I am quite new to Radiance and have not used the exposure setting so far.
So far I had expected to get a "well illuminated" scene with some shadows
cast.
I have to read a bit on how exposure works.
Does it work like an ambient light to sort of adjust the overall brightness
of the image?
From what I understand so far exposure only affects
the view of the rendered image, not the actual physical properties
like illuminance, radiance and irradiance.
And if rtrace is used to "view" the image instead of rvu the
illumination would be consistent.
Is that correct?
Kind regards,
Joe
···
Am 08.08.2017 10:01 schrieb Jan Wienold:
Hi Joe,
why do you think it is too bright? Is the luminance to high? I guess
you just have to adapt your exposure setting for the image.
The physics for gendaylit and your parameters is correct:
1. test of luminance of the sun (ray in sun direction):
Yes; exposure affects the appearance of the image but not the underlying
photo/radiometric values, or pixel data if you will. A Radiance image is
this amazing collection of spot meter data, basically. I’m not sure I
understand your last statement there, about the “illumination [being]
consistent”.
You can take a Radiance luminance image and run it through pcond to
'tonemap' the image and create all kinds of valuable effects. The simplest
is the '-h' option, which will map the pixel luminance according to human
vision capability. Thus, creating an image that conveys the appearance of
the scene as viewed by a human observer in that scene.
The main takeaway is that the raw Radiance images are physically accurate,
yet you are viewing them on dynamic range-limited devices, so they will be
hard-pressed to “look right”.
From what I understand so far exposure only affects
the view of the rendered image, not the actual physical properties
like illuminance, radiance and irradiance.
And if rtrace is used to "view" the image instead of rvu the
illumination would be consistent.
Does pcond -h something like automatically calculate the maximum luminance
and adjust the exposure? Or will it also apply the different perception of red, green and blue?
Is there a parameter for rvu to automatically set the exposure using the maximum Luminance
calculated?
···
Am 08.08.2017 17:03 schrieb Guglielmetti, Robert:
Yes; exposure affects the appearance of the image but not the underlying
photo/radiometric values, or pixel data if you will. A Radiance image is
this amazing collection of spot meter data, basically. I’m not sure I
understand your last statement there, about the “illumination [being]
consistent”.
You can take a Radiance luminance image and run it through pcond to
'tonemap' the image and create all kinds of valuable effects. The simplest
is the '-h' option, which will map the pixel luminance according to human
vision capability. Thus, creating an image that conveys the appearance of
the scene as viewed by a human observer in that scene.
The main takeaway is that the raw Radiance images are physically accurate,
yet you are viewing them on dynamic range-limited devices, so they will be
hard-pressed to “look right”.
From what I understand so far exposure only affects
the view of the rendered image, not the actual physical properties
like illuminance, radiance and irradiance.
And if rtrace is used to "view" the image instead of rvu the
illumination would be consistent.
Does pcond -h something like automatically calculate the maximum
luminance
and adjust the exposure? Or will it also apply the different perception
of red, green and blue?
Something like that, yeah. When low-luminance scenes are detected (by
inspecting the pixels’ values), the loss of color acuity is modeled.
Disability glare and veiling reflections are modeled as well. The exact
goings-on are detailed in Chapter 7 (Roadway Lighting) of the Rendering
with Radiance book, and the -h option to pcond is simply a combination of
luminance-based tweaks to the pixels, discussed in that chapter. The
output images lose all validity at the pixel value level, but they make
for compelling images on low dynamic range displays.
You will definitely want to look at Rob Shakespeare’s work (he’s
presenting an update at this years’ Radiance Workshop) with using Radiance
images and tone mapping to simulate scenes as perceived by aging eyes.
It;s awesome stuff, and a great illustration of the value of Radiance
images and the tools used to create and analyze them.
Is there a parameter for rvu to automatically set the exposure using the
maximum Luminance
calculated?
In rvu you can hit ‘e’ and the pick a point near where you know max
luminance to be and it will scale the exposure accordingly, albeit
linearly. You can also use ‘pextrem’ at the command line to get the
maximum and minimum pixel values (and x,y locations) of an input image.
Another utility you may be interested in is ‘phisto’, which will create a
histogram of a series of input images and supply a value you can use to
expose the input images uniformly (sorta).
