6 Replies Latest reply on Dec 20, 2010 11:33 AM by MadManChan2000

    Fuzzy Dark Objects Against Bright Red Background

    Noel Carboni Level 8

      For as long as there have been digital cameras (Canon ones, at least, which are the ones I've looked at more images from), there's been a tendency to blur bright red objects - sometimes severely.  Ever take an image of a car and notice missing detail in the taillights?


      Let me say at this point that Camera Raw seems to do this less than most. Thank you for that, Adobe!


      I understand how de-Bayering works, and that with a Bayer pattern camera there's really only half as much information directly measured for the red and blue colors as for green, but what I'm talking about seems if anything only indirectly related to that...  Note that you don't really see the same issues with Blue objects.  It seems to be more related to the transitions between colors.


      Here's a sample image converted in Photoshop...  Felt letters on a cloth background.  Note that the dark letters look significantly fuzzier against the bright red background than do most of the lighter letters.  You might have to blow the image up by clicking on it to see this clearly.




      All noise reduction, sharpening, and lens distortion correction were turned off for this conversion.


      I thought of the possibility that the dark letters might be just merging more with their shadows more than the lighter ones...  But I don't think this is the entire issue here.  There are really no shadows visible behind most of the letters near the top, yet the dark ones are still fuzzier than the others at the top as well.  Visually things just don't look quite right, and this is an extreme example.


      From what I have been able to see, the 2003 process blurs the dark letters very slightly less than the 2010 process does.


      I'm not really complaining about ACR's performance in this area - in fact I find I can recover more detail in red objects than with other converters.  I'm just curious.


      Your thoughts?


      Here's the raw file, by the way:  http://Noel.ProDigitalSoftware.com/temp/IMG_3468.zip



        • 1. Re: Fuzzy Dark Objects Against Bright Red Background
          Vit Novak Level 3

          Well, as you mentioned, there are R, G and B pixels and there are 2x more greeen pixels than R or B. There are various interpolation methods, but in general, resolution of the photo comes only or at least mostly from the green chanel. So edges, where change in green chanell is small, appear less sharp. Another thing, ACR sharpening is less intensive in dark areas, to avoid noise boosting


          About difference between R and B objects, you should check the colors as your camera see them, in the sensor color space (use dcraw for instance), where ratios between the colors are different than in output color space, so if you check your photo for instance, you will see that red letters or background are much more saturated than 'blue'


          The worst scenario for bayer sensor is red/blue edge. Several years ago, I made some pictures of red bridge over blue sea, with my 400D (10 Mpix) and older Canon film camera, and elements of the bridge with blue background appear considerably more sharp on the film. But it's onyl extreme case, occuring rarely, except in some white papers for the Foveon sensor

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          • 2. Re: Fuzzy Dark Objects Against Bright Red Background
            Noel Carboni Level 8

            Yes, you've made the point well about the need for the green channel to show fine luminance detail, and to be fair the violet letter on red background in the image above is pretty fuzzy (red and blue data, but little green data).


            I've always also suspected, however (and you touched on it with your comment about selective sharpening), is that the red channel may be more noisy and subject to more noise reduction - in Canon cameras at least - because the light levels reaching the red-filtered photosites are somewhat suppressed by the Antialiasing/IR suppression filter (which is blue-green colored), and thus the measurements must already be boosted to compensate.



            • 3. Re: Fuzzy Dark Objects Against Bright Red Background
              Vit Novak Level 3

              You are correct


              Checked one typical daylight photo (5200K) on 400D, and RGB multipliers, measured by the camera (=WB as shot, as reported by dcraw) are  2.22, 1.00, 1.44.


              On my Canon A620 compact, RGB multipliers for the same scene are 1.70, 1.00, 1.60


              On the other side, when 400D WB is set to tungsten (3200K), RGB multipliers are 1.50, 1.00, 2.41, so in that case, there is more blue noise

              • 4. Re: Fuzzy Dark Objects Against Bright Red Background
                MadManChan2000 Adobe Employee

                The central problem is the red/blue boundary. As you've already noted above, red/blue boundaries (with weak green) in the native camera coordinates will have lower resolution than other boundaries.


                I have not seen any evidence that Canon is performing noise reduction on the red channel any differently than the other two channels.


                It is true that the red channel is relatively "dark" compared to the green channel, but this is not the reason for a fuzzier edge.

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                • 5. Re: Fuzzy Dark Objects Against Bright Red Background
                  Vit Novak Level 3

                  Hi, Eric


                  My last reply to Noel's post was only about higher WB multiplyer for red channel than for green and blue for daylight, which mean, I suppose, that noise in whitebalanced image is higher in red channel. Of course, I don't know the details about noise reduction in Canon (whether on sensor itself or during processing) or in ACR. Only thing I noticed is that ACR sharpening has less effect in the dark tones, like SmartSharpen in PS


                  However, I also noticed that Clarity tool, which looks to me like USM with larger radius, has also less effect in dark tones, so dark parts of the picture are not 'clarified'. Any reason for this (except because of noise)? I personally preffer ordinary USM with larger radius in PS, instead of Clarity tool for that reason

                  • 6. Re: Fuzzy Dark Objects Against Bright Red Background
                    MadManChan2000 Adobe Employee

                    Hi Vit, yes, you are right that the white-balanced image will generally have more noise in the red & blue channels, for exactly the reason you mention.


                    Yes, you are right that Clarity is designed to adjust local contrast in the midtones only. I believe the reasons for this had to do with clipping & noise.