and in most image file formats.

> From my own standpoint, of simply wanting to produce nice, realistic,
> perceptually-pleasing images on my own system...

Keep in mind that "realistic" and "perceptually pleasing" are sometimes 
conflicting goals. And in my opinion, the best way of getting 
"realistic" images is staying as close as possible to the physical 
reality. That may include use of features like "brilliance" in some 
cases where it improves the approximation to nature, but not as a 
general cure-all.

> ... then using assumed_gamma of 2.0 is just...easier and more
> intuitive...while 1.0 creates hurdles and difficulties that have to
> be constantly addressed: having to specify "non-linear" color
> values;

The way I work, that's not a big deal - I just tweak the numbers until 
it looks the way I want it, and I don't care whether in the end "that 
middle gray that I want" is encoded by 0.5 or by 0.2.

> ... having to add and tweak other POV values (like brilliance)
> because of lighting anomolies;

I don't think I've ever needed to use brilliance to get more realistic 
surfaces in any of my scenes - but then again most of my scenes have 
been technical rather than artistic so far.

> having to deal with image_map images (created in any typical graphics
> program) that don't render correctly unless they are in the .png
> format with an embedded gamma of 1.0.

I seem to remember that there's indeed something wrong in POV-Ray in 
that area, but since I don't exactly remember what it is I'll keep my 
mouth shut for the moment. (What you say certainly sounds odd, it would 
only be correct if the contents of the PNG are indeed encoded with a 
gamma of 1.0, which is rare (at least for images that don't come out of 
POV-Ray).)

> My wish, from an artist's standpoint:  WOULDN'T IT BE NICE IF...POV allowed
> an alternate way of working, one that allowed the use of "linear" color
> values (as in Photoshop, and as in my initial POV gray-band test scene),
> CHANGED those values internally to work in its own ideal color space of
> assumed_gamma 1.0,

You could do that yourself by defining a macro (but, frankly, I don't 
see much use in that, given what I said above about tweaking numbers):

#macro MyColor(r, g, b)
   rgb <pow(r, 2.0), pow(g, 2.0), pow(b, 2.0)>
#end

or even

#macro MyColor(r, g, b)
   rgb <pow(r/255, 2.0), pow(g/255, 2.0), pow(b/255, 2.0)>
#end

> and then re-massaged THOSE values before spitting them back to the
> monitor, so that they appeared perceptually correct in the user's
> chosen gamma environment?

POV-Ray already does that (if you use assumed_gamma 1).

> (Of course, my own use of assumed_gamma of 2.0 mimics that behavior!
> But as has been pointed out, that's not the correct way of working.
> A real conundrum.)

It mimics that behavior *and* causes the internal calculations to be 
done in a physically incorrect way. That's a price you (as an artist) 
may be willing to pay, but I (as a physicist) am not.

> I'm curious; would you yourself use the brilliance trick to visually
> correct the rendered image (as seen in the POV preview render),or
> would you leave it as-is because it's physically correct? I'd
> genuinely like to know, as it will help me to further understand how
> you (and others) use and interact with POV.

If I'm convinced that the surface I want to simulate is better 
approximated by a diffuse-reflector-with-brilliance than by an ideal 
diffuse reflector, I would use it, but otherwise I tend to accept what 
POV-Ray delivers as "realistic" even if it looks different than I had 
expected. Again, the fundamental belief behind that is that using a 
physical model that as closely as possible approximates nature will lead 
to the most realistic images. And getting a realistic image is usually 
my primary goal (over getting an artistically pleasing image).

  -Christian

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Christian Walther <cwa### [at] gmxch> wrote:
> Kenneth wrote:

> > Such an omission makes me wonder if
> > "correcting" for lighting anomolies...when using assumed_gamma of 1.0...was
> > ever even considered!
>
> I don't know - reading
> <http://www.povray.org/documentation/view/3.6.1/346/> in my opinion
> describes quite well what brilliance does. Isn't what's described there
> exactly "correcting the lighting"? I don't see a fundamental difference
> between those two uses...

