[Radiance-general] Glass Transmittance not showing

[Thomas Bleicher]

On 7 Oct 2008, at 04:11, Jhun Calixihan wrote:

> Hi!
>
> I am new to Desktop Radiance and wish to ask if anybody knows why I  
> could
> not get any differences in luminance rendering clear and dark glass.

[...]

> But I get the same image rendered, the objects behind the glass  
> looks the
> same.  Looks like there is no difference in the transmission thru  
> the glass,
> or is there something wrong with my approach?  But when I click on  
> the image
> to display luminance I get different cd/m2 numbers.
>
> Any advise would be helpful.  Thanks in advance!

I think you only have a problem in understanding the image format and  
its
representation on screen.

Radiance stores the calculation result as an image where every pixel has
RGB value and an exponent giving the 'brightness' of the point. This
format can store an illuminance range that exceeds the capabilities  
of your
display by far. To get a visual impression (the image you would expect
from the rendering) the image viewer has to convert the real values  
in the
picture file to RGB values for your computer screen.

If you have two different pictures with the same scene but lit with  
different
amounts of light this adjustment will present you the image optimised  
for
the display, which probably means that the real brightness of the scene
is changed to something that gives the best representation for your  
eyes.
The images 'look' similar (or indeed identical) but the real content  
of the
image is different (as you have found out with the cd/m2 values).

You can think of this process of something like your eyes do in  
different
lighting conditions: At first you will feel blinded in a bright room  
but after
a while your eyes adjust to the light levels and you can see  
normally. If
you move to an identical but dimly lit room you will think it's too  
dark. After
some time you will get used to it and you can see in this room just  
like in
the room before.


To help you with your problem:

While you view the image in an image viewer for Radiance *.pic file
format you have to set the 'exposure' to the same value for both images.
If you use ximage (on Unix) use the '-e' option for this.

If you can't do this in your picture viewer use the tool 'pcompos' to  
combine
the two pictures into one and view them at the same time:

pcompos -a 2 bright.pic dark.pic > brigh_and_dark.pic


Regards,
Thomas