[Radiance-general] Brightness to cd/m2?

[Iris Moonen]

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
>
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