"Kenneth" <kdw### [at] earthlinknet> wrote:

> A Kodak "photographic gray card" -- neutral gray, in other words-- has an
> actual reflectance of only 18% (not 50% as would be assumed.) This
> corresponds to a PERCEPTUAL brightness of "half as bright as white,"
> whether viewed with the eye or by photographic film.  As Poynton says in
> his gamma discussion, "A source having a luminance of 18% of a reference
> luminance will appear half as bright." This is the human eye's PERCEPTUAL
> response to luminance....which isn't linear.  A POV-generated gray value of
> <.5,.5,.5>, applied to an object with finish{ambient 1 diffuse 0}, should,
> it seems to me, reproduce on screen in the same way...half as PERCEPTUALLY
> bright as (POV) white.  "Raw" monitor gamma/OS
> gamma/display_gamma/assumed_gamma all contribute to make that a reality, of
> course. So POV's assumed_gamma must be chosen to take that POV <.5,.5,.5>
> gray and place it squarely in the "middle" of our PERCEPTION of
> black-to-white.
>

The 18 % refers to the amount of light that is reflected from the grey card
in relation to the light that hits it. 100 % reflection is a rare thing in
the real world, a white sheet of paper will reflect about four times as
much as the grey reference, and a black sheet of paper will reflect about a
quarter as much as grey. This corresponds to two 'stops' to either side of
gray, in photographic terms.

This middle gray on a grey card is produced, I think, by mixing equal
amounts of black and white paint (assuming they are of equal strength), so
it is a 50 % gray in 'paint' terms. I have compared the tone of a grey card
to that of a grayscale chart that you can get in a paint store, and it
matches that 50 % tone exactly. So the paint industry and the photo
industry uses the same scale, it seems.

Perception is a difficult matter, but as far as I know our senses are
logarithmic in nature; doubling the amount of light is perceived as the
same change, no matter where you are on the scale (sorry if my english is
unclear).

H

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Kenneth, thanks for stirring this up again (seriously).  The previous
discussion about this died early.  In case you're feeling patronised
by the other responses, I did read your entire post and I understand
your argument.  I used to agree with you, but now I'm going to try to
convince you to change your ways.

First, two points that are away from your topic.  Thanks for the link
to Norman Koren's test images.  They are the best I have seen and they
allowed me to tweak my apparent gamma down to 2.2.

Second, your test scene demonstrates our eyes' edge-enhancement very
well: the right sides of the 50% bands appear much darker than the
left.  I had to run a colour picker over them to make sure I *was*
seeing things.

Regarding gamma abuse: I feel your pain.  Until this year I did not
have assumed_gamma in my scenes, which led to the behaviour you're
choosing.  I started the scenes before POV (DKB) knew of it.
pigment{Orange} looked orange, shadows looked good, gray10 was very
dark, and my happy little universe was properly lit.

Before we go further you should take a mathematical look at what POV
does to a 0.5 pixel.  It will save you making more test scenes and
examining them in PS.  As pointed out elsewhere in this thread,
section 3.3.3.3 tells what you need to know: with assumed_gamma
omitted or set to display_gamma, your output pixel will be exactly as
set in the pigment.  With display_gamma/assumed_gamma at 2.0, every
pixel value is raised to the power of 1/2.0.  0.5 grey will be raised
above 0.70.

Worse, 0.05 grey is taken to 0.22, which is why the difference between
the darkest two bands in your test seems ridiculous.  To get a
near-black in my scenes that will react to diffuse light but still
look dark, I have to play with input values around 0.03.  I see your
point: it seems daft.  It's just the way it is.

You have illustrated a mind-set very well.  You expect 50% to look
half way between black and white.  I felt the same way about orange.
I felt it shouldn't be yellow.  But that was naivete:  I know now that
trusting the names in colors.inc is foolishness.  I don't think you're
misunderstanding anything, and you certainly didn't deserve to be
spoken down to.  As someone who has recently taken the blue pill, been
assimilated, clicked Start, I feel qualified to respond.

Here is a summary of a summary I have put in a blog entry, which I
won't link here because it is below this NG's audience and will only
draw lofty criticism.  Tek begged us to use {assumed_gamma 1}
elsewhere in these groups after spending a chunk of his life learning
the lesson, and I will do the same:

- Set your assumed_gamma to 1.0.  It will save you pain when you move
  to 3.7.

