On Tue, 26 Jan 2010 01:30:02 -0800 (PST), David wrote:
[first question already answered]
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 256×256 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