Digital tools sharpening devoted many publications. Brief description of basic procedures can be found, for example, the web-page Sharpening Techniques. More detail with issues relating to the sharpening can be found in many monographs (see [1, 2]).
Most of sharpness in digital images raise with staffing tool Adobe ® Photoshop ® Unsharp Mask. Despite its universality, this method is not without flaws and a few, such as ghosting and accentuation of noise. Application to the “sharpleniem” intelligent search procedures circuits (see [1, pp. 332]) also does not always lead to an acceptable outcome. In cases where the Unsharp Mask can not achieve a satisfactory result, management recommended to use alternative procedures.
The purpose of this article – to remind the reader of the existence of such alternative procedures, a brief description of their nature, as well as to provide all interested the basic information for your own experiments. In this case, the reader should not forget that the information published in the “Small non-scientific tests” information no claim to completeness, nor the details. Anyone interested in the issue under consideration in its entirety, the author refers to numerous other articles and books devoted to digital image processing.
Summary of alternative procedures
As mentioned above, the Unsharp Mask filter is not always ideally suited to improve the sharpness of edges, because it is not always cope with the localization of the contours. The basic idea of all the alternative procedures is simple: they are meant to identify the circuit and add them to the original image.
All the alternative approaches, in fact, consist of three phases:
1. Initially, the additional layer, one way or another, we obtain
image with accentuated contours.
2. Then ask for a new layer of one of the blending modes, which can emphasize the contrast (Overlay, Soft Light, Hard Light).
3. If necessary, select the desired transparency of the additional layer.
This article will explore ways to identify the contours on the basis of filter High Pass (filter emphasizes high spatial frequencies) and Emboss (filter detection relief).
I. PROCEDURE Sharpen BASED HIGH PASS FILTER
1. Duplicate the base layer (Duplicate Layer).
2. Filter => Other => High Pass. We select the radius R so as to identify the optimal path. As an initial approximation, you can use R = 2 … 10.
3. For the new layer select the desired mode overlay (Overlay, Soft Light, Hard Light).
4. If necessary, select a new transparent layer.
II. PROCEDURE Sharpen BASED FILTER EMBOSS
1. Duplicate the base layer (Duplicate Layer).
2. Filter => Stylize => Emboss. We select the optimal value of Angle. Set the option Hight in the range 2 … 5, and the Amount parameter is approximately 100%.
3. For the new layer select the desired mode overlay (Overlay, Soft Light, Hard Light).
4. If necessary, select a new transparent layer.
Comparison of different procedures based on the test images
In the upper part of Fig. 1 shows the original test image for the study of various algorithms for sharpening. It consists of:
1. Strips of grayscale tones (in increments of 10%);
2. Fragment of black and white worlds, containing strokes with different spatial frequencies;
3. fairly large colored stripes on a colored background;
4. minor color noise that fills the white space between the Mira and the colored stripes.
In the lower part of Fig. 1 shows the same image after applying the filter Gaussian Blur (R = 0,7). It was at this image and will apply different procedures for sharpening.
Fig. 1
In the upper part of Fig. 2 shows the result of the filter Unsharp Mask (Amount = 210%, Radius = 1, Threshold = 0). Sharpness on long circuits almost completely restored, but graded strip has noticeably undesirable separation boundaries.
In the lower part of Fig. 2 shows the result of repeated application of the same filter with the same parameters. Contour sharpness completely restored. However, things on the graded strip became more apparent. In addition, digital noise on the white area has become very noticeable.
Fig. 2
In the upper part of Fig. 3 shows the result of the filter High Pass (R = 5) on an additional layer (blending mode Overlay; Opacity = 100%). Artifacts on the graded strip less pronounced. Noise on a white field is not visible. Sharpness restored more gently. In general, we can say that this method allows you to restore good sharpness with minimal undesirable edge effects.
In the lower part of Fig. 3 shows the result of the filter Emboss (Angle = 1350, Height = 3, Amount = 100%) on an additional layer (blending mode Overlay; Opacity = 100%). Artifacts on the graded strip visible, though not explicitly, as in the case of the filter Unsharp Mask. Sharpness restored softer. Guidelines recommend the use of this method for a slightly blurry image (soft focus).
Fig. 3
What conclusion can be drawn from the results? The conclusion is simple: different tools work differently. None of them is not ideal. Each of them has both its advantages and disadvantages. If you do not like some kind algorithm, try to use an alternative procedure. A combination of several different ways is also a good idea.
