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Wasn't sure if this belonged in the "POV for Windows" newsgroup, so I posted
it here...
In all the time I've been working with POV for Windows, I've used Photoshop
--on my Mac--to create images for use as heightfields and image_maps. And
I've saved the images as either a .bmp or a .png file type, depending
on...whim. Sometimes I've seen "anomolies" in my POV-rendered scenes, but
just passed them by (I was too busy learning POV!)...not realizing until
now that the .png file type itself might be introducing these anomolies. I
say MIGHT...
Another news thread got me to thinking about all this; it's here...
http://news.povray.org/povray.binaries.images/thread/%3Cweb.4373fd35d28cb00fed802ab30%40news.povray.org%3E/?mtop=220515
I create my image_map images in Photoshop 4.01 on the Mac (prehistoric, I
know) then port them over to my PC for inclusion in a POV scene file. (This
is how I'm used to working, as I began computer life on a Mac long before
buying a PC.) And most times, I don't alter my image_map images in my PC's
newer version of Photoshop (v5.5). Ah, but sometimes I do...and I've
finally discovered the cause of the anomolies--specifically with .png
images vs. .bmp images vs. gamma. AND how to fix them.
I ran some experiments, which I detail here (and I've purposely left out the
"use assumed_gamma of 1.0" suggestion altogether, for clarity.)
First, the set-up:
I have an older PowerMac system and a not-as-old PC (a Pentium II), both
with 17" CRT monitors.
In my PC, where I run POV for Windows, I've set monitor gamma, POV's
assumed_gamma and display_gamma all to 1.8--in order to match my Mac's
monitor/computer combo (my "standard candle" for these tests) to make
images on both platforms display equally. The PC monitor's gamma is
configured using the Adobe Gamma control panel app (and tested using POV's
gamma setup info.) SO...gamma is set to 1.8 everywhere on both computer
systems, and everywhere(?) within POV-Ray. And I took the time to
(visually) match both monitors' color, brightness and contrast.
Now, the experiments...
1) Create image_map in Photoshop on the Mac, with PS's own gamma set to 1.8
(in the FILE:COLOR SETTINGS:MONITOR SETUP dialog box.) Save PS image as
..bmp file. Port it over to PC. Run POV scene, of an object mapped with this
image_map, and with object's finish set to ambient 1 diffuse 0. View final
image in POV's preview window, and output rendered image as .bmp file type
(POV's default output for Windows.) Port that rendered image back over to
Mac, bring it up in Photoshop. Results: rendered image is IDENTICAL on
both monitors (as identical as my eyes can see), and identical to original
Mac-created image. As would be expected.
NOW, in PC, I start Photoshop 5.5, and in FILE:COLOR SETTINGS:RGB SETUP, set
gamma to 1.8 and check the box called "Display Using Monitor Compensation."
Then in FILE:COLOR SETTINGS:PROFILE SETUP, I leave everything blank--no
"embedded RGB profiles", "assumed profiles" or "profile mismatch handling"
whatever.
2) in PC, open .bmp image in Photoshop 5.5, do nothing to it, then re-save
it (as .bmp again.) Run POV scene as before (again, output as .bmp file
type), port that file over to Mac, bring it up in Photoshop. Results:
Again, the rendered scene is IDENTICAL on both monitors.
3) In Photoshop on the Mac, save original image as .png file type rather
than .bmp. (This, I assume, "embeds" a "display gamma" of 1.8 into the .png
file.) Port this over to PC, run POV scene with this. Results: IDENTICAL
again (both within POV scene itself--compared to previous .bmp image_map--
and on Mac.)
Now comes the problem...
4) In PC, open the newly-created .png image in Photoshop 5.5, add nothing
to it, and save it (as .png again.) Run POV scene with it. Results within
POV: The image_map image is now darker, with more contrast...looking like a
major gamma alteration. Not a subtle change, but a big one. Yet opening
this .png image again in Photoshop on the PC shows its
contrast/brightness/gamma to be unaltered! (Turning off "Display Using
Monitor Compensation" has no effect either.) Porting this "altered" .png
image back over to the Mac, and viewing it in Photoshop there, also shows
it to be unaltered. (BTW, the rendered POV image still looks identical on
both Mac and PC.)
I'm really mystified as to what's going on: Is Photoshop 5.5 altering the
gamma value within the .png image, without being told to do so? (But then,
why would it look unaltered on a different version of Photoshop running on
a different computer/monitor combo?) Or is POV handling .png images oddly?
OR, is Photoshop 5.5, the way I have it configured, "stripping" gamma info
from the file when it's saved? WHATEVER the reason, it's an irritating
problem.
There are two different ways I've found to get this "altered" .png image to
render "unaltered" in POV:
1) In POV, leave both assumed_gamma and display_gamma at 1.8, but change
monitor's own gamma to 1. (Actually, making assumed_gamma any value has no
effect here.) Not a good way to work, though; EVERY image on the PC
monitor, in whatever app, is now way too bright.
