[Radiance-general] rvu exposure setting

Tue May 15 16:32:06 PDT 2012

Hi Stefano,

Sorry, I thought maybe you were coming from one of the other rendering tools where you need to set exposure as a rendering parameter.  get it wrong and you wasted valuable computation time.

There are two options for automated post-processing in Radiance: pfilt sets the exposure to the image average and pcond mimics human visual response.  You could just run your images through one of those tools and they would select a good exposure for each image.  

rpict -ab ... mymodel.oct > image.hdr
pfilt image.hdr > exposureadjusted_image.hdr
or 
pcond image.hdr > humanvision_image.hdr

If you want all your images to use the same exposure, you can use pfilt with a single pass (-1) and set exposure with "-e".  In this case, I'd suggest having separate render and post process scripts so you can easily change the exposure - it might require a bit of fiddling to find a single exposure that gives decent results for all images.

Andy

On May 15, 2012, at 4:07 PM, Stefano Moret wrote:

> Thanks for your replies.
> 
> @Andy: I'm aware of that thing, but I was wondering how to get good exposures in cases in which I couldn't interactively set it, like when I'm using scripts to automate the images creation process;
> @Rob: ok, I'm posting it on Openstudio forum.
> 
> Cheers,
> Stefano
> 
> --
> 
> Stefano Moret
> California Lighting Technology Center
> University of California, Davis
> 633 Pena Drive
> Davis, CA 95618
>  
> 530-747-3846
> smoret at ucdavis.edu
> 
> On 15/mag/2012, at 14.04, Andy McNeil wrote:
> 
>> Hi Stefano,
>> 
>> It sounds to me that you think exposure needs to be set before rendering.  It doesn't.  The image format used by Radiance contains more data than typical image formats allowing you to adjust exposure after rendering.  With rvu you can type "e", then enter then click on any pixel to adjust the overall image exposure to the value of that pixel. Or you can type "e 1" to adjust to the exposure to the image average (" e 2 " is twice the image average).  Or "e -1" to reduce exposure by one f-stop (+1 to increase).  You can adjust exposure as much as you like in rvu.
>> 
>> For a rendered hdr image you can adjust exposure after rendering with pfilt.
>> 
>> Andy
>> 
>> 
>> On May 15, 2012, at 12:49 PM, Stefano Moret wrote:
>> 
>>> Dear Rob,
>>> 
>>> Thanks for clarifying this. So which would be the best way of getting a good image with a "correct" exposure if I want, for example, to automate the images production process with a script? 
>>> My question ultimately ends up referring to the Ruby script Daylightsim.rb for Openstudio exported models (which was probably written by you?) which is giving me in output very dark/bad quality .hdr images.
>>> 
>>> Thanks,
>>> Stefano
>>> 
>>> --
>>> 
>>> Stefano Moret
>>> California Lighting Technology Center
>>> University of California, Davis
>>> 633 Pena Drive
>>> Davis, CA 95618
>>>  
>>> 530-747-3846
>>> smoret at ucdavis.edu
>>> 
>>> On 14/mag/2012, at 18.47, Guglielmetti, Robert wrote:
>>> 
>>>> Hi Stefano,
>>>> 
>>>> The ambient value is used as a proxy estimate for the interreflected light, to be used for pixels when the other ambient investigation methods have been exhausted (ab, as, ad, lw, etc). It's a "value of last resort", if you will. So in one sense, if you drive the simulation with aggressive ambient parameters, the av setting becomes fairly irrelevant because a "good" av will be derived deterministically. Of course in keeping with the "there's no such thing as a free lunch" theme, cranking up the other ambient parameters results in long simulation times. There are some rules of thumb regarding "good" av settings for indoor and outdoor scenes that you can start with (consultation of the "rad" manual page will reveal these), but for a good image or numeric result you should be asking Radiance to do some raytracing to tease these values out for all the little corners of your space. 
>>>> 
>>>> As far as using rvu for a testbed, this is an excellent way to interactively see the effects of the various parameters on appearance and render time. If you change a setting though, you need to redraw the image with the "new" command, or by clicking "redraw" on the new Qt-based rvu that comes with the NREL installers, to see the effect. Speaking of the NREL Radiance installers, we are posting updated packages tomorrow that address the missing .cal files issue that is present in the current installers. 
>>>> 
>>>> - Rob
>>>> 
>>>> ________________________________________
>>>> From: Stefano Moret [smoret at ucdavis.edu]
>>>> Sent: Monday, May 14, 2012 6:47 PM
>>>> To: radiance-general at radiance-online.org
>>>> Subject: [Radiance-general] rvu exposure setting
>>>> 
>>>> Hi,
>>>> 
>>>> I’ve a basic question which I haven’t really succeeded to get answer to from the different tutorials. When I visualize an octree with rvu I usually set –av and –ab as first parameters.
>>>> As far as I understand, the “ambient value” should set the brightness of the scene, but I don’t see any change by setting it to different values, and to get a nice image I usually use the –pe option to set the exposure.
>>>> Could you please enlighten me on the usage of “ambient vale” option to get a good exposure in my images?
>>>> 
>>>> Thanks,
>>>> Stefano
>>>> 
>>>> --
>>>> 
>>>> Stefano Moret
>>>> California Lighting Technology Center<http://cltc.ucdavis.edu/>
>>>> University of California, Davis
>>>> 633 Pena Drive
>>>> Davis, CA 95618
>>>> 
>>>> 530-747-3846
>>>> smoret at ucdavis.edu<mailto:smoret at ad3.ucdavis.edu>
>>>> 
>>>> 
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