Correcting/Reducing Rasterization

Discussion in 'Printing' started by oldracer, Oct 28, 2012.

  1. oldracer

    oldracer Mu-43 All-Pro

    Oct 1, 2010
    USA
    I have some street photography:

    P1040234-Edit.

    that has an issue. The near-horizontal wires show a sort of beads-on-a-chain defect from rasterization:

    rasterization.

    Normally I would not pixel-peep to this level (4:1) but when I print the photo even at 8.25x11" the rasterization is fairly noticable. For me, it is enough to distract from the photo.

    So, the question is: Are there any post-processing tricks that can fix this up? I am wondering about maybe quadrupling the number of pixels and then using some kind of Photoshop trick. But before I started messing around on my own with such a big image I thought I'd ask whether anyone else has seen and fixed this kind of thing.
     
  2. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    What is your resolution set at?

    A trick I have used for softening this sort of thing is to put a small amount of blur on it. If it is just the wires that are doing this try using the blur tool set to a brush size just larger than the wires and manually trace over them. The risk is also bluring the sky and making it look worse. Another trick I use when an entire image is pixelated is too double the resolution, then put a low gausian blur over the entire image and then resharpen it. It often blends the offending pixels together then sharpens out better looking edges. Works wonders if you scan a picture from a magazine!
     
  3. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    The correct term is "aliasing" not "rasterization". What you are looking for are ways to "anti-alias" the wires.

    It isn't possible to completely eliminate the problem. When there are few pixels used to render a portions of the image, like these wires, the problem is more noticeable. Sometimes is can be reduced by upsampling in Photoshop using Bicubic-smoother.
     
  4. oldracer

    oldracer Mu-43 All-Pro

    Oct 1, 2010
    USA
    Resolution initially at the default 240 but I also tried 600. Printing took longer but there was no difference in the results.

    Thanks. That's the kind of thing I was looking for, but of course without the dog work of tracing every wire. :-( Oh, well. So much for trying to be lazy. It will probably do what I want without messing anything else up.

    Now that you have me thinking along those lines, I wonder how it would work if I resampled to double the pixel density and then ran the smear tool along the wires to blend the black beads. What do you think?

    I'm somewhat familiar with ime-domain sampling, Nyquist rates, etc. but I haven't really tried understand spatial aliasing. I thought it was more like the low frequency "moire" aliasing you see when you resample a screened photograph at too low a sample density or when a texture or pattern in the subject images onto the sensor too close to the dot pitch of the sensor.
     
  5. dwig

    dwig Mu-43 Top Veteran

    624
    Jun 26, 2010
    Key West FL
    That happens with digital images also. Moire can be a big issue when fine pattern (distant fences, ...) interact with the photosites in the sensor. AA (Anti-Aliasing) filters are part and parcel of the camera sensor bundle, some weak and some strong, to reduce the issue.

    In this case, portions of the wire span parts of different numbers of pixels resulting in the uneven rendering. When the resolution is higher the relative differences in the number of pixels spanned is reduced.

    "Rasterization" is a term reserved for processes where vector based description of image data are converted to raster (aka bitmap) data. Vector based data is what is used in electronic font data and "drawing" programs like Illustrator, Corel DRAW, ... . See Vector Art

    The main reason I wanted to make this point is that using the wrong term prevents you from sucessfully searching for information about the problem and its "fixes" and from finding the tools/controls in editing programs that can aid in the process.