That's true; but as I read it, POV's discusion of that keyword treats it
more as a "special effect" than as a fundamental tool for adjusting the
surface reflectance of lighting on an object (actually, it would have to be
ALL objects) in a scene. It's just surprising that such an important,
all-encompassing use is not even touched on. The interesting thing to me is
that the default value is 1.0 (the value used when it isn't specified.) I'm
guessing this is because assumed_gamma is recommended to be 1.0 as well.
And when it is, then making brilliance equal to display_gamma (or
system/monitor gamma)..as you suggested...does indeed produce "realistic"
light/object interaction (the way I like to see it, anyway.)
>
> I guess you're just the first one to see physically correct rendering of
> an ideal diffuse reflector (which is just an approximation to real-world
> surfaces, admittedly, but not that bad for many types of materials, if
> combined with highlights) as an "anomaly".

Ah, this is where we "see" things fundamentally differently. :-) Based on
how light seems (to me) to interact with objects I look at, I think that
assumed_gamma of 1.0 actually DOESN'T create "an approximation to
real-world surfaces"--without using the various SDL  tweaks so far
discussed, anyway. (And I'm not totally convinced that ALL the "anomolies"
I see can be completely eliminated this way; don't know.)  It's
strange--yet interesting!--that we have such a different take on this. But
I have to assume that (many?) others are in your camp, so to speak.  (This
is why I was hoping to have a sort of "roundtable discussion" of lots of
users' opinions of Ard's two images.) I wonder--is the POV community
equally divided on this particular issue, or am I one of the few wandering
madmen??

I do understand all that you've said about physically-correct
lighting/object interaction, using assumed_gamma of 1.0.  It all makes
sense, physically; but, used by itself and with no SDL tweaks, the overall
effect just doesn't corresponded to what I think I should see in the POV
preview render.  Alas!  To tweak or not to tweak, that is the question! :-P

>
> > Photoshop is, in effect, insulating me from the need to worry
> > about gamma correction. A beautiful, intuitive way of working!!
>
> Right, but I suspect this is more by accident than by design. 127 is
> converted by the display's response to approximately (127/255)^2.2 = 21%
> light intensity, which by the response of the human visual system is
> converted to approximately "half the perceptual brightness".

That's, I believe, a feature BY design, in order for it to be an
easy-to-use, intuitive WYSIWYG graphics program...one of its big original
selling points. (And probably the main reason I was never really cognizant
of gamma correction until I started using POV; it was all, or mostly, kept
behind the scenes.)
>
>
> > ...and then re-massaged THOSE values before spitting them back to the
> > monitor, so that they appeared perceptually correct in the user's
> > chosen gamma environment?
>
> POV-Ray already does that (if you use assumed_gamma 1).

Hmm. Again, I think (?) we're possibly coming at that from two different
perspectives...your's physically-based, mine artistically (or perceptully)
based. I disagree, using my "linear-value" grey-band test scene as a prime
example.
>
> > (Of course, my own use of assumed_gamma of 2.0 mimics that behavior!
> > But as has been pointed out, that's not the correct way of working.
> > A real conundrum.)
>
> It mimics that behavior *and* causes the internal calculations to be
> done in a physically incorrect way. That's a price you (as an artist)
> may be willing to pay, but I (as a physicist) am not...
> And getting a realistic image is usually
> my primary goal (over getting an artistically pleasing image).

Yes, I understand the difference.

A most interesting discussion!!

Ken

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Kenneth wrote:
> That's true; but as I read it, POV's discusion of that keyword treats it
> more as a "special effect" than as a fundamental tool for adjusting the
> surface reflectance of lighting on an object (actually, it would have to be
> ALL objects) in a scene.

That's because that's what it is. POV-Ray's default diffuse reflection 
calculation (with brilliance 1) is based on a (simplified) physical 
model (that says "of the total light power incident on every small 
surface piece, a constant fraction is absorbed, and the rest is 
uniformly reflected in all directions"). The physical interpretation of 
a surface with only diffuse reflection and a brilliance greater than 1 
on the other hand would be that less light is reflected (i.e. absorption 
is stronger) for shallow incidence than near normal incidence, and I 
doubt that's a property than many real-world surfaces have. Brilliance 
was introduced as an artistic tool, without any claims of physically 
based "realism". (If near-specular reflection, i.e. highlights, are 
taken into account, however, it may indeed be that the reflective 
properties of some surfaces are best approximated by a brilliance 
different from 1. Say, a diffusely reflecting surface beneath a thin 
varnish coat that specularly reflects more light at shallow angles 
(Fresnel reflection), leaving less for the diffuse reflector below.)