Comparison of different procedures based on real pictures
Of course, to draw conclusions only on the basis of a fleeting analysis of test images is difficult. In some cases, show the real picture may be more useful. Let’s see what can be discussed when applying filters to the real picture.
Fig. 4. It looks like a Moscow skyscraper Square Rebellion from the territory of the Moscow Zoo. This image was obtained with a film scanner Minolta Dimage Scan Elite. No treatment (except Resize) was carried out.
Fig. 5. The image after applying the filter Unsharp Mask (Amount = 192%, Radius = 0,5, Threshold = 3). Setting the threshold (Threshold) at level 3 would avoid the graininess in the image of the sky.
Fig. 6. The image after applying the filter High Pass (R = 5) in a separate layer.Overlay mode: Soft light; opacity = 73%.
Fig. 7. The image after applying the filter Emboss (Angle = 1350, Height = 3, Amount = 100%) in a separate layer. Overlay mode: Overlay; opacity = 75%.
You have to understand that all these images are only for reference. On the basis of any categorical conclusions should not be done.
Final words
1. Sharpen – a procedure with many pitfalls. If you are preparing an image for printing, it should be remembered that no monitor (either CRT or LCD) can not display the picture exactly as it is printed. Moreover, the sharpness of the image will depend on the type of paint, and the type of paper, and the type of printing process. If you have enough experience, then applying the method of trial and error should be your rule.
2. Those who want to experiment can easily create its own procedure for sharpening.For example, you can try instead of the High Pass filter and use the Emboss filter Glowing Edges. Do not get discouraged when you see that the direct replacement of the filter does not give an immediate positive result. Try to add a filter Glowing Edges suitable additional treatment. Try to change the Opacity setting in the extreme.Experiment!
Another possible way to create own sharpening is to use a custom filter (Filter => Other => Custom). In the article about the bokeh, already mentioned, that this filter allows us to calculate the two-dimensional convolution with an arbitrary user function blur point (PSF). To increase the sharpness suitable PSF, which resembles the function sin (X) / X (if the language of mathematical formulas frightens you, look at the book Kathryn Eismann: at the very end of Chapter 8, you will find a good initial approximation for the filter Custom, based on the sharpening). Instead of full-custom filter program can use Photoshop filters from other manufacturers. For example, a free Custom Convolution Filter on company Reindeer Graphics Inc provides for modeling PSF matrix size of 7 x 7.
At the very beginning of the article mentioned on the application before sharpening intelligent search procedures contours. Such procedures should also include a free plugin Select Edges of the already mentioned companies Reindeer Graphics Inc (see [6] at the end of the article). This tool allows you several ways to create a selection containing the boundary and contour. In selected areas to improve the sharpness can be one of the traditional methods, including traditional procedure of Unsharp Mask.
And, of course, boundless field for your own experiments when you try to combine different approaches and methodologies. Happy creativity!
Literature and Internet links
1. Eismann K. Retouching and image processing in Photoshop. Second edition.: Trans. from English. - M.: Publishing house “Williams”, 2004. - The strengthening of field devoted to Part III of Chapter 8 of this book. The remainder of this guide is also worthy of attention the reader interested in digital processing.
2. Margulis D. Photoshop 6 for professionals: the classic guide to color correction.Per. from English. - Moscow: 000 RTV-Media, 2001. - The classic guide, which needs no introduction. However, you should pay attention to the fact that in addition to color correction in the text of the book touched upon other issues of image processing. In particular sharpening is discussed in Chapter 4 (“Sharpness with exquisite accuracy”).
3. Sharpening Techniques. Short compilation of the main ways of sharpening. In English.
4. Michael Reichmann. High Pass Sharpening. Article on a site The Luminous Landscape.
5. Efremov I. What is bokeh and ni-sen? Why are the photographs branches dvoyatsya? This article can podcherpnut little initial information about the mathematical convolution operation.
6. Select Edges (free plugin for Adobe ® Photoshop ® from the company Reindeer Graphics, Inc). Includes a short guide in English format Adobe ® Reader ® (pdf).SelectEdges.8bs file should be copied to a folder Plug-ins software Photoshop.CAUTION This is NOT a filter! After copying the folder tool is available from the menu Select => OPTIPIX => Select Edges.