4 Replies Latest reply on Jan 2, 2015 11:32 PM by Eadig

    Color information and image quality

    Eadig Level 1

      Happy new year

       

      Color corrections of digital images seem to be increasingly important these days,

      and as a part of the workflow I would like to understand the relationship between the color information and image quality correctly.

      Photoshop CC CIB states the following concerning them:

       

      JPEG format retains all color information in an image, but compresses file size by selectively discarding data.

      The greater the compression, the lower the image quality.

       

      Does this imply that the quality of an image changes by the level of compression (such as JPEG) even though the same RGB or CMYK tonal range information is retained for the image in a file regardless of the level of compression?

       

      Could not find relevant info on this in Help.

       

       

      Replies would be appreciated as always,

       

      Ead

        • 1. Re: Color information and image quality
          Mylenium Most Valuable Participant

          There is no causal relation of color vs. any form of compression. You can have millions of colors in a blurry and totally useless image. With regards to JPEG images, simply experiment and compare the results.

           

          Mylenium

          • 2. Re: Color information and image quality
            Eadig Level 1

            Thank you, I began understanding that after you'd mentioned that you can have millions of colors in a blurry, useless image; I see that the same amount of color information can be retained at different image qualities which may make the image be blurry to the point of being not useful or be crisp to the point of being too sharp to be usable.

             

            I've done enough experiments with different qualities of JPEG to figure the differences, and found it convenient to keep saving color-corrected images in DNG format and select JPEG near the final stage of the workflow.

             

            Ead

            • 3. Re: Color information and image quality
              G.Hoffmann Level 3

              JPEG splits colors RGB into Luminance Y and Chrominance Cb,Cr

              (by more accurate nomenclature: inverse gamma-encoded numbers R'G'B').

               

              JPEG compression does not shrink the ranges of Luminance and Chrominance.

              JPEG reduces the number of tonal levels in the range, less for Y, more for Cb,Cr,

              mainly by reducing higher frequency components.

               

              http://en.wikipedia.org/wiki/JPEG

               

              http://docs-hoffmann.de/jpeg131200.pdf

               

              Best regards  --Gernot Hoffmann

              • 4. Re: Color information and image quality
                Eadig Level 1

                Great, that's the sort of thing I wanted to know about.

                It makes sense to think that because the ranges of Luminance and Chrominance remain the same, the color information remains the same, but as the the high frequency components are reduced by compression, the file size can be reduced with it.