gray point algorithm

gray point algorithm

Post by Cynthia Tedor » Thu, 09 Dec 2010 12:06:14



I need to know what specific calculations Photoshop applies to an
image when, using the gray point eyedropper in the Levels dialog box,
I select a gray pixel in my image. Specifically, I need to know the
algorithm the software applies to each of the R, G, and B channels in
each pixel when a gray point is selected. For example, in a microscope
camera software I use (Olympus Microsuite), when you select a gray
point pixel, the software calculates the average 0-255 intensity value
across the R, G, and B channels for that pixel. It then calculates a
correction factor for each channel in that pixel by dividing the
intensity value in each channel by the average intensity value. These
correction factors are then applied to the rest of the image. So, for
example, if the RGB values for the selected gray point pixel are
(R,G,B)=(100, 200, 200), then the average intensity=100+200+200)/
3=167.  The correction factor for the R channel=167/100, for the G
channel=167/200, and for the B channel=167/200. These correction
factors are then applied to the rest of the image. Does Photoshop use
this same algorithm, or something different? I need this level of
detail for describing my methods in a scientific paper I'm writing.
Thanks so much!
 
 
 

gray point algorithm

Post by Paul Simo » Thu, 09 Dec 2010 14:33:35



Quote:>I need to know what specific calculations Photoshop applies to an
> image when, using the gray point eyedropper in the Levels dialog box,
> I select a gray pixel in my image. Specifically, I need to know the
> algorithm the software applies to each of the R, G, and B channels in
> each pixel when a gray point is selected. For example, in a microscope
> camera software I use (Olympus Microsuite), when you select a gray
> point pixel, the software calculates the average 0-255 intensity value
> across the R, G, and B channels for that pixel. It then calculates a
> correction factor for each channel in that pixel by dividing the
> intensity value in each channel by the average intensity value. These
> correction factors are then applied to the rest of the image. So, for
> example, if the RGB values for the selected gray point pixel are
> (R,G,B)=(100, 200, 200), then the average intensity=100+200+200)/
> 3=167.  The correction factor for the R channel=167/100, for the G
> channel=167/200, and for the B channel=167/200. These correction
> factors are then applied to the rest of the image. Does Photoshop use
> this same algorithm, or something different? I need this level of
> detail for describing my methods in a scientific paper I'm writing.
> Thanks so much!

I think the situation may be far more complex that what you describe.  Image
data stored in anything but RAW image format is processed by the camera or
associated software which include camera response function and color space
transformation.  In photoshop and other color managed applications, color
management is done by means of .icc profiles to display RGB values on a
monitor or printer, for example.  The interactions between stored image
"colors" and various input and output devices is complex and not so simple
as you have described above.  I suggest you look at "Real World Color
Management" by Bruce Fraser et al, Peachpit Press.

Mention .icc profiles and color management to the Olympus salesman and see
what kind of reaction you get.

Paul Simon

 
 
 

1. picking a gray point in Levels ..using a gray card

If I take a digital picture which includes a Kodak gray card, can I select
that with the gray eye dropper in Levels and get a true color match?

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