Pvalue for converting to luminance

Hi all,

I tried to apply the factor of 179 to the scaled brightness from
pvalue. However, it seems that the value is still slightly off from
the actual one read from psfview.

The value from pfsview is 2.579e+4

while from pvalue is 2.803e+4

i used pvalue -b -o sample.hdr

Is there an exact value for the scale ?

Thanks
Cw

Hi Cw,

I'm not sure how pfsview converts colors to luminance values, but it may apply a different set of coefficients from those used in Radiance (and pvalue). This could explain the discrepancy. The best way to check is to compare values for a gray pixel, which should be the same regardless of the color -> luminance conversion.

Also, are you 100% sure you are checking the exact same pixel(s)?

-Greg

···

From: "chang cw" <[email protected]>
Date: September 5, 2008 7:27:46 PM PDT

Hi all,

I tried to apply the factor of 179 to the scaled brightness from
pvalue. However, it seems that the value is still slightly off from
the actual one read from psfview.

The value from pfsview is 2.579e+4

while from pvalue is 2.803e+4

i used pvalue -b -o sample.hdr

Is there an exact value for the scale ?

Thanks
Cw

Hi Greg,
Thanks for replying so fast.
So i assume that the value read off from ximage is the same at that
from pvalue ?

Using -b -o option and a factor of 179 ?
Sorry but i could get not the radiance source compiled for ximage ;(

Thanks
Chew Wai

···

On Sat, Sep 6, 2008 at 10:36 AM, Greg Ward <[email protected]> wrote:

Hi Cw,

I'm not sure how pfsview converts colors to luminance values, but it may
apply a different set of coefficients from those used in Radiance (and
pvalue). This could explain the discrepancy. The best way to check is to
compare values for a gray pixel, which should be the same regardless of the
color -> luminance conversion.

Also, are you 100% sure you are checking the exact same pixel(s)?

-Greg

From: "chang cw" <[email protected]>
Date: September 5, 2008 7:27:46 PM PDT

Hi all,

I tried to apply the factor of 179 to the scaled brightness from
pvalue. However, it seems that the value is still slightly off from
the actual one read from psfview.

The value from pfsview is 2.579e+4

while from pvalue is 2.803e+4

i used pvalue -b -o sample.hdr

Is there an exact value for the scale ?

Thanks
Cw

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Hi Chew,

Yes, pvalue -o -b * 179 and the ximage 'l' command should report the same values for the same pixels, though it's a bit difficult to know if you are in fact checking the same pixels -- the ximage 'p' command can help with that.

I guess this is moot since you couldn't compile ximage. I believe Ecotect offers a Windows-ready substitute free of charge.

Best,
-Greg

···

From: "chang cw" <[email protected]>
Date: September 5, 2008 7:39:49 PM PDT

Hi Greg,
Thanks for replying so fast.
So i assume that the value read off from ximage is the same at that
from pvalue ?

Using -b -o option and a factor of 179 ?
Sorry but i could get not the radiance source compiled for ximage ;(

Thanks
Chew Wai

HI greg,

Thanks, i believe for RGB, the conversion to radiance units is the same factor ?

I think as compared to PFSview, the pixels for Pvalues are shifted -1
-1 for x and y.

Thanks

Cw

···

On Sat, Sep 6, 2008 at 12:15 PM, Greg Ward <[email protected]> wrote:

Hi Chew,

Yes, pvalue -o -b * 179 and the ximage 'l' command should report the same
values for the same pixels, though it's a bit difficult to know if you are
in fact checking the same pixels -- the ximage 'p' command can help with
that.

I guess this is moot since you couldn't compile ximage. I believe Ecotect
offers a Windows-ready substitute free of charge.

Best,
-Greg

From: "chang cw" <[email protected]>
Date: September 5, 2008 7:39:49 PM PDT

Hi Greg,
Thanks for replying so fast.
So i assume that the value read off from ximage is the same at that
from pvalue ?

Using -b -o option and a factor of 179 ?
Sorry but i could get not the radiance source compiled for ximage ;(

Thanks
Chew Wai

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Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

I tried to apply the factor of 179 to the scaled brightness from
pvalue. However, it seems that the value is still slightly off from
the actual one read from psfview.

Hi,

from the manpage of pfsinrgbe[1]:

···

--
"IMPORTANT NOTE

There is an incompatibility between viewers of RGBE format in terms of
absolute luminance values. The ximage viewer from Radiance package
expects radiance values in RGBE files and uses the WHITE_EFFICACY
constant equal to 179.0f to convert from radiance to luminance. This in
practice means that the values displayed with pfsview maybe higher than
expected. To compensate for this incompatibility we suggest to use
pfsabsolute command:

pfsinrgbe measured_luminance.hdr | pfsabsolute 1 179 | pfsview

This is not necessary when RGBE files are saved using pfsoutrgbe,
Radiance package, mkhdr or Photospheare. This is also not relevant when
only relative absolute values are considered."
--

Can you please try the work-around and tell us if you get the expected
results than?

CU Lars.

[1] http://www.mpi-inf.mpg.de/resources/pfstools/man1/pfsinrgbe.1.html

Sorry.. you use this on hdr images generated by radiance, assembled from
fotographs, or..?

Hi I use it on photos and generated using psftool

Actually want to find out if there are any tools that can read out the values using psftool into text mode

Tks
cw

···

-----Original Message-----
From: Lars O. Grobe <[email protected]>
Sent: Saturday, 6 September, 2008 19:42
To: Radiance general discussion <[email protected]>
Subject: Re: [Radiance-general] Pvalue for converting to luminance

Sorry.. you use this on hdr images generated by radiance, assembled from
fotographs, or..?

Actually want to find out if there are any tools that can read out the values using psftool into text mode

I think the authors did not implement this simply because it already
exists - pvalue has been around for a while. So you could write your own
pvalue tool, in something like the pfsout-tools, but with a text format
target. Another simple way would be to combine pfsabsolute and
pfsoutppm, to create an ascii-text coded grey-scale image with linear
mapping, and read out the values than. But the accuracy would be very
bad (8bit).

Hi Lars

Greetings from Singapore :).

Thanks for the suggestion. i think will stick with pvalue then.

So when is the next time you dropping by Singapore?

Best Regards,
cw

···

2008/9/7 Lars O. Grobe <[email protected]>:

Actually want to find out if there are any tools that can read out the values using psftool into text mode

I think the authors did not implement this simply because it already
exists - pvalue has been around for a while. So you could write your own
pvalue tool, in something like the pfsout-tools, but with a text format
target. Another simple way would be to combine pfsabsolute and
pfsoutppm, to create an ascii-text coded grey-scale image with linear
mapping, and read out the values than. But the accuracy would be very
bad (8bit).

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Radiance-general mailing list
[email protected]
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