[Radiance-general] Brightness to cd/m2?

[Greg Ward]

Hi Iris,

Thanks for sharing your command.  The new command should in fact give you the average radiance in watts/m^2, which when multiplied by 179 should tell you the average luminance in cd/m^2 as you desire.  If the answer is not what you expect, then something else is going wrong.

-Greg

> From: Iris Moonen <moonen.iris at gmail.com>
> Subject: Re: [Radiance-general] Brightness to cd/m2?
> Date: October 28, 2015 7:18:23 AM PDT
> 
> Hello,
> 
> Before I used "pvalue -d -b -h -H picture.hdr | total". This gave me
> numbers such as 89057. After these mails I've been trying to use
> "pvalue -o -b -d -h -H picture.hdr | total -m" but now I get values
> which are about 0.2832. The cd/m2 values I should get should be about
> 700 cd/m2, which I don't get by multiplying either of these 2 values
> with 179. What do I do wrong?
> 
> With kind regards, Iris
> 
> 2015-10-26 18:21 GMT+01:00 Greg Ward <gregoryjward at gmail.com>:
>> Hi Iris,
>> 
>> You really need to share your entire command chain with us.  The total program will compute averages, but only with the "-m" option.
>> 
>> Cheers,
>> -Greg
>> 
>>> From: Iris Moonen <moonen.iris at gmail.com>
>>> Subject: Re: [Radiance-general] Brightness to cd/m2?
>>> Date: October 26, 2015 10:08:32 AM PDT
>>> 
>>> Hello,
>>> 
>>> I've tried that but it doesn't say exposure in the header, and while
>>> using this the values still stay this high. I did notice that when I
>>> change the image size, the values differ. I use "| total" because I
>>> need the average cd/m2 value. Is it possible that I need to divide
>>> this number that I get by pvalue | total before multiplying it with
>>> 179?
>>> 
>>> With kind regards,
>>> Iris
>>> 
>>> 2015-10-23 18:31 GMT+02:00 Greg Ward <gregoryjward at gmail.com>:
>>>> Hello Iris,
>>>> 
>>>> If the picture has been re-exposed by some other program (such as pfilt), then there will be an EXPOSURE= line in the header, which you can see with the getinfo command.  In any case, you can use the "pvalue -o -b" option to undo this exposure and convert to brightness, then multiply the output by 179 to get luminance in cd/m^2.
>>>> 
>>>> This is of course assuming your picture was generated by Radiance in the first place using a properly calibrated scene.  If it is just an HDR image you got from somewhere, it is unlikely that the pixels have an absolute basis.
>>>> 
>>>> Best,
>>>> -Greg
>>>> 
>>>>> From: Iris Moonen <moonen.iris at gmail.com>
>>>>> Subject: [Radiance-general] Brightness to cd/m2?
>>>>> Date: October 23, 2015 9:19:32 AM PDT
>>>>> 
>>>>> Hello,
>>>>> 
>>>>> I have a question. I want to get cd/m2 values by using the pvalue. Now
>>>>> I'm not sure how to get those cd/m2 values - I read somewhere that you
>>>>> should multiply the values you get from pvalue with 179, but if I do
>>>>> that I get crazy high values (about 15941203). Do I do something
>>>>> wrong? I have been trying to search for what pvalue exactly gives you
>>>>> (I know it's the brightness value but I can't find what unit it is
>>>>> in), but had no luck on that so far. I also wanted to know if this
>>>>> value is dependent on the area of your luminance picture or not.
>>>>> 
>>>>> Thank you so much in advance!
>>>>> Iris
>>