- Grapple all your colour values down so the fully-lit pigments look
  right again.  Squaring them does the trick.  Try out your colour
  picker: your fully-lit pixels will be as they were with
  assumed_gamma 2.

- That left my half-lit faces too bright, so I reduced their roughness
  and phong_size.  I've switched to the first person here because YMMV.

Leave your display_gamma at 2.0.  Then, if you send us a PNG, our
systems will brighten it slightly so that our displays, which darken
images more than yours does, show us exactly what you see.

Sorry if this sounds preachy but, basically, deep down we're all
trying to save you pain.  Well no, USENET posters being like 4WD
drivers, some of us are showing off, making up for small tackle
(that's a different blue pill, by the way), and taking advantage of
anonymity to behave in ways that would see us beaten to a geeky white
pulp if we tried it at the pub.

But perhaps there is something worthwhile in it.

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"Ard" <ard### [at] waikatoacnz> wrote:
> Kenneth, thanks for stirring this up again (seriously).

Thanks for your very well-considered response.  Believe me, I donned a very
thick suit of armor before deciding to post my discussion! I figured I'd be
seen as either the new Einstein of POV-dom, or else a lowly protozoa ;-)
But I LIKE a lively dicussion!

Let me apologize to everyone for originally posting such a long discourse;
it's just that I wanted to cover things thoroughly.  It DID start out even
longer!

Something you said (and which Christian Walther also discussed) is probably
at the CORE of my basic misunderstanding of the topic.  As I mentioned in
my post...

"We all assume --don't we?--that POV's
color/brightness values, as used in a typical PIGMENT block, are meant to
reproduce brightness levels such that <.5,.5,.5> represents "half as
perceptually bright as" <1,1,1>."

Your own response has given me MUCH to think about...
>
> Before we go further you should take a mathematical look at what POV
> does to a 0.5 pixel.... with assumed_gamma
> omitted or set to display_gamma, your output pixel will be exactly as
> set in the pigment.  With display_gamma/assumed_gamma at 2.0, every
> pixel value is raised to the power of 1/2.0.  0.5 grey will be raised
> above 0.70. Worse, 0.05 grey is taken to 0.22, which is why the difference between
> the darkest two bands in your test seems ridiculous.  To get a
> near-black in my scenes that will react to diffuse light but still
> look dark, I have to play with input values around 0.03.  I see your
> point: it seems daft.  It's just the way it is.
>

and Christian Walther responded...

      >No. At least I don't assume that, and I think most other people with
      >some background in physically-based color theory and computer
      >graphics don't either....Of course that means that you and I are
      >starting from different premises...

That really does open my eyes, and explain a lot. Essentially, I've been
writing POV scenes using the wrong assumption! I confess it never occured
to me that I should be choosing "non-linear" color and brightness values in
my POV scenes...which would make perfect sense when using an assumed_gamma
of 1.0.  And would make my grey-band test incorrect. (Am I putting the facts
together correctly?) If so,
then...WOW...that's a big conceptual change for me.

Of course, the question that immediately comes to my mind is, "Gee, why NOT
simple linear POV values?" But I need to assimilate this paradigm shift
before posting further...

Thanks again (everyone!)

Ken

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"Zeger Knaepen" <zeg### [at] povplacecom> wrote:


> but then why do some image_maps look a lot darker than they should?
>

Tim Nikias discusses this same question in a different post, in more detail,
and it raises some interesting points...

http://news.povray.org/povray.binaries.images/thread/%3C439b2aa2%40news.povray.org%3E/

Ken

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"Ard" <ard### [at] waikatoacnz> wrote:

>
> - Grapple all your colour values down so the fully-lit pigments look
>   right again.  Squaring them does the trick.  Try out your colour
>   picker: your fully-lit pixels will be as they were with
>   assumed_gamma 2.
>
YES INDEED, this does work!  I went back to my "linear" POV grey-band test
scene, squared all the RGB values (in effect, making them non-linear), and
the resulting POV render, using assumed_gamma of 1.0, looks exactly the way
it should.

FASCINATING!

So...if I had chosen 2.2 as my overall system gamma (rather than 2.0), then
I would have had to alter each of  my "linear" grey-band values by raising
them to the power of 2.2, rather than just squaring them...to get the same
pleasing visual result.  Correct?

