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

[Dan Reetz]

Hey Blochi,
actually, this is all very good stuff, especially the note about RLE
encoding. However, when I'm referring to cd/m2, I mean the output of
our Brightside HDR display, not software estimation.

What I really need to know is what the actual values of each pixel
are, as stored in the Radiance HDR file. Summer emailed me off-list
with the output of a custom program, and the values line up with what
I was hoping, so I guess that part of the problem is solved.

I'll sign up on the other list for more technical/file
format/precision questions, thanks for the pointer.

Thanks all.
Dan Reetz

On 6/1/07, Blochi <Blochi at blochi.com> wrote:
> OK, it does look like the RLE encoding is the key.
> Uncompressed Radiance keeps the values: www.Blochi.com/files/
> 4cdm_calibrated_in_psphere2.zip
> Just opening and saving it as RLE encoded Radiance (i.e. in
> Photoshop) brings it back to the 0.696646 that was in your file. So,
> it seems to exceed the precision that the encoding can handle.
>
> Sorry for this monologue.
>
> Blochi
>
> On Jun 1, 2007, at 12:31 PM, Blochi wrote:
>
> > errr... sorry, apparently that calibration fell off with when
> > saving it RLE compressed. Uncompressed it's 7.9 MB, so try
> > www.Blochi.com/files/4cdm_calibrated_in_psphere.exr
> >
> > Blochi
> >
> > On Jun 1, 2007, at 12:17 PM, Blochi wrote:
> >
> >> 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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