Anti-Aliasing

Anti-aliasing of images is a bit tricky to describe, though for compensating for low resolution images, it is a helpful trick to be aware of. As described in an earlier post relating to limitations with resolution for web images, sometimes things can look more clear by being more blurry. Anti-aliasing works in a similar way.
     Aliasing refers to the unwanted "jagged" look that can be seen with pixels in high contrast areas of an image. The closeups below show a comparison between an image (top) with aliasing, and one (below) with anti-aliasing. Though the resolution (pixels per inch) is the same for both of these, the one with anti-aliasing appears more clear.

The terminology and methods used for enabling anti-aliasing may differ depending on the program being used. This may be found as an option within paint programs, layout programs, PDF generating settings, settings for merging layers, as filters, as effects, etc. Regardless of how it is found within the programs, the main point of this article is to point out its positive effect when used for certain types of images.
    The examples below is from a scanned document which consists primarily of only high contrast text and graphics. By name, "anti-aliasing" was not used through any of the tools or options available to me in the paint program I was using. What was done instead was to adjust the tones of the image in such a way as to manually have less aliasing in the image.

The sample above (top) is with tones adjust to soften the edges between the areas of black and white. Basically some of the gray is allowed to remain as a thin layer of division between the two contrasting colors. If this is removed (second example above), and the tones are adjusted for more contrast between the black and the background white, the image becomes much more "jagged" and rough looking. (And in addition, as seen in the illustrative areas of the image, some detail is lost.)  

Above is an even closer view that more clearly shows the tones being used between the areas of high contrast. As noted earlier, even though the resolution (pixels per inch) remains the same for each, the illusion of added clarity seems greater for one over the other. It is odd to think that the blurry looking one ends up appearing more clear and detailed. 
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Note, earlier post on sharpening filters also relates to perceiving added clarity through seemly odd ways. The two of these kind of go hand-in-hand, and are of interest for both web publishing (where resolution of images is limited) and for print production (for optimizing image quality).    

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Halftone Grays & Dot Gain

A bit more about halftone patterns and their dots....
     Many of the adjustments that are made for image tones relate directly to how they will print as halftone dots. As seen below, for lighter tones we are speaking of increasingly smaller black dots, and for darker tones increasingly smaller white dots. In print production there are certain limitations and tolerances that will effect how these dots will appear.

For conventional printing, the ink that is being used (such as black) will absorb into the paper which can result in having the image swell in size. The printed black dots become larger ("dot gain"), and the printed areas behind the "white dots" become wider. This too can happen with digital printing where toner (rather than ink) lays on, and is baked onto, the paper's surface. This layer of toner has a thickness to it which too can effect the sizes of the dots. 

      There are many considerations for anticipating how much "gain" to expect in production. Much of this goes beyond what is needed at this point for this discussion. One consideration though that should be mentioned is noting the stock selection. The paper being used can have a huge effect on what will happen once ink or toner is applied. Generally speaking, coated stocks are better for holding printed detail than uncoated papers. 

For refining tones with taking dot gain into consideration, would like to first consider the darkest areas of the image. In the example above, at some point the white dots will close up and no longer be visible. Much of what is to the right will print as 100% black (without the small white dots). In addition to this, many of the other darker tones, and the mid tones, may print darker than anticipated. 

  

 

For the lighter tones of an image, the opposite can happen. At some point the dots become too small to print. Rather than things printing darker (as mentioned previously for the mid and dark tones), some of the very light areas may print lighter. If tonal detail is found in these lightest areas of an image, this may be lost or diminished once printed.  

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What to do?

    Though the description of the problem may seem confusing, the solution really is not that bad. One way of looking at it is that rather than having a color palette (in this instance of grays) that run from pure white (0%) to full black (100%), you would have a slightly adjusted palette (with different values). 

