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Technique for Removing Halo Effects from Strongly Sharpened Images This writer has always had a strong interest in the image sharpening process and recently published on this web site, at the kind invitation of Edwin Leong, an article dealing with sharpening by the stair-step process with two different resizing methods. Presently, however, a new technique for improved sharpening is introduced which can be used with virtually any image sharpening technique of the user's choice. Something of which most readers who have experimented with sharpening techniques is aware, is that any attempt to sharpen an image strongly inevitably leads to the formation of strong white lines surrounding the image detail. These create an ugly effect in the final output image and normally force the user to use a lesser strength of enhancement to restore a visually acceptable image. The writer will show, however, that there is indeed a relatively simple procedure by means of which this objectionable halo can be removed while still retaining the desirable aspects of image sharpening. This will be illustrated by an example which will indicate clearly the steps employed and which the reader might like to apply to an image of his own choosing together with his favorite basic sharpening technique. Consider the composite image shown here which comprises five image steps. In step 1 the image I1 has been increased in size to 200% using bicubic resizing. This particular image has been given the self-explanatory name of I1(Bicubic 200%).
The second image I2(UM 2.0 500%) is the result of sharpening I1 using the well-known Unsharp Mask filter in PaintShopPro7 at the relatively strong settings of 2.0 radius and 500% intensity level. The resulting increased sharpness is very evident as is also the white halo effect surrounding many of the image features. Note also that there is a general increase in the color level over most of the image. The third image I3 is what this writer always refers to as a Laplacian image which in this case is obtained by subtracting I1 from I2 and adding the standard grey level intensity of 128. Thus I3 = I2 - I1 +128. In this image the halo effects are very pronounced. The fourth image I4 illustrates the magical process which eliminates the white halo effects and is thought to be a new application which deserves a clear description even though at root it is remarkably simple. What is done is to open up the "Curves" feature under "Color" in PSP7 where the default curve is seen to be a straight line at 45 degrees from the point (0,0) on through the mid grey level point of (128,128) and then on to the maximum level of (255,255) which if applied leaves the image totally unchanged. However what one does to eliminate the halo is to leave the initial line running from (0,0) to (128,128) but change the second half of the line to run from (128,128) to (255,128) . This curve when applied in this case to I4 totally wipes out all values exceeding 128 and thus completely eliminates the halo effect in the now altered image I4 = I3-halo. The final image I5 is obtained by adding the above image I4 to the starting image I1 together with subtraction of the 128 grey level in I4. It will be noticed sharpening occurs now without the usually unwanted color change and also that there is now no observable halo effects in the image. This process - while illustrated here using an unsharp mask sharpening filter can be equally well applied to almost any other sharpening procedure since it is almost universal in sharpening filters that sharpening occurs as the result of the addition of Laplace corrections in some form to the original picture even when the filter designer is not totally aware of this process taking place. For those readers who wish to try this process use whatever sharpening process you like prior to applying this method for halo removal. Questions about Robert's method can be sent directly to him. |
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