Emission Scale

[Jay-ko]

After a chat with Zom-b i wanted to know more about this parameters.

What is exactly "Gain" in emission scale?

"Gain" parameters is so weird for me.

I understand it's a multiply coeficient but why we can change the unit by luminance and luminous emitance?

If you choose "luminance" you obtain "nits" units, and i don't know what it is.

If you choose "luminous emitance" you obtain "lux" and i like "lux".... it's like on my lamp's package in my "buy more" shop and even like on wikipedia when in want to know how much "light" i can have on a surface lighted by a sky in that condition or that one etc....

http://en.wikipedia.org/wiki/Lux

So.. you can read on this article that you find 1000 lux if you are lighted by an overcast sky.

ok, so this emission scale is a "scale" or it's a multiplying coeficient?

I put 1000 on it and chose an overcast texture for my background.
If it's a scale, when my EXR sky texture have a +1 intensity pixel, I normally obtain 1000 lux on the ground... If my texture exr is globally at ~+4, i need to correct it by 250?

Today it's overcast outside my home, so i take my camera, i put the automatic mode, i used 200 iso and I obtain 1/60 shutter speed.
= i need 3,5 ev ajust in indigo to have a correct picture.

Why i need to correct it?

So, in conclusion, if we are in case of physically realistic lighting, what is the correlation between the pixel intensity of the exr background texture and the emiting scale.

[OnoSendai]

Hi Jay-ko, lots of questions there!

Gain is just a multiplier. Gain of 10 means multiply emission spectral radiance by 10.


As for emission scale, it overrides any use of gain etc.. Only the relative values from the EXR file will be used, i.e. the emitted spectral radiance will be scaled so that e.g. the desired luminous emittance is achieved.

As for your comparisons with the real camera, was the aperture radius (f-stop) the same?

The correspondence between real camera output and Indigo output is somewhat approximate anyway, because the cameras use unpublished tonemapping algorithms and correspondences between sensor energy and image output.

[galinette]

The correspondence between real camera output and Indigo output is somewhat approximate anyway, because the cameras use unpublished tonemapping algorithms and correspondences between sensor energy and image output.

Yes for the tonemapping, but for clamping level, they have to follow the ISO standard! (if they want to name it ISO)

[OnoSendai]

The correspondence between real camera output and Indigo output is somewhat approximate anyway, because the cameras use unpublished tonemapping algorithms and correspondences between sensor energy and image output.

Yes for the tonemapping, but for clamping level, they have to follow the ISO standard! (if they want to name it ISO)

Good point. I will look into this, maybe the camera tonemapping calibration can be improved.

[Jay-ko]

Thank you for your explanations!

[Zom-B]

Good point. I will look into this, maybe the camera tonemapping calibration can be improved.

Something that I also wonder about is that White Point calibration is done before tonemapping is applied resulting in tinted images for certain profiles...

Maybe that could be controled by a ceckbox or so.

[Jay-ko]

Hey, so much time are passed since this original question but it is always still open for me.

About white point it seems that it's done on the color (LAB?) share in the IGI before any tone mapping. and placed on the X and Z of the picture.
In that such a case it is clever because it is a method that can avoïd any tone mapping modifications.

It could be very interesting to have a better visualisation of the color space like this kind of :