Applying such a feature indifferently to all surfaces in a scene, no 
matter what material, makes no sense IMO, if the goal is realism. You 
can still do it if you like by adjusting the default texture (see 
<http://www.povray.org/documentation/view/3.6.1/239/>):

#default { finish { brilliance 1.7 } }

> The interesting thing to me is that the default value is 1.0 (the
> value used when it isn't specified.) I'm guessing this is because
> assumed_gamma is recommended to be 1.0 as well. And when it is, then
> making brilliance equal to display_gamma (or system/monitor
> gamma)..as you suggested...does indeed produce "realistic" 
> light/object interaction (the way I like to see it, anyway.)

That's purely a coincidence. I still haven't checked the source to see 
how brilliance works exactly (the documentation doesn't mention it), but 
the easiest way to do it would be a power function. Since a power 
function with exponent 1 is identity, using 1.0 has the same effect as 
if the feature didn't exist at all. And since assumed_gamma (by 
coincidence) is used in the exponent of a power function as well, 
brilliance is able to cancel its effect.

> Ah, this is where we "see" things fundamentally differently. :-) Based on
> how light seems (to me) to interact with objects I look at, I think that
> assumed_gamma of 1.0 actually DOESN'T create "an approximation to
> real-world surfaces"--without using the various SDL  tweaks so far
> discussed, anyway. (And I'm not totally convinced that ALL the "anomolies"
> I see can be completely eliminated this way; don't know.)  It's
> strange--yet interesting!--that we have such a different take on this.

Yes, that's interesting. I have to say that Ard's first sample image 
doesn't look nearly as wrong to me as it seems to look to you. If I 
imagine standing in a completely darkened room with one bright point 
light source and a dull white painted ball, I indeed expect to see 
something more like the upper than the lower image. Or consider the 
first-quarter moon. It's approximately a diffusely reflecting sphere 
illuminated from a single direction in an otherwise dark space - and 
doesn't it look more like the upper image than the lower one?

> But I have to assume that (many?) others are in your camp, so to
> speak. (This is why I was hoping to have a sort of "roundtable
> discussion" of lots of users' opinions of Ard's two images.) I
> wonder--is the POV community equally divided on this particular
> issue, or am I one of the few wandering madmen??

I'd be interested in that too - I'd guess the latter, not because I 
consider you a madman :), but because I think POV-Ray's "numerical" user 
interface (as opposed to the "graphical" way other 3D applications work) 
appeals more to "scientist" types than to "artist" types.

>>>Photoshop is, in effect, insulating me from the need to worry
>>>about gamma correction. A beautiful, intuitive way of working!!
>>Right, but I suspect this is more by accident than by design.
> That's, I believe, a feature BY design, in order for it to be an
> easy-to-use, intuitive WYSIWYG graphics program...one of its big original
> selling points.

I disagree. The habit of encoding intensity by a power function rather 
than linearly (I call this "gamma encoding") was there before Photoshop. 
It originated from the fact that it approximately models the response of 
a CRT to the applied voltage. The video hardware of the first computer 
displays just linearly converted 127 to "half the voltage", and with 
that voltage the CRT phosphors happened to glow with about 20% of full 
intensity, which happens to be about "half the perceived brightness". 
That's a nice coincidence, and it's the reason why we still use gamma 
encoding even though nobody does a straight linear conversion from video 
memory values to CRT voltage anymore.

>>>...and then re-massaged THOSE values before spitting them back to the
>>>monitor, so that they appeared perceptually correct in the user's
>>>chosen gamma environment?
>>
>>POV-Ray already does that (if you use assumed_gamma 1).
> 
> Hmm. Again, I think (?) we're possibly coming at that from two different
> perspectives...your's physically-based, mine artistically (or perceptully)
> based. I disagree, using my "linear-value" grey-band test scene as a prime
> example.

I think you're misunderstanding me. I'm talking about what POV-Ray does 
at the output end, assuming that you feed it what it expects at the 
input end. Unlike other points in our discussion, there's no place for 
disagreement here. These are facts.

POV-Ray interprets the 0.5 in the middle of your gray-band as "half the 
intensity" (that's not what you meant it to be, but that's just the way 
POV-Ray works). It then calculates 255*0.5^(assumed_gamma/display_gamma) 
= 186 (assuming that your display_gamma is 2.2 and assumed_gamma is 1) 
and writes that value to the output file. When the file is displayed, 
that value is copied into video memory and makes your screen pixels glow 
with (186/255)^2.2 = 0.5 of full intensity (assuming your display is 
properly calibrated to a gamma of 2.2). That's the correct value, given 
what was input at the beginning, even though it's not "half the 
perceptual brightness".

