Also talk about Photoshop’s USM sharpening filter – filter usage

Also talk about Photoshop’s USM sharpening filter. The name of the USM sharpening filter comes from the traditional film synthesis technology. That is, “enhance the border effect of the image by blending the blurred negative with the original positive.” (Quoted from the Photoshop Book). No matter how confusing the name is, USM Sharpening is our most requested sharpening filter. When you use it, Photoshop increases the contrast of adjacent pixels, that is, it works by identifying borders and increasing the contrast around them. Just like when we adjust the focus of a camera to make the picture clearer.
The USM sharpening filter has three options:
1. Quantity: The degree to which the image is sharpened. Higher values ​​produce more pronounced sharpening effects. When sharpening, the results of using the filter repeatedly starting with a smaller value are essentially the same as using the filter once starting with a larger value. The advantage of starting with a smaller value is that it is easier to observe and use the Ctrl F shortcut key for repeated use to accurately find the ideal value. When the file is large and the resolution is high, the number will be larger; otherwise, the number will be smaller.
2. Radius: The width of the sharpened edge. Smaller values ​​produce sharper edge effects. Larger values ​​produce high contrast and wider edge effects. The ideal radius depends on the size of the image, its resolution, and the quality of its borders. Use lower radius values ​​for screen graphics and online images. For example: 0.5; for images with particles, scan lines or JPEG compression marks, use a higher radius value. For example: 1-1.5; use higher radius values ​​for large files and high-resolution images.
3. Threshold: Controls the effect of confirming edges in the image (that is, sharpening the brightness change value of two adjacent pixels). Low values ​​make pixels clear, high values ​​repel pixels. When the threshold is 0, each pixel value of the image is sharpened. Increasing this value causes Photoshop to calculate and identify boundaries, looking for differences between adjacent pixels to sharpen. Therefore, when sharpening a grainy image, increasing the threshold should produce a satisfactory sharpening effect. However, this is not the actual situation. The transition between sharpened and unsharpened pixels is stiff and not soft. To solve this problem , using masking technology is ideal.

Several issues you should pay attention to when using the USM sharpening filter:
1. To accurately see all the details of sharpening, the screen image should be enlarged to 100% display. Sharpening should be performed after the image color correction, cropping (redefinition of pixels) and other designs are completed.
2. Sometimes sharpening the red or green channels alone can produce obvious effects. And can reduce the noise generated during sharpening. This is because these two channels contain more image detail, with the blue channel having the least.
3. In order to reduce the halo produced during sharpening, the image can be converted to Lab mode, and then the brightness channel can be sharpened separately.
4. If you want to print out, it will be better if the sharpening radius value and quantity value are smaller. For printing, sharpen larger.

The steps for sharpening images by generating and using boundary mask techniques are as follows: Figure 1
1. Convert the image to Lab mode and copy the brightness channel in the channel palette.
2. Select the duplicated Luminance channel and invert it by selecting Filter → Stylize → Find Edges and then pressing Ctrl + I (or Image → Adjustments → Invert). As shown in Figure 2
3. Widen the border, select Filter→Other→Maximum, and enter a radius value of 4 (you should keep trying according to the resolution of the image. The higher the resolution, the larger the value, and vice versa).
4. To thicken the border, select Filter → Noise → Middle Value, and enter the radius value as above {or smaller}.
5. Soften the hard boundaries. Select Filter→Blur→Gaussian Blur and enter the radius value as above. At this point, a precise and soft boundary mask is generated. As shown in Figure 3
6. Return to the layer layout and copy the layer.
7. Ctrl-click the mask channel generated above. Load selection.
8. Click the Add Mask icon at the bottom of the layer palette to add a mask.
9. Select the brightness channel, and open the eye icon of other channels, run the USM sharpening filter, enter the amount: 350; radius: 1; (add or subtract according to the size and resolution of the file) and always set the threshold to 0.
10. After sharpening, select the brush tool according to the sharpening effect, and use black and white to further edit the mask. Add black to the areas that don’t need to be sharpened, and white to the areas that need to be sharpened. You can also lower the opacity of the top layer to reduce the amount of sharpening. As shown in Figure 4
This method is effective for sharpening grainy images.

Figure 2

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Figure 4

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