Converting from ProPhoto RGB to other RGB spaces

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-default
Posts: 1916
Joined: Thu Mar 26, 2015 1:53 am

Postby -default » Wed Jan 27, 2010 5:36 pm

(originally posted to alt.graphics.photoshop)

On Tue, 26 Jan 2010 01:30:02 -0800 (PST), David wrote:

> When I edit/convert-profile from  ProPhoto RGB to Adobe RGB - the
> photo is becoming brighter, the histogram is clearly shifted to the
> right. I tried all the intents.
>
> Question 2: why does it happen?

Well this turns out to be a very interesting answer.  (Whether it's correct
in your actual case or not is another story :-)

Re the histogram shifting:
ProPhoto RGB has a gamma of 1.8, and Adobe RGB is 2.2.  The difference in
histograms is as expected.  A larger gamma value requires larger channel
values to achieve the same visual brightness, so the histogram would move
to the right.  But this does not explain the increase in apparent
brightness that you are seeing.  Normally that would stay the same in a
profile conversion.

Here's a possibility re the increase in visual brightness:
The following explanation may apply if you are dealing with very saturated
colors.  On most monitors, in ProPhoto, Photoshop must "clip" saturated
colors near the primaries, setting them to the purest red, green, or blue
that your monitor can display.  Converting to a smaller color space results
in a smooth scaling of RGB values, in effect adding white to some of the
pixels, and the result is a brighter image.

If you are as curious as I was, or more so, try the following experiment:

1) Create an empty ProPhoto RGB image about 256x256 pixels in size.
2) Use the gradient tool to draw a "rainbow" gradient.
3) Dupe the image, and name it "Adobe RGB"
4) Go to Edit>Convert to Profile, and select Adobe RGB. (sRGB will show the
effect more clearly)
5) Toggle the Preview button to see the lightening of colors, particularly
in deep blues.
6) Press Print Screen, or use your favorite method of screen capture
7) Paste the screen image into a new image, zoom to 100%, and use the info
palette to compare the screen RGB values from both images numerically

Exercise for the reader: try the same experiment converting to lab mode,
and notice the very dark band in the middle of the blue stripe.  Cool, huh?
Thought Lab was lossles, din'cha?, LOL.

Now back to our regular program.  Let us know what the answer actually
turns out to be.
--
Mike Russell - http://www.curvemeister.com

mikemeister_admin
Posts: 4925
Joined: Fri Sep 20, 2013 8:29 pm

Postby mikemeister_admin » Wed Jan 27, 2010 7:02 pm

I also saw this when comparing colour spaces, as you can clearly see on the ProPhoto RGB plot near the bottom of this page
http://www.broadhurst-family.co.uk/lefteye/MainPages/c16-colourwatcherandspaces.htm

In fact I'm coming to the conclusion that changing colour spaces can really change colours in ways that one does not necessarily want.  Grey is the only 'safe' colour, just the brightness changes.

Zog



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