why do photos need color correction prior to printing ?

Started Jan 24, 2013 | Questions thread
technoid Senior Member • Posts: 2,161
Re: Embedded color profile?

Vernon D Rainwater wrote:

Hugowolf wrote:

Vernon D Rainwater wrote:

technoid wrote:

It doesn't make any difference what working space you use when printing so long as the image is within the gamut of the working space. OTOH, there is no reason to use a different Gamma working space. Changing Gamma only changes the RGB values, not the actual color.

I have some question regarding your last line. My limited experience (that I posted a thread a few weeks ago) regarding changes in RGB values and the colors on the screen (for selected colors) definitely displayed different colors when changing from aRGB to sRGB.

It depends on what you mean when you say 'changing from aRGB to sRGB'. If you convert from one space to another, you should see no change in displayed colors. The RGB numbers for those colors will change, but the colors should not.

If, however, you make the mistake of simply assigning a different profile, then you are basically telling the software that those RGB number values mean something different, that they represent a different color, and then different colors should be displayed.

BTW: I take it you mean AdobeRGB, aRGB would be unusual. I seem to rememember there was or is an aRGB around, that isn't AdobeRGB.

Brian A

Brian, yes I am using aRGB to indicate AdobeRGB. Your comment is my first to read that there are both aRGB and AdobeRGB with each being different.

The link for the post I referred to is at: http://forums.dpreview.com/forums/post/50258683

Whatever method I used (in Photoshop) to change from aRGB to sRGB resulted in somewhat different RGB values for selected colors and the displayed colors reflected the difference.

In your example you were converting from aRGB (used as a shortcut for Adobe RGB 1998) to sRGB. The RGB values change because the colorspaces differ. The Color istelf also is forced to change in those cases where the converted color is outside the colorspace gamut. With Tri-point colorspaces (almost all non printer ones) matrix math is used to determine the RGB coordinates of the the color in the new space. Colors out of gamut will always have either negative values or values exceeding 255 (for 8 bit). Convention is to clip the negative ones at zero and limit the others to 255 max.

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