     For example, if printing on coated paper, it could be that your adjusted palette of grays goes from 4% to 96%. When adjusting tones for your image for highlights, all tonal detail should be at least 4% or higher (anything less may not print and appear as pure white). For darkest tones, these can be adjusted so that they are at most 96% (anything greater will print as full black; the small "white" dots not being visible). Keeping all the tonal detail within 4-96% should do the trick. If printing on uncoated stock, different values may be needed - such as 7-92%. (The print shop may have some suggestions for what values would be best for the equipment and stock being used for the job.)  

     Anther consideration is with all those tones between the extreme ones. If an image is mostly dark tones, rather than running everything up close to the edge (on the borderline of being black), maybe they can be lightened with the darkest areas of the image being set much lower, such as in the lower 80%s. For all images, it may be good to anticipate that all mid to dark toned areas will appear a bit darker once printed. 

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Once you get past all of the technical details, when it comes to adjusting tones, it really is not that bad. It becomes less of a value-specific process and becomes more intuitive. You soon develop "an eye" for seeing tones in a different way - expecting to see mid to dark toned areas as lighter shades gray; highlights as darker shades of gray; white as a very light gray (of a certain value); and black as a dark gray (of a certain value). This is not to say that some details cannot go beyond these limits. Much of this is a judgment call. In some instances highlights may be small and in the middle of the shot, and full white may be fine. At other times, some of the darkest points may be details that are linear in nature and full percentage of black may be fine. Decisions on all of this can become natural and will become less about the numbers and more about the aesthetics of what you are seeing - with adjusted eyes. 

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Tones: Highlights

For examples relating to adjusting tones for highlights, the following photo comes to mind for two reasons. One being the snow, which can add many challenges for tones. The other is for pointing out the effects of highlights when they are found along the edges of the composition - and with realizing that unless there are some printed tones, then the highlight color in the image will be the same as that of the paper. Unless an outlining border is added to the photo, it may create an odd silhouetted shape with its background.

As noted on the Tonal Adjustments & Shadows post, clipping (loss of tonal detail) can occur when scanning photos. In this instance though, maybe the scan captured all that was in the original photo, and the loss was from when the photo was taken with the camera being too flooded with light from the snow. Either way, most of the image is with highlights consisting of white (0% black) pixels. 

The main adjustment, above, was with the darkening of all highlights. The lightest tone for any of the pixels is now at about 6% black. This should (depending on press tolerances and stock selection) print as a very light gray (almost white) - yielding a silhouetted rectangle shape (of the complete photograph) onto its paper background. Though it may seem odd to make gray something (snow) that was pure white, the result is that the snow still appears as being white and we now have a bit of contrast with all of the photo to its background.

     Adjustments were also made to decrease the shadow tones found in certain areas (such as lower left of the shot), and to darken the mid/light tones in others (namely for the snow areas, and select areas of the children). Care was taken to not over compensate so as not to lose the character of the original photo. 

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Tonal Adjustments & Shadows

Family photo, Kutztown, PA., early 1900s.

This old photo is typical of many which have very high contrast. When scanning, there are limits to how much detail can be captured in the darkest and lightest areas areas of an image (due to limitations of the scanner). 

The view of levels shows a high population of very dark pixels to the far left of the chart (above). This "peak" found just along the edge of the chart is where clipping has occurred with loss of tonal detail. 

Another way of showing this loss of tonal detail is to show just the pixels that are 100% black (above). This photo though, can still be adjust for optimum quality for print production. 

The above images was with gray levels adjusted to make the darkest areas of the image to be at about 90% gray (rather than 100%). For the lightest areas of the photo, the levels were adjusted to make them a bit lighter (to compensate for the overly dark highlights in the original). To bring out some of the tonal details in other areas, dodge and burn tools were used, in selected areas, to add contrast and to darken some of the shadows and midtones. In particular, this was to make more noticeable the tonal detail found in the buildings on the right side of the street, some of the roof tops, steeple, hills in the distance, etc. Final touch was to add a bit of sharpening to add some clarity.    

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Different programs can be used for making these type of adjustments to images. Hopefully what is described here will allow you to make similar refinements using the paint or photo editing program of your choosing.
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