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Best RGB value of 'mid-grey' ?

 
 
Sorby
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      05-17-2004
My local printers print 10"x8" and 12"x10" but I need a 10"x10".
So I have widened the original square-format photo so there is a 1" strip
either side.
These strips are both white but it occured to me this may skew the
auto-colour correction when I get the lab to print them.
Should I change the colour of these white strips to a mid-grey?
If so, what is the value of mid-grey in RGB terms?

Thanks

--
Sorby


 
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nixjunk
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      05-17-2004
>My local printers print 10"x8" and 12"x10" but I need a 10"x10".
>So I have widened the original square-format photo so there is a 1" strip
>either side.
>These strips are both white but it occured to me this may skew the
>auto-colour correction when I get the lab to print them.
>Should I change the colour of these white strips to a mid-grey?
>If so, what is the value of mid-grey in RGB terms?
>
>Thanks
>
>--
>Sorby
>


There should be no default correction unless you ask for it for digital images.
Check to be sure. The only reason you should change it to a grey is to make it
easier to see where to cut in case the area in the picture along the border is
white or black.






 
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Bart van der Wolf
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      05-17-2004

"Sorby" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> My local printers print 10"x8" and 12"x10" but I need a 10"x10".
> So I have widened the original square-format photo so there is a 1" strip
> either side.
> These strips are both white but it occured to me this may skew the
> auto-colour correction when I get the lab to print them.
> Should I change the colour of these white strips to a mid-grey?
> If so, what is the value of mid-grey in RGB terms?


In RGB terms it is 128,128,128.
In photographic 18% average reflection terms it is 117.

Bart

 
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David J Taylor
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      05-17-2004
"Bart van der Wolf" <(E-Mail Removed)> wrote in message
news:40a8d42f$0$65124$(E-Mail Removed)4all.nl...
[]
> > If so, what is the value of mid-grey in RGB terms?

>
> In RGB terms it is 128,128,128.
> In photographic 18% average reflection terms it is 117.
>
> Bart


Assuming what value for gamma?

David


 
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Bart van der Wolf
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      05-17-2004

"David J Taylor" <(E-Mail Removed)-this-bit> wrote in
message news:rW5qc.2276$(E-Mail Removed)...
> "Bart van der Wolf" <(E-Mail Removed)> wrote in message
> news:40a8d42f$0$65124$(E-Mail Removed)4all.nl...
> []
> > > If so, what is the value of mid-grey in RGB terms?

> >
> > In RGB terms it is 128,128,128.
> > In photographic 18% average reflection terms it is 117.
> >
> > Bart

>
> Assuming what value for gamma?


2.2 as in sRGB, Adobe RGB or whatever most digicams use when creating a
JPEG.

I've given a more detailed explanation in an earlier, somewhat related,
thread called "D70 Photoshop and gray cards".
Since the OP already has a Gamma adjusted image he wants to add info to, 128
is good enough (still depending on what the Lab does to the data).

Bart

 
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David J Taylor
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      05-17-2004
"Bart van der Wolf" <(E-Mail Removed)> wrote in message
news:40a8f6b0$0$563$(E-Mail Removed)4all.nl...
>
> "David J Taylor" <(E-Mail Removed)-this-bit> wrote in
> message news:rW5qc.2276$(E-Mail Removed)...
> > "Bart van der Wolf" <(E-Mail Removed)> wrote in message
> > news:40a8d42f$0$65124$(E-Mail Removed)4all.nl...
> > []
> > > > If so, what is the value of mid-grey in RGB terms?
> > >
> > > In RGB terms it is 128,128,128.
> > > In photographic 18% average reflection terms it is 117.
> > >
> > > Bart

> >
> > Assuming what value for gamma?

>
> 2.2 as in sRGB, Adobe RGB or whatever most digicams use when creating a
> JPEG.
>
> I've given a more detailed explanation in an earlier, somewhat related,
> thread called "D70 Photoshop and gray cards".
> Since the OP already has a Gamma adjusted image he wants to add info to,

128
> is good enough (still depending on what the Lab does to the data).
>
> Bart


Thanks, I haven't done the sums to see if you are correct! Yes, 117
sounds good to me for an 18% reflectance.

Cheers,
David


 
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Tony Spadaro
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      05-17-2004
A Kodak grey card will be roughly 160 RG&B for a 2.2 Gamma CCalibrated
monitor. I don't see why you need the background to be grey though. You can
use anything you like and yes colour of the rebate will affect your
perception of the colours in the photo, but they have no "physical" effect
on them. Once you trim to the picture area all will be equal. Of course you
could use the rebate to "matte" the picture. When I have pictures with white
areas at or very near the edge I put a few pixel wide black border on the
picture. IF the edge is black I use white and if there is black and white
both to deal with I have a nice dark red, etc.

