[HDR-photo] Re: Measuring *really* small values.

[Blochi]

Maybe this is not the best test image for this. How about putting in  
some reference patches of 0.5 and 1.0 intensity? A plain 0 background  
might be throwing it off as well...
Photosphere measures the center patch as 0.0696646 cd/m2, and the  
surrounding black as 1^-39 cd/m2   ... I calibrated your image in  
Photosphere for the center patch to be 4 cd/m2, try this file:  
www.Blochi.com/files/4cdm_calibrated_in_psphere.hdr

Get Photosphere from www.anyhere.com, and also your question is  
better suited for this list: http://www.radiance-online.org/mailman/ 
listinfo/hdri

Blochi


On Jun 1, 2007, at 11:49 AM, Dan Reetz wrote:

> Hey Ron,
>
> In this particular case, noise is not an issue- the HDR image is a CG
> render from Autodesk Maya, which consists of one bright rectangle and
> surrounding black space at exactly 0,0,0 for R,G, and B, respectively.
> I need super-low values because I am trying to drive our HDR display
> at very low luminances. On this display, a pixel value of .0.000389099
> will actually result in 24.8cd/m2, and I need to get that down to
> about 4.
>
> But if I were working in camera space, I would definitely take those
> things into consideration. Thanks!
>
> Dan Reetz
>
> On 6/1/07, Ronald Murray <kandroma1 at mac.com> wrote:
>>
>> When you read this and other values, do you factor in device noise  
>> levels? There really should be a convention of stating a luminance  
>> value along with some measure of variation (+/-).
>>
>> Selecting larger or smaller patches of the area of interest will  
>> give you a sense of this, and will let you know how much decimal  
>> places in the tool readouts matter.
>>
>> Ron Murray
>>
>>
>> On Friday, June 01, 2007, at 11:54AM, "Dan Reetz"  
>> <danreetz at gmail.com> wrote:
>> >Sorry to reply to my own message.
>> >
>> >If I open the file in Photomatix Basic, and open the Histogram,  
>> is the
>> >value under Absolute (2.90E-04) the actual pixel value?
>> >
>> >Thanks,
>> >Dan Reetz
>> >
>> >On 6/1/07, Dan Reetz <danreetz at gmail.com> wrote:
>> >> Hi all,
>> >>
>> >> I have a Radiance encoded HDR file, and I want to know the  
>> values of
>> >> the pixels within. The problem is that they seem to exceed the
>> >> 3-decimal place accuracy of the tools that I usually use.  
>> (Meaning, if
>> >> I use the Info tool in Photoshop or HDRview, they show up as all
>> >> zeros).
>> >>
>> >> Can anyone suggest a tool that can accurately read back the  
>> values in
>> >> the center rectangle of this image?
>> >>
>> >> http://danreetz.com/for_HDR-photo/4cdm.hdr (170kb)
>> >>
>> >> Thanks,
>> >> Dan Reetz
>> >>
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>> >
>> >
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