Reducing Noise in Photoshop -- Part III

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS3 (Beta) Used in this Tutorial

Gaussian Blur/Median Filter

Figure 1: Noisy Image

The Median filter noise reduction technique (covered in the previous article) works well with images that have primarily luminance noise. However, it does not work as well with images that have color noise. Luckily, the technique can be modified by adding one additional layer that will handle color noise.

Figure 1 shows an image of a doll. A 100% crop of the image reveals a problem with luminance and color noise (see Figure 2). In this image, the color noise is particularly noticeable as blue specks.

Figure 2: Crop of Noisy Image

Figure 3: Layers Palette with Blur Layer

As in the previous technique, it is best to duplicate the Background layer and perform the noise reduction work on the duplicated layer. Accordingly, the first step is to duplicate the Background layer by dragging the Background layer to the Create a new layer icon on the Layers palette. The new layer is renamed the Blur layer (see Figure 3).

Figure 4: Gaussian Blur Dialogue Box

The Gaussian Blur filter is now used to blur the color noise on the Blur layer by choosing Filter/Blur/Gaussian Blur. The Gaussian Blur dialogue box appears as shown in Figure 4. The radius should be set at the minimum amount that will blur the color noise. A setting of 6 worked well for this image. Figure 5 shows the image with the Gaussian Blur applied.

Figure 5: Crop of Image after Gaussian Blur

Of course, the image in Figure 5 is too blurred to use as is. The trick is to change the Blend mode of the Blur layer to the Color Blend mode. The Color Blend mode blends the luminance of the Background layer with the hue and saturation of the Blur layer. The final result has the luminance of the Background layer (including the luminance noise), but the color noise of the Background layer is replaced by the smoothed color of the Blur layer. Figure 6 shows the image crop after the Blend mode was changed to the Color Blend mode. Moving the cursor over the image will show the image before the Blur layer was added. Moving the cursor back and forth over the image shows how the Blur layer has helped to significantly reduce the color noise (you will need a good monitor to see the difference).

Figure 6: Crop of Image after Color Blend Mode Applied

Figure 7: Layers Palette with Median Layer

So far, the procedure has helped reduce the color noise, but the luminance noise is still a big problem. Thus, the next step is to use the Median filter to reduce the luminance noise. For this, it is necessary to create a new layer that merges the effects of the Background and Blur layers. With the top layer selected, a new layer is created by clicking the Create a new layer icon at the bottom of the Layers palette. Then, the other layers effects are merged into the new layer (hold down the Alt key [Option key on a Mac]; while holding down the left mouse button, select Layer/Merge Visible). This layer is renamed the Median layer. The Layers palette after this step is shown in Figure 7.

Figure 8: Median Dialogue Box

The Median filer is launched (choose Filter/Noise/Median). The Median dialog box appears as seen in Figure 8. The radius must be set to a point where the noise is reduced, but the detail is maintained as much as possible. A radius setting of 5 works well for this image. Figure 9 shows the crop of the image with this setting.

Figure 9: Crop of Image after Median Filter

Figures 10 and 12 show crops of the image before the noise removal steps. Figures 11 and 13 show the same crops after these steps. A comparison of these images shows that the procedure has been very effective in reducing the noise.

Figure 10: Crop before Noise Removal

Figure 11: Crop after Noise Removal

Figure 12: Crop before Noise Removal

Figure 13: Crop after Noise Removal

Once this image has been sharpened, it may print just fine. However, to further improve the image, some may wish to apply the edge technique that was introduced in the previous article. Since the steps are the same, the procedure will not be repeated.

Reduce Noise Filter

Figure 14: Noisy Image

The Reduce Noise filter offers another option for dealing with noise. One of the advantages of this filter is its ease of use due to the fact that both luminance and color noise can be handled by the same filter.

Figure 14 shows an image that was shot with a high ISO. A 100% crop of the image reveals the resultant noise (see Figure 15). The Noise Reduction filter will be used to improve the noise problem in this image.

Figure 15: Crop of Noisy Image

Figure 16: Layers Palette with Noise Layer

The process starts off by duplicating the Background layer by dragging the Background layer to the Create a new layer icon on the Layers palette. The new layer is renamed the Noise layer (see Figure 16).

Figure 17: Reduce Noise Dialogue Box (Basic)

Next, the Reduce Noise filer is launched (choose Filter/Noise/Reduce Noise). The Reduce Noise dialog box appears as seen in Figure 17. In the basic mode, there are five controls:

Strength: Reduces luminance noise.
Preserve details: Helps preserve detail.
Reduce Color Noise: Reduces color noise.
Sharpen Details: Applies sharpening to the image.
Remove JPEG Artifact: Reduces noise created by JPEG compression that occurs when JPEG files are saved.

A couple of things should be kept in mind when using this filter. First, the Strength and Preserve Details settings work against each other. Higher Strength settings will remove more noise, but it will also remove more detail. Higher Preserve Details settings will preserve more detail, but it will also preserve more noise. It is necessary to play with these two settings to find the best combination that reduces the noise but leaves as much detail as possible. Second, it may be best to leave the Sharpen Details setting at 0. That way, the noise reduction and sharpening can be handled independently.

This particular image had quite a bit of luminance noise, so the Strength setting was set very high. Conversely, the image does not have much detail; thus, the Preserve Details setting was set very low. The Reduce Color Noise was set to a moderate level. Lastly, the Sharpen details was set to zero and the Remove JPEG Artifact was turned off.

Figures 18 and 20 show crops of the image before the noise removal steps. Figures 19 and 21 show the same crops after these steps.

Figure 18: Crop before Noise Removal

Figure 19: Crop after Noise Removal

Figure 20: Crop before Noise Removal

Figure 21: Crop after Noise Removal

My opinion is that this technique is not as thorough in removing noise as the previous two techniques. However, it is quick and easy.

Figure 22: Reduce Noise Dialogue Box (Advanced)

The noise reduction on this image was done with the Basic Noise Reduction Dialogue Box. Clicking the Advanced option brings up more alternatives (see Figure 22). Once in the Advanced mode, moving to the Per Channel tab allows the Strength and Preserve Details controls to be applied to each channel independently.

This ability is of value when there are different amounts of noise in the three channels. However, this option will not be used with the current image.

If one wants the maximum amount of detail, the edge technique can be used to add more detail back into the image.