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The Improved Links Pages are at
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"nixjunk" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> >My local printers print 10"x8" and 12"x10" but I need a 10"x10".
> >So I have widened the original square-format photo so there is a 1" strip
> >either side.
> >These strips are both white but it occured to me this may skew the
> >auto-colour correction when I get the lab to print them.
> >Should I change the colour of these white strips to a mid-grey?
> >If so, what is the value of mid-grey in RGB terms?
> >
> >Thanks
> >
> >--
> >Sorby
> >

>
> There should be no default correction unless you ask for it for digital

images.
> Check to be sure. The only reason you should change it to a grey is to

make it
> easier to see where to cut in case the area in the picture along the

border is
> white or black.
>
>
>
>
>
>



 
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Dave Martindale
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      05-19-2004
"David J Taylor" <(E-Mail Removed)-this-bit> writes:

>Thanks, I haven't done the sums to see if you are correct! Yes, 117
>sounds good to me for an 18% reflectance.


It also depends on the image exposure. Does 255 represent the
brightness of a mythical 100% reflectance object, or a more typical
piece of white paper/cloth (90% reflectance), or did you allow more
headroom for specular reflections and place white at 200 or 230 instead
of 255?

If you assume that 255 is 100% reflectance matte white, then 18%
reflectance is 0.18^(1/2.2) * 255 = 0.459 * 255 = 117. But other
exposure conditions yield different mid-grey values.

Dave
 
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David J Taylor
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      05-19-2004
"Dave Martindale" <(E-Mail Removed)> wrote in message
news:c8g1kb$6he$(E-Mail Removed)...
> "David J Taylor" <(E-Mail Removed)-this-bit> writes:
>
> >Thanks, I haven't done the sums to see if you are correct! Yes, 117
> >sounds good to me for an 18% reflectance.

>
> It also depends on the image exposure. Does 255 represent the
> brightness of a mythical 100% reflectance object, or a more typical
> piece of white paper/cloth (90% reflectance), or did you allow more
> headroom for specular reflections and place white at 200 or 230 instead
> of 255?
>
> If you assume that 255 is 100% reflectance matte white, then 18%
> reflectance is 0.18^(1/2.2) * 255 = 0.459 * 255 = 117. But other
> exposure conditions yield different mid-grey values.
>
> Dave


Yes, and the 2.2 is certainly another element which can be different
according to the actual display - i.e. what gamma you should apply when
going from linear sensor space to gamma-corrected space. Should you apply
the theoretical gamma and require a gamma 2.2 display (which is what I
believe you should do) or adjust the gamma in the camera (its so-called
"contrast" control) to suit your own display?

The number of ways you can get things wrong without really trying!

Cheers,
David

Cheers,
David


 
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Dave Martindale
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      05-19-2004
"David J Taylor" <(E-Mail Removed)-this-bit> writes:

>Yes, and the 2.2 is certainly another element which can be different
>according to the actual display - i.e. what gamma you should apply when
>going from linear sensor space to gamma-corrected space. Should you apply
>the theoretical gamma and require a gamma 2.2 display (which is what I
>believe you should do) or adjust the gamma in the camera (its so-called
>"contrast" control) to suit your own display?


In this case, I'd say that doesn't matter. The thread is about what
number you should put into an image *file* to represent mid-grey, and
that depends on the standard gamma of the *file*. If the file uses
sRGB encoding (and that's probably the default if there's no ICC info),
then the file's gamma is 1/2.2, and that's the end of the matter.

Now, if the display has a highly nonstandard gamma, correcting that is
the job of the lookup table in the video card, or the display software.
Thus a 117 pixel code in the file might end up as 110 or 125 going into
the video DAC - but that's independent of what's in the file.

(And, in fact, most CRTs have a gamma of 2.5 or higher, while the data
in the file is 1/2.2, but no correction is done to reconcile the
difference. In fact, this is deliberate, because it gives a slight
contrast boost that most viewers prefer to completely accurate tonal
reproduction when the display is viewed in a dim surround. So
complete physical accuracy generally isn't desired!).

You've also got to be awfully careful with controls called "contrast".
In photography, and probably in digital cameras, a "contrast" control
really adjusts contrast (gamma). In video, "contrast" is really video
gain which adjusts what a photographer would call brightness. Meanwhile
the video "brightness" control is really black level.

Dave
 
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