hdri Gamma


#1

ok heres an image i created

http://www.deviantart.com/view/9173086/

I think it pretty self explanatory, so here`s the question the one with the gamma correction does it look right? I get the feeling that it looks washed out no? any toughts?


#2

You are only using a .jpg for the IBL. Why are you using gamma correction on it?


#3

no only the top image is a jpg the left one is a hdr no gamma correction, and the right has a gamma correction of 2.2


#4

Your top image looks kinda washed out.
But you render it with a jpg,right ?

My understanding is,that you have to shoot a reference on location,
to have something to compare to.
Otherwise how can you tell which is right or wrong ?

.


#5

ok check it out this is what I am trying to understand

can you open an hdri image with imf_disp

well if you can youll see that maya defaults to a gamma of 1.0

this turns out to be darker, I am trying to adjust the gamma of the hdri so that it matches my original 2.2 without clipping the values, I beleieve the reason the bottom right image looks washed out is because the gamma node clipped the black and white values


#6

Wouldn’t the Gamma just be adding to the values, instead of actually increasing the pixels values from eachother to increase the darker shades and brighter whites which come with floating point images, converting a jpg to a hdr requires extensive overlays, boosting the brightness without brightening the darks.


#7

check this out open ure fav hdri image using img_disp command from the command prompt, youll see that when maya opens an hdri image its gamma is set to 1.0 so that in turn makes ure hdri image much darker since it was created with a 2.2, (most images are) that is the problem at hand

I have created this real world example to see if u guys can help me please
both of these images are using an hdri image…
look carefully at the background and you can see the difference
http://www.deviantart.com/view/9185104/


#8

hm… you are right.
Just checked this, and it dims the highlights too.
This is probably because the gamma node outputs rgb values only,
so it ignores the energy values.
Can you use the color gain value instead in your file texture.
Note: IF you have access to XSI and you have the xsi shaders for maya,
you can use the [size=2]sib color math exponent node i think.[/size]
[size=2]Just did a small test and looked good.[/size]
[size=2][/size]
[size=2].
[/size]


#9

finally some1 thats on the same page as me, nope i don`t have neither of those nodes, but maybe i can use a regular maya math function, exactly what does a gamma node do?

any1 else have any ideas?


#10

the color gain is not a node :slight_smile:
it is inside your hdr texture, or any file texture.

(I have never before used a gamma node on a hdr texture.
I use it with a luminance node though, to control the saturation.)

What dows the Gamma node do ?
new == old ** (1.0/Gamma)

.


#11

Can’t we just change the gamma of the image from 2.2 to 1 in, for example, PhotoghenicsHDR? I tried it, not sure if it helped much, but I think it got a little better. (Although it did make that other problem worse, the one where the hdr is overly saturated so the resulting rendering becomes oversaturated. So I also had to reduce saturation.)


#12

Hi stahlberg, tell you what I tried that route I downloaded photogenics 30 day trial and i edited adjusted my gamma to the right amount and to my surprise it looked identical to the one maya had with the gamma node, Can some1 actually confirm for us if the gamma node is clipping my hdri? cause if it isnt then problem is solved


#13

No,
no problem solved ,because if this is the case,
them im really dissapointed that mayas gamma correct node cannot
proccess floating point values
And I thought for a moment that most of the hypershade nodes calculate everything in floating point.
maybe this is not the case for hdr pictures. :surprised :argh: :shrug:
Anyway I did a test with a set range utility instead, and I had somewhat interesting results.

Instead of the gamma correct node, try a set range utility.

Old Min = 0
Old Max = 1
New Min = 0
New Max = I tried 2.5

After you render take a closer look of your highlights.
They dont look clamped.
And the result is different, when I use the color gain attribute in the file (hdr) texture.

Then you can pipe the original hdr with a luminance node to make it B&W and use
a blend colors utility to control the saturation.

EDIT :
I just realise, that for best results you sould render in something highter than 8-bit
(16-bit,float)
then color corect that in post.

.


#14

is there maybe a mel command that will output the values my gamma node is spitting out so i can see what the values look like and if it`s clipping them?


#15

guys if any1 finds an answer to this let me know please


#16

I don’t think the gamma node is clipping on you. However try unchecking the box
“clip final shaded color”
This clips the outColor of shaders to zero to one if on. (In my opinion it really should be off by default) For example if the incandescence of an object is “super bright” and it is seen through a semi transparent object it should punch through more than a mearly white object, but with this toggle on it will look as if it were simply white. It is a bit confusing, because in terms of UI one would think it is simply clipping the color before passing it to the final image buffer( but this must happen, so it could not be an option).

Duncan

#17

hi duncan glad to see you to the rescue :slight_smile:

anyways I do have this thread in highend3d and some1 told me it`s impossible to change the gama on an hdri
is this true?
check out the thread he seems very knowledgeable, and I will try out the clipping option
http://www.highend3d.com/boards/showflat.php?Cat=&Board=MayaMR&Number=183288&page=1&view=collapsed&sb=5&o=0&fpart=1#Post183640

oh and thanks ure were my last hope for salvation, tech support said theyd log a bug and get back.... I looked for the clipping final shaded colorsI am using mental ray for this, and its set to render raw images


#18

I looked at the gammaCorrect node and it does not seem to clip its output to zero to one. Perhaps the confusion comes from expecting the gamma to boost values below 1.0 to be greater than 1.0. What the gamma does is modify the curve of the color values. This curve always goes through 1.0 and zero, thus a value of 1,1,1 or 0 0 0 will not be affected by gamma. If you wish to brighten values beyond one you can scale the color intensity before passing it into the gamma node.

For example on the texture you could increase the colorGain… With an hdri I believe this would be equivalent to simply increasing the exposure time on a camera. The gamma is more to get the right shape to the response(what is right for you would depend on a lot of factors), as opposed to changing brightness.

If you have 6.0 you might try the remapColor node. This allows you to adjust the gamma curve directly. You can also set the indice values beyond 0-1 if desired by typing in the selected value field.

Duncan


#19

thanxs duncan I do have remap color node but it doesn`t seem to work on mr, when I render using maya render it renders fine, but when I switch to mr my geo disappears, It does have an alpha though so its just the shading is completely black


#20

I am fairly certain none of the new remap nodes work with mental ray.