[Radiance-general] accurate light levels

[Thomas Bleicher]

On 14.01.2006, at 16:59, william reynolds wrote:

Welcome to Radiance.

> hi there
>
> As I said in my other question today (spliting a render by light  
> sources)
> I am quite new to radiance, and I am having some issues setting  up my
> renderings. I have radiance installed and working fine, but I dont  
> quite
> understand how I make sure the brightness of the final render is  
> realistic.
> There are a few reasons for this:

You don't have to worry much about the "brightness" of images. The  
Radiance
image file format can store a much greater dynamic range than your  
monitor
can display or your eyes can see.

> I'm not sure what exposure setting to use with pfilt - is it as simple
> as setting '-e 1' so as not to adjust the exposure? I have tried this
> and my images all come out very dark, but this might be because I  
> don't
> have enough light in the scene (see next paragraph, below).

The information within the images is not lost until you convert
the Radiance image (traditionally called *.pic, more resently *.hdr)
to *.tga (or similar) format which may loose all the details. You can
convert to some *.tif formats that support the full range but this
depends on the application you will use to process these images
afterwards.

And yes, using ra_t16 (for *.tga) or ra_tiff (for *.tif) it is as
simple as "-e +2" or "-e -2" to make the image brighter or darker.
Your images may in fact appear dark at the first glance when you render
a scene designed for 100 sources with one active source only.

Open your rendered image in "ximage" which is designed to handle
*.pic images. Hit the "a" key or "h" key to perform an adjustment
of the image exposure on screen, which should give you a better
rendering (on screen) of the image information. Read the man-page
of ximage for further details on the controls. Then read the man-page
of "pcond" which will create an new addjusted image.

Please note that most image manipulation tools bundled with Radiance
will work as filters and dump their output to your terminal window
when not told otherwise. Usually you will redirect the output to a
new file or "pipe" it into another program. These are basic Unix
concepts you should know to get the most out of your new Linux box.

Examples:

     pcond -h+ oldimage.pic > newimage.pic

     pcond -h+ oldimage.pic | ra_t16 -3 > newimage.tga

> Has anyone used radiance with any theatrical light fixtures?
> and would they be prepared to share the .rad files for them?
> I have found the .rad descriptions of architectural fixtures,
> but i'm not familiar with any of them, and even if I was I need
> to be able to use accurate models of the specific instruments
> I'll be using in theatres. I would simply build my own files,
> using the iestorad utility, but rather unhelpfully the instrument
> manufacturers dont seem willing to give out ies data.

Rob Shakespeare, coauthor of "Rendering with Radiance" (see below)
has experiance in this field, as have some others here on the list.

IIRC there was a post a few weeks ago about theatrical lights.
I'm not shure if the poster was willing to share his homemade
messurments of some fixtures. Hope he will answer personally.

> Has anyone made any more materials information available to the
> public? I have looked through the material.rad list of materilas
> in the lib folder, but I'm specifically looking black paint and
> heavy black cloth type materials. If not, then does anyone have
> any recomendations of how to go about determining good values to
> use in my own materials definitions? I dont have access to any
> way of measuring reflected light levels in an experiment.

You can find a short introduction to the radiance materials
at other things here:

     http://radsite.lbl.gov/radiance/refer/usman1.pdf

and here (follow link "Documentation" and start with the
"radcourse_basic.pdf" document)

     http://luminance.londonmet.ac.uk/learnix/

While browsing for the link to Axel Jacobs more resent tutorials
I found this overview of materials:

http://www.learn.londonmet.ac.uk/student/resources/radiance/ 
material_db.shtml

Everyone highly recommends "The Book" about Radiance:

     http://radsite.lbl.gov/radiance/book/

It even includes a chapter about "Dramatic Lighting" (stage lighting).

> Thanks for your help with my probably rather basic questions
> Will


Everyone has his/her share of problems with Radiance ;)

Thomas