OR...
2) In PC's Photoshop, go to FILE:COLOR SETTINGS:RGB SETUP, change its gamma
to 1, and leave the "Display Using Monitor Compensation" box checked. Now
open the previously-altered .png file. Right away, the image "brightens up"
and gets washed out. But I re-save it, as .png again. In POV, it now
renders perfectly, unaltered from its original Mac-created appearance! Not
sure I understand that...but it's fixed, so I can't complain. Is Photoshop
now "tagging" the .png image with a "display gamma" of 1? But that doesn't
make sense either. (To get rid of the "washed out" look in Photoshop, I
UNCHECK the "Display Using Monitor Compensation" box. Now I see an
UNALTERED image in both Photoshop and in POV rendering.) Actually, the
rendered image_map looks MOST like the original Mac-created one if PS 5.5
gamma is set to .9 rather than 1. Odd.
Let me reiterate that I'm visually cross-checking the image_map image using
three different, simultaneous "techniques": The original (as well as
altered) .png image file in Photoshop 4.01 on the Mac, the "altered" .png
image in Photoshop 5.5 on the PC, and the rendered version of the image_map
in POV's preview window. Using 2) above, they all look identical once
again.
CONCLUSIONS: Any Photoshop images I create on the Mac for image_maps will
be saved as .bmp, not .png, so that I don't have to deal with this crazy,
unfathomable .png idiosyncracy!
OR
If I ever open a Mac-created .png image again in my PC's Photoshop 5.5 and
then save it, I'll first change PS 5.5 settings to gamma of 1 (or .9) and
NO "Display Using Monitor Compensation."
Some postscripts...
Bringing up the Mac's (unaltered) .png image in PC's Photoshop -- with PS
gamma set to 1.8 (and monitor set to that as well) -- shows no change,
whether using "Display Using Monitor Compensation" or not...so Photoshop is
matching the PC monitor's gamma value very well (which is set with Adobe
Gamma, so that's not surprising!) And it matches the Mac's display of the
image as well.
Interestingly, if I bring the altered (OR unaltered Mac-version) .png image
up in Window's Picture Viewer app, it doesn't quite match the PC's
Photoshop display of the image (with "Display Using Monitor Compensation"
on.) PV's version is a bit brighter. Changing Photoshop's gamma to 1.45
equalizes them. Strange.
Bringing up the PC's POV rendered scene (a .bmp image) in Picture Viewer
exactly matches the POV preview image.
Post a reply to this message
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On Tue, 29 Nov 2005 09:11:08 +0100, Kenneth <kdw### [at] earthlink net>
wrote:
> "Fredrik Eriksson" <noo### [at] nowherecom> wrote:
>> I am not sure if this is what is causing your problems, but PC software
>> has a tendency to assume that the monitor has a gamma of 2.2. Adjusting
>> your monitor to make it have a gamma of 1.8 can make such software
>> produce
>> bad results.
>>
>
> Yet both .bmp and "unaltered" .png image_map images look identical on
> both
> my PC and Mac (both monitors set to gamma of 1.8), in both versions of
> Photoshop and in the final POV rendered scene on the PC. It's only the
> "altered" .png file that causes problems.
Exactly. The bmp has no gamma information. The unaltered png has either no
gamma information, or a gamma of 1.8 (the "correct" value). The altered
png probably has a gamma of 2.2, which makes it misbehave because the
image data is actually gamma 1.8.
Interestingly, I found this tidbit in the PS help files:
"Note: Images created in Photoshop 5.0 or later use the Windows gamma
value (2.2) by default and are at the correct gamma for display in Windows
with no adjustment."
I assume they meant just the Windows versions of PS though.
>> > I've purposely left out the "use assumed_gamma of 1.0" suggestion
>> > altogether
>>
>> I take this to mean that you have tried it, and it made no
>> difference....
>>
> Actually, no. I'm still not totally convinced of its usefulness...mainly
> because I'm STILL trying to understand its pros and cons. The set-up I
> now
> have seems to "work for me, " so I haven't gone that route yet.
> Although,
> I do (sort of!) understand that a scene file to be shared with others may
> need it, for rendering consistency.
That is indeed its purpose, as far as I understand it.
>> Have you examined the files to see what the gamma chunk says?
>
> I have to admit ignornace there; how does one go about doing that?
Off-hand, I can only point to one program, and I wrote that myself...
http://hem.bredband.net/b230591/hdr2exr/hdr2exr_win32.zip
Run it like this:
hdr2exr --verbose suspect_image_file.png %TEMP%\dummy.png
It should then tell you the gamma value encoded in the file.
You can also use that program to correct an erronous gamma value.
--
FE
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