Photoshop:
Within and beyond unsharp mask I.
Within and beyond unsharp mask I.
A key issue in improving your photo is sharpening. In case of a digital or a post-digitalized photo you may often have to supervise and alter the job done by the image-improving algorythm of the software, which was not to your liking, or you may have been dissatisfied with the result of the poorer quality equipment you had, and you would like to bring out the maximum with regards to sharpness. Several softwares offer methods for correcting photos that are stored in the form of bits, and Adobe Photoshop also does. Most often these are "one-button" user-friendly solutions, which will easily produce spectacular effects, but you will just as often face such side effects as increased noise along with sharpening or the annoying halo effect.
We are launching a new series about the major pitfalls and also a few sophisticated tools of sharpening with a software. We will leave behind possibilities of Sharpen and Unsharp Mask in Photoshop, and venture to achieve better results by combining them with other steps. The first example deals with color noise and introduces a sharpening tool that helps avoid this anomaly.
Applying Unsharp Mask filter has a big disadvantage, namely boosting color noise. In RGB mode, ideal for computer visualization, red, green and blue color channels together amount to the brightness value of the photo too. Sharpening will have the same effect on these channels too. Sharpening increases the contrast difference between adjacent pixels, and consequently the until then unnoticeable noise will increase as well.
Here’s an example. The photo on the left shows part of the sky in a digital photo before sharpening, while the one on the right shows the same part after strong sharpening. It is of course not our aim to sharpen the homogenous sky, but any sharpening in the photo will have this negative effect and will be most annoying on homogenous surfaces. The resulting change in the color of the sky is clearly detectable here. In every channel the contrast difference among the pixels have increased, which appears as color noise eventually. The homogenously colored sky turns into an unnatural whirl of colors.
The solution lies in omitting color channels and sharpening the bright contents solely. It's not possible to carry it out in RGB mode because each color channel contains brightness and color information at the same time. Photoshop, however, provides an option with which to modify brightness data solely. To do so, switch from RGB color mode to Lab Color mode with Image/Mode/Lab Color command.
By looking at Channels palette you’ll see that there are three channels in Lab mode as well. However, Lightness channel differs from ‘a’ and ‘b’ channels. The latter two contain the color data of the image which was the “duty” of all three channels in RGB mode, while the former contains all of the image content apart from colors. Consequently, no changes in Lightness Channel will affect the colors of the photo. Color noise will not increase either.
As a first step, duplicate the original layer and make all changes on the new one and leave the original one unchanged. The usual starting instruction in a recipe “Take an egg.” goes like this in Photoshop: "Duplicate the layer." To do so, use the Layer/Duplicate Layer command. Afterwards click on Layers tab to check. You should see a Background layer and its copy in the opening window. Select the duplicate layer.
Return to Channels palette and click on Lightness layer (Ctrl+1) as shown in the previous picture. The photo appears as a grayscale photo, but there’s no real change in the image only you can now see the lightness content.
Now comes sharpening. Select Filter/Sharpen/Unsharp Mask filter. Unsharp Mask is the most frequently used sharpening method in Photoshop. With this tool you can set the strength of sharpening in detail and also the number of pixels involved in the modification. You can see three sliders in the appearing window: Amount, Radius and Threshold. These are used for setting the appropriate sharpness. Amount sliders is for setting the strength of sharpening between the amounts 1-500%. Radius controls the scope of sharpening, with decimal accuracy. Threshold represents the number of pixels involved in sharpening. When set to a lower value, sharpening affects a bigger area in the photo, while at a higher value, sharpening will appear in a smaller area.
Use excessive sharpening now, because later you will soften it a bit. The edges can be accentuated, a bit harsh. We set Amount to 388% and Radius to 0.5 pixels. Threshold is 7, which softened the final result, yet the overall effect is still excessive sharpness. These numbers are of course not carved in stone, it is the type of the photo, its details and your sense of beauty that casts a limit to the numbers.
When finished, click on Lab channel (Ctrl+~) and so all the channels will be selected and colors of the photo become visible again. Now it is clear that the colors have not changed along with sharpening, you’ll see no increase in color noise.
The advantages of Lab mode are no longer needed, you can switch back to RGB mode. The actions taken between these two modes have no negative effect on the photo. Let’s apply Image/Mode/RGB Color command for color mode switch. You will get a warning message reminding you of the two layers and that they should be united. There’s some more work to do with the layers separately, therefore choose Don’t flatten option.
Next move on to Layers palette. You can see the above-mentioned two layers here: the original background layer and its duplicate, which was changed. Select the top layer (this is the active one by default), and set the ratio of the unmodified and the new layers with Opacity. The value of Opacity is 100% by default, which means a non-transparent upper layer. By decreasing this value, the photo becomes more and more transparent, and this way sharpness also decreases. The photo you have just sharpened a bit too much can be softened according to your taste, and further annoying effects of sharpening can be decreased too. The appropriate value may vary again, as you like it. Try to find the balance between the too soft and the too harsh edges. Find the stage where the effect of the sharpening is visible but not yet loud. The effect of the upper layer has been set to 80% in the sample photo.
Now you can compare the result and the starting picture result.
The edges and details appear sharper, more accentuated, but there is no detectable change in colors and the colored homogenous surfaces. In case of a more drastic sharpening, you may bump into further problems at this point. One of them is increased luminance noise. Luminance noise can be a by-product of sharpening applied in the whole image and in Lightness channel, or the effect may increase if such a noise was already there in the original photo. It is again the most conspicuous in homogenous surfaces. Its presence is usually not disturbing in areas with tiny details. The next tutorial will introduce a sharpening method which will make you forget about increase in noise.