Ken

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"Kenneth" <kdw### [at] earthlinknet> wrote:

> So...if I had chosen 2.2 as my overall system gamma (rather than 2.0), then
> I would have had to alter each of  my "linear" grey-band values by raising
> them to the power of 2.2, rather than just squaring them...to get the same
> pleasing visual result.  Correct?

Yep.  When you start messing with gamma, particularly investigating the GAMA
chunks in PNGs, the number 0.45455 comes up a lot because it is 1/2.2.  POV
essentially raises every pixel value to the power of 0.45455 before writing
it to disk and placing it it in the preview.

To test all this I produced a scene of four coloured boxes and rendered it
into a 2x2 PPM.  That is human-readable, so I didn't need to load it into
an editor to run a colour picker over it, risking another application
applying gamma to the values.

You can go around raising things to the power of other things but when it
comes down to it, I think accepted POV practice is to munge numbers until
it looks right.

Post a reply to this message

if (! horse->dead)
 flog(it)
else
 flog(it)

So tweaking RGB values in pigments brings them back into line, if they are
fully lit.  But adding {assumed_gamma 1} makes the edges of soft shadows
and spotlights sharper and faces oblique to incident light brighter.  This
is why I have had to tweak surface highlights and spotlight radii to
compensate for adding gamma to old scenes.

Demonstrated in p.b.i at
http://news.povray.org/povray.binaries.images/thread/%3Cweb.43a0a486c3c8eadaed802ab30%40news.povray.org%3E/

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"Ard" <ard### [at] waikatoacnz> wrote:
> Yep.  When you start messing with gamma, particularly investigating the GAMA
> chunks in PNGs, the number 0.45455 comes up a lot because it is 1/2.2.  POV
> essentially raises every pixel value to the power of 0.45455 before writing
> it to disk and placing it it in the preview.

One thing that everybody should keep in mind when looking at gamma-corrected
images such as PNG is that many applications do *not* handle the PNG gAMA
chunk according to the specs.  Two of the main culprits are:
- All versions of Internet Explorer (including the beta version 7.0)
- Old versions of Photoshop (older than 7.0 or CS)

MSIE is known for having very bad support for transparency in PNG images
(this is fixed in MSIE 7.0) but also for its bad support for gamma
correction.  According to the PNG specs, the browser should apply the
correction when displaying a PNG image including a gAMA chunk and should
not apply it when the image does not include it.  Or to be more consistent,
if the browser applies a gamma correction to other colors such as those
from HTML, CSS or other images (JPEG, GIF) then it should do the same
for PNG images that have no gAMA chunk.  MSIE fails to do that consistently.
In addition, MSIE seems to assume that the display gamma is a bit below 2.0.
According to http://www.libpng.org/pub/png/pngapbr.html#msie-win-unix MSIE
uses an implicit display gamma of approximately 1.93.

Other browsers such as Firefox or Opera (version 7 or later) process the PNG
gAMA chunk correctly.  However, they may rely on the operating system for
some display functions so it is possible that a PNG image does not look the
same in Firefox for Windows, for Linux or for OS X.

So given that the most common applications for displaying PNG images do not
handle the gamma information correctly, one should be careful before stating
that POV-Ray handles this in the "right way" or not.

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"Ard" <ard### [at] waikatoacnz> wrote:
> if (! horse->dead)
>  flog(it)
> else
>  flog(it)
>
> So tweaking RGB values in pigments brings them back into line, if they are
> fully lit.  But adding {assumed_gamma 1} makes the edges of soft shadows
> and spotlights sharper and faces oblique to incident light brighter...
>
 YES! Very true indeed. (BTW, I did get around to running my grey_band test
scene with an overall system gamma/display_gamma of 2.2, altering the
gray-band values by raising them to the power of 2.2, and the scene did
render just as beautifully and realistically as before.)

OK. So here are the facts so far: to make my POV "linear" grey band test
reproduce in POV's preview window with a correct , real-world appearance,
and using the recommended assumed_gamma of 1.0, then my grey values need to
be specified in a NON-linear way--i.e., instead of <.5,.5,.5> being used to
represent "half as perceptually white" as <1,1,1>, the values instead need
to be raised to the power of [whatever system gamma I've chosen.]
Apparently for the sole purpose of letting POV work in its ideal "linear"
gamma-of-1.0 color space

Ugh. Well, it works, and I accept it...I'M NOT HAPPY... but I'll accept it.
For the moment anyway.