Of course, in the gray band test, if you pretend that POV-Ray interprets 
0.5 as "half the perceptual brightness" and use assumed_gamma = 
display_gamma, the correct result (a pixel glowing with "half the 
perceptual brightness") comes out too - but as soon as any physical 
lighting calculations have to be done, such as in the sphere test, their 
results will be incorrect, because linear-to-perceived-brightness 
numbers are put into formulas intended for linear-to-intensity numbers. 
(But you seem to like the incorrect results better than the correct 
ones, and I can't argue with that.)

>>>(Of course, my own use of assumed_gamma of 2.0 mimics that behavior!
>>>But as has been pointed out, that's not the correct way of working.
>>>A real conundrum.)

It just occurs to me that the assumed_gamma feature (whose purpose I've 
never really understood, given that there's only one correct value for 
it in my view of POV-Ray as a light physics simulator) may have been 
introduced just for what you're trying to do - making POV-Ray interpret 
the numbers it's given not as linear-to-intensity, but as something 
else, e.g. linear-to-perceived-brightness. Unfortunately that just 
doesn't work correctly the way it's currently implemented. Such a 
feature should affect the input end (before the physical calculations 
are done), while the current implementation affects the output end 
(after the calculations).

> A most interesting discussion!!

I agree - but it's starting to take way too much of my time :). I'll try 
hard to keep my further answers a bit shorter. But I think most of the 
points I wanted to make have been made by now anyway.

  -Christian

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Christian Walther <cwa### [at] gmxch> wrote:

> ...I have to say that Ard's first sample image
> doesn't look nearly as wrong to me as it seems to look to you... consider the
> first-quarter moon. It's approximately a diffusely reflecting sphere
> illuminated from a single direction in an otherwise dark space - and
> doesn't it look more like the upper image than the lower one?

Yes, I do have to agree with you there. I was thinking of the same example:
a half-moon in a dark sky DOES appear to be like Ard's assumed_gamma of 1
image. Which has caused me some head-scratching. But I wonder...is it
*possible* that that's because the BRIGHTEST part of the moon--coupled with
the dark-to-black night sky background-- presents too MUCH brightness and
contrast (contrast with the background)  for our eyes to "make sense" of?
Just a conjecture.  But your argument is definitely worth thinking about.
I suppose a conclusive "proof" for me would be to set up a white sphere
against my Kodak neutral-gray card...to lessen the contrast... and just
LOOK at what a single, far-away light source does to it. Who knows, I may
be surprised!

> > I wonder--is the POV community equally divided on this particular
> > issue, or am I one of the few wandering madmen??
>
> I'd be interested in that too - I'd guess the latter, not because I
> consider you a madman :), but because I think POV-Ray's "numerical" user
> interface (as opposed to the "graphical" way other 3D applications work)
> appeals more to "scientist" types than to "artist" types.

You may very well be correct, from what I read not only in this discussion
but others in the newsgroup. Yes, I suppose I would be squarely in the
"graphical" camp---and sometimes I do find it odd MYSELF that I actually
like the numerical way of working in POV!!  Yet it definitely appeals to
me. That left brain/right brain thing, I suppose.
> f
>
> >>>...and then re-massaged THOSE values before spitting them back to the
> >>>monitor, so that they appeared perceptually correct in the user's
> >>>chosen gamma environment?
> >>
> >>POV-Ray already does that (if you use assumed_gamma 1).
> >
> > ...I disagree, using my "linear-value" grey-band test scene as a prime
> > example.
>
> I think you're misunderstanding me. I'm talking about what POV-Ray does
> at the output end, assuming that you feed it what it expects at the
> input end...
>
> POV-Ray interprets the 0.5 in the middle of your gray-band as "half the
> intensity" (that's not what you meant it to be, but that's just the way
> POV-Ray works)... When the file is displayed,
> that value is copied into video memory and makes your screen pixels glow
> with (186/255)^2.2 = 0.5 of full intensity (assuming your display is
> properly calibrated to a gamma of 2.2). That's the correct value, given
> what was input at the beginning, even though it's not "half the
> perceptual brightness".
>
Ah, I do now understand what you meant. (And nicely-explained, too!) Yes, we
are in agreement.

Many, many thanks, both to you and to the others who've contributed, for
taking the time to go into this topic in such detail. I've learned a great
deal.

