Sharpening Images Better
Copyright 2003 - JMA - All Rights Reserved
Sharpening is a very important part of digital imaging manipulation. To get your images to look their best, both on screen and in hard copy, you need to understand why sharpenng is necessary, when it should be appliedand what kind of sharpening method will work best for your photos. In digital images there are two places where we need to be concerned with image detailDigital cameras contain an optical filter in front of the image sensor (CCD) which is a low pass filter. The filter is used to improve overall image quality, but in the process it also blurs the imagedetail slightly. A similar softening effect occurs when the image is being processed for output to a printer or a web page. With printers, the softening occurs as a result of putting ink on paper. With web pages, the softening results from taking a high resolution image and resizing it to a lower one. Once you have determined what you will do with image, you will be able to decide when and how to apply sharpening. If the goal is to sharpen the blurring effects caused by the low pass filter in the camera, then sharpening should be applied before the image is resized or re-sampled. If you are trying to overcome the softening effects produced by the output process, then sharpening should be applied after the image is resized. In all cases, sharpening should be done after all other image manipulation and corrections have been made, because certain correction filters can magnify or distort the sharpening effects.
In this tutorial we will examine how to apply sharpening to a single channel, namely the Lightness channel using PSP 8 Unsharp Mask filter. This technique wll provide significantly better results than when you apply the USM filter at the RGB level.
Figure 1. The Raw Image
The image of the clock was taken with a digital camera under ambient light conditions. The exposure was 1/15 sec at f 5.0 with an ISO setting of 200. In camera sharpening was off. Notice the softness in the image.
Figure 4. Splitting the Channels.
A more sohisticated method for sharpening images is to split the image into the Hue,Saturation, Lightness channels and apply the USM to the Lightness channel. To convert the image into HSL go to the Main Menu bar and select Image > SPlit Channel > Split to HSL to produce the three grayscale images shown on the right.
Figure 2. The Unsharp Mask.
The unsharp mask comes with three settings, Radius, Strength and Clipping. The Radius defines how many pixels will be included around the area to be sharpened. Radius values should be kept low for web images and high for printed output. The Strength determines the amount of contrast that will be applied to the edges. A high value will brighten the edges of the pixels and a low value will darken them. The Clipping value specifies what level of color change the program will accept for sharpening. A value of zero (0) will cause the program to sharpen all the pixels in the image. As the number increases, the amount of contrast between adjacent pixels will determine which of them are sharpened. The image on the right shows the effects of the USM at the default settings.
Figure 3. The Clarify Filter
The Clarify filter adds a sense of depth by adjusting the intensity distribution in the image without artifacts associated with sharpening edges. It gives the image a crisp focused look to make objects stand out and can often produce the same visual effects as the Unsharp Mask Filter. To apply the Clarify filter, go to the Main Menu bar and select Adjust > Brightness and Contrast > Clarify. The image below on the right was produced using the default settings of the Clarify filter aas shown.
Clarify dialog window
Unsharp Mask dialog window
TIP: Before you sharpen any image, make sure that you make all your global changes and color corrections first, then save a copy of the image unsharpened at its full resolution. Sharpen the image when you are ready to sent it to the printer or the web and always view the image at 100% when you apply any sharpening filters or techniques.
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Figure 5. Sharpen the Lightness Channel
The Lightness Channel carries the Brightness and Contrast portion of the image's pixel information. Since sharpening increases the contrast between pixels, applying the Unsharp Mask to only the Lightness channel will modify only that information. To apply the USM filter, go to the Main Menu bar and select Adjust > Sharpness > Unsharp Mask. For this image the Radius was set to 1, the Strength to 100 and the Clipping value to 4 as shown.
Figure 6. Recombining the Channels - The End Results
Once the sharpening has been applied to the Lightness channel, the three channels need to be merge again to form a single RGB image. To combine the channels go to the Main Menu bar and select Image > Combine Channel > Combine from HSL. With the image combined back into the RGB mode we can see how well the details in the clock have been brought out without the halo effect typically encountered when using sharpening filters. If you are satisfied with the results, you can delete the three Grayscale images and save your sharpened image. If not you can delete the RGB image, undo the sharpening effects on the Lightness channel and re apply the USM with different settings.
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