Ah, but what about lighting? As you've mentioned, strange things happen. My
test scene made no use of lights.  The gray bands were
"self-illuminated"--{ambient 1 diffuse 0}. When light sources ARE included,
things turn...ugly! Your test images over at p.b.i. show very clearly what
happens.

Funny thing. I was JUST getting ready to post my own example scene to show
these effects (and discuss them), but happened to see your examples first,
and found that your test is EXACTLY the same as mine!!! Even down to the
direction of the light source!   Adventurous minds think similarly. ;-) But
I'd like to explore this little test, as it goes to the very HEART of my
initial "perceptual" argument and discussion.  (For the edification of
others: It's just a white sphere, illuminated by a single white
point-source light placed very far away, and with no ambient lighting to
muddle things.  You can't get much simpler than that!!) But in its very
simplicity, it clearly and "honestly" shows the effect of lighting on an
object...and the fundamental difference in "render realism" between using
assumed_gamma of 1.0 as opposed to my use of 2.0 (or 1.8 or 2.2). And it
duplicates, though in a completely different way, the "distorted"
brightness levels I was seeing in my original "linear" gray band test
scene.

Everyone, take a look at Ard's assumed_gamma of 1 image--that's an order!!--
or just run such a test.  I'd really like to know how each of us perceives
the illumined sphere...how the light reacts with the curving surface. And
what we each do to correct for it...if in fact we DO correct for it. (I
do...by using assumed_gamma of 2.0! Ard uses a different method.) But
others may not do so at all...??? I'm actually going somewhere with this
argument, though it may not seem so right now; kind of like a chess game!

It's important to note that with this test, there is obviously no way within
POV to alter any of the gray values OR the lighting to be "non-linear" (as
I did to my original gray-band test scene per Ard's suggestion.) An absurd
idea anyway.

More later!

Ken

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Kenneth wrote:
> OK. So here are the facts so far: to make my POV "linear" grey band test
> reproduce in POV's preview window with a correct , real-world appearance,
> and using the recommended assumed_gamma of 1.0, then my grey values need to
> be specified in a NON-linear way--i.e., instead of <.5,.5,.5> being used to
> represent "half as perceptually white" as <1,1,1>, the values instead need
> to be raised to the power of [whatever system gamma I've chosen.]

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 - a linear relationship is always between two 
quantities, and saying that a quantity is "linear" without specifying to 
what other quantity it is linear only works as long as there's an 
implicit agreement about what that other quantity is. Obviously there's 
no such agreement between us two.

> Ah, but what about lighting?
> [...]
> Everyone, take a look at Ard's assumed_gamma of 1 image--that's an order!!--
> or just run such a test.  I'd really like to know how each of us perceives
> the illumined sphere...how the light reacts with the curving surface.

I just did the math, and if I assume that 1. the sphere is an ideal 
diffuse reflector and that 2. Ard's image is encoded with a gamma of 2.2 
(it is not specified in the PNG), I come to the conclusion that Ard's 
upper image (with assumed_gamma 1) is physically correct, while the 
lower one (without assumed_gamma) isn't.

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...

> And what we each do to correct for it...if in fact we DO correct for
> it. (I do...by using assumed_gamma of 2.0! Ard uses a different
> method.) But others may not do so at all...???

...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.)

What you're trying to do with your "assumed_gamma 2.0" is changing 
POV-Ray from taking linear-to-intensity numbers to taking 
linear-to-perceived-brightness numbers because these seem more natural 
to work with to you. However, assumed_gamma affects the output end of 
POV-Ray, not the input end, so you're also forcing POV-Ray to do its 
internal calculations with the linear-to-perceived-brightness numbers, 
and that just doesn't lead to physically correct results.

Inputting linear-to-intensity numbers makes POV-Ray do its calculations 
in a physically correct way, and using assumed_gamma 1.0 makes POV-Ray 
encode its results in the output file correctly (with your display_gamma 
value). As Christoph already said, don't misuse assumed_gamma to adjust 
your rendered image's brightness to a visually pleasing value. The 
proper way of doing that is changing the objects and light sources in 
the scene itself.

  -Christian

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