 Ken

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Christian Walther <cwa### [at] gmxch> wrote:
>consider the
>first-quarter moon. It's approximately a diffusely reflecting sphere
>illuminated from a single direction in an otherwise dark space

I had to jump in here, even though I haven't been following the discussion
closely. The moon is illuminated by two sources: the sun and the earth. The
reflection of sunlight off of the earth (the albedo) is why we can see the
dark portion of the moon. The earth reflects approximately 39% of the
sunlight that hits it (which is where the title of Vangelis' album
"Albedo 0.39" comes from).

Over and out.

Post a reply to this message

Tim Riley <timothyrileyatnetscapedotnet> wrote:

>
> I had to jump in here, even though I haven't been following the discussion
> closely. The moon is illuminated by two sources: the sun and the earth. The
> reflection of sunlight off of the earth (the albedo) is why we can see the
> dark portion of the moon. The earth reflects approximately 39% of the
> sunlight that hits it (which is where the title of Vangelis' album
> "Albedo 0.39" comes from).
>

I had actually forgotten about that!  Thanks. But I seem to remember reading
somwehere that the moon's surface material is VERY dark...which would
explain why the "darK" part of the moon seems almost black, compared to
it's much brighter half-moon shape.  In other words, the dark half is
reflecting much less than 39%. Wheras, the sun is SO bright that it makes
the moon's intrinsically-dark material positively GLOW with brilliance! ;-)
So I would *assume* that the albedo isn't contributing very much to the
bright half's appearance, relatively speaking.

Ken

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Kenneth nous apporta ses lumieres en ce 2005-12-24 02:56:
> Tim Riley <timothyrileyatnetscapedotnet> wrote:
> 
> 
>>I had to jump in here, even though I haven't been following the discussion
>>closely. The moon is illuminated by two sources: the sun and the earth. The
>>reflection of sunlight off of the earth (the albedo) is why we can see the
>>dark portion of the moon. The earth reflects approximately 39% of the
>>sunlight that hits it (which is where the title of Vangelis' album
>>"Albedo 0.39" comes from).
>>
> 
> 
> I had actually forgotten about that!  Thanks. But I seem to remember reading
> somwehere that the moon's surface material is VERY dark...which would
> explain why the "darK" part of the moon seems almost black, compared to
> it's much brighter half-moon shape.  In other words, the dark half is
> reflecting much less than 39%. Wheras, the sun is SO bright that it makes
> the moon's intrinsically-dark material positively GLOW with brilliance! ;-)
> So I would *assume* that the albedo isn't contributing very much to the
> bright half's appearance, relatively speaking.
> 
> Ken
> 
You're right. On the bright side of the moon, the earth's contribution can be
considered as 
negligeable, and practicaly ignored. You also need to consider the relative size and
the distance 
between the earth and the moon: if the earth was the size of a baseball and the moon
about that of a 
ping-pong ball, the distance separating them would be a little more than 2 meters,
reducing earth's 
contribution further.

-- 
Alain
-------------------------------------------------
When the bosses talk about improving productivity, they are never talking about
themselves.

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Kenneth wrote:
> Tim Riley <timothyrileyatnetscapedotnet> wrote:
> 
>>The earth reflects approximately 39% of the
>>sunlight that hits it (which is where the title of Vangelis' album
>>"Albedo 0.39" comes from).
>
> I had actually forgotten about that!  Thanks. But I seem to remember reading
> somwehere that the moon's surface material is VERY dark...which would
> explain why the "darK" part of the moon seems almost black, compared to
> it's much brighter half-moon shape.  In other words, the dark half is
> reflecting much less than 39%. Wheras, the sun is SO bright that it makes
> the moon's intrinsically-dark material positively GLOW with brilliance! ;-)
> So I would *assume* that the albedo isn't contributing very much to the
> bright half's appearance, relatively speaking.

Actually I'd say that the primary reason why the contribution to the 
bright part is negligible is that only a very small part of the light 
reflected by the earth hits the moon (but the moon's low albedo plays a 
role too (it's 12%, according to Google's first hit 
<http://www.asterism.org/tutorials/tut26-1.htm>)). Keep in mind that the 
earth's albedo being 39% doesn't mean that 39% of the light that hits 
the earth is reflected to the moon. It is reflected in all directions, 
and the moon only covers a very small range of directions.

  -Christian

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