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Sharpening & SmoothingSession

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AISIG MeetingJuly 17, 2007

Sam Saeed

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Exercise outline

A. Signal, Noise and the Ratio.

B. Create a dark area selection.

C. Soften the dark area selection’s edge.

D. Reduce noise in the dark area.

E. Create a bright area selection.

F. Soften the bright area selection’s edge.

G. Sharpen the bright area.

H. Adjust image brightness.

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Signal, Noise and the Ratio

As we open the camera shutter to expose the CCD to light, the signal is created and its level will increase to form an image with varying brightness across the frame. Due to several reasons, some of the electrons will accumulate in a random fashion and not in accordance with the image being formed, creating what we call noise. The relationship between the signal and the noise is the Signal to Noise Ratio (S/N).

The higher the ratio, the better is the image. This manifests itself in the brighter areas of the picture where details may be found and enhanced. While the darker areas may contain detail, the S/N is usually low and the noise makes its presence noticeable with adverse effects.

To combat noise and increase signal, several methods may be implemented such as cooling the CCD to subzero temperatures, increasing sub-exposure time, combining many frames, using different stacking methods, etc. Once we have an image at hand and start manipulating it by means of stretching, further details will come into view including dark areas’ noise becoming more visible.

To enhance the appearance of the image, we’ll need to reduce this noise to a level as low as possible by blurring the dark regions. In the brighter portions of the picture where the S/N is higher, details may be improved by applying different methods of sharpening to large areas and fine edges.

By using Photoshop, we can separate the image into areas of different brightness. Then we apply the appropriate filters for the desired effects. This should be cautiously performed to maintain a natural appearance and avoid the introduction of digital artifacts.

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Create a dark area selection1. From the menu, Go to Select-Color

Range.

2. Choose fuzziness value of 40.

3. Click on the darkest spot in the image.

4. If selection boundaries are too grainy, use the middle eye dropper (+) and click on a neighboring dark area to increase the selection and loose the grainy appearance.

5. Click OK to create the actual selection.

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Soften the dark area selection’s edge

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1. At this point, the selection is created with a semi hard boundary. We will smooth the edge in the following 2 steps.

2. Go to Select-Modify-Contract, and choose a value of 4 and click OK.

3. Go to Select-Feather, and choose a feather radius value of 2 and click OK to finalize the selection.

4. You may want to save the selection as a new channel, in case you need it later. Go to Select-Save Selection and name it “Dark Area”.

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Reduce noise in the dark area1. While the dark area is selected,

go to Filter-Blur-Gaussian Blur.

2. Choose a Radius value of 1 pixel. Generally, the useful range is between 0.5 and 2 pixels. The higher the value, the more unnatural look the image may have.

3. Click OK.

4. Release the selection and examine the results.

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Another Noise Reduction Method

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1. While the dark area is selected, go to Filter-Noise-Reduce Noise.

2. Choose a Strength value of 10.

3. Preserve Detail of 25%.

4. Sharpen Detail of 0%.

5. Check the Remove JPEG Artifact if you’re working with a JPEG file. Otherwise, leave it unchecked.

6. Click OK.

7. Release the selection and examine the results. This filter is available in CS2.

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Create a bright area selection1. From the menu, Go to Select-Color

Range.

2. Choose fuzziness value of 40.

3. Click on a spot with a high brightness value in the image.

4. Use the middle eye dropper (+) and click on neighboring areas of higher and lower brightness to increase the selection.

5. Use the right eye dropper (-) to loose the stars and the darker background if they become part of the selection.

6. Click OK to create the actual selection.

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Soften the bright area selection’s edge1. At this point, the selection is created

with a semi hard boundary. We will smooth the edge in the following 2 steps.

2. Go to Select-Modify-Contract, and choose a value of 4 and click OK.

3. Go to Select-Feather, and choose a feather radius value of 2 and click OK to finalize the selection. (Now, the selection may look like the image to the right).

4. You may want to save the selection as a new channel, in case you need it later. Go to Select-Save Selection and name it “Bright Area”.

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Sharpen the bright areaUsing the Unsharp Mask filter, sharpening may be applied in 2 stages.

Stage 1, Large Detail:While the bright area is selected, go to the Unsharp Mask filter and choose the following values:Amount 80%Radius 4 pixelsThreshold 3 Click OK

Stage 2, Fine Edges:While the bright area is selected, go to the Unsharp Mask filter and choose the following values:Amount 50%Radius 2 pixelsThreshold 0 Click OK

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Another Sharpening MethodInstead of the Unsharp Mask, now we’ll try the Smart Sharpen Filter (located under the Filter-Sharpen Menu) to sharpen the image. This filter has excellent edge detection and sharpening without introducing unwanted halos. Once the filter is selected, choose the Advanced button and the following values for each tab:

Sharpen Tab:Amount 60%Radius 4 pixelsRemove Gaussian BlurCheck More Accurate

Shadow & Highlight Tabs:Fade Amount 100%Tonal Width 50%Radius 8 pixels

This filter is available in

CS2.

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Final Results

Before After

A minor brightness adjustment may be applied now.