What is the principle of PS sharpening?

Sharpening is a topic that many photographers can't avoid when they retouch their pictures later. Sharpening is the last step in the later photography workflow, which plays a very important role. In PS software, there are many ways to sharpen images, but the general mode and algorithm are the same. Here I take USM sharpening as an example to illustrate the principle of sharpening.

How to produce sharpening effect? First of all, please remember an idea: the sharpness of the picture, the contrast from the edge is increased. It doesn't matter if you don't understand it now. Remember this sentence first. Let's use the following example to prove this point.

Please look at a picture first:

This is a close-up of a plant photo. I enlarged the leaves separately. Let's observe the contrast before and after sharpening. Obviously, after sharpening, the clarity and texture of the right image are greatly improved compared with the original image on the left, giving people a clear and sharp feeling.

Except for the contrast of the clarity of the two pictures, it seems that I can't see why. It doesn't matter. If I enlarge the sharpened picture, what will you find?

Look, at the position indicated by the arrow in the figure, a black line, a white line and two lines appear at the junction of the leaf edge and the background. There is a black line in the background and a white line at the edge of the leaves.

Due to the contrast of black and white lines, the contrast of the edges of leaves is increased, and the subject and background are affected by the visual error caused by these two lines, which seems to increase the sharpness.

This is a sentence that everyone remembers earlier: the feeling of sharpening the picture, the contrast from the edge increases. It is this contrast between light and dark that gives us a sharp sense of vision.

Sharpening algorithm: I can't fully understand the principle of sharpening just by looking at this contrast, so I will tell you how PS software achieves sharpening effect. What I'm going to say is likely to confuse you even more. You can't even believe it: "USM sharpening in PS software actually uses a fuzzy algorithm to achieve the sharpening effect."

What logic is this? Are you confused? The relationship between sharpening and blurring is simply miserable. How to use fuzzy algorithm to achieve sharpening effect? Then let's take a look at the principle and study how PS software is done. Let's look at the picture first:

This is the first step in the whole sharpening process. Let's remember a formula first:

A. on the original image, use the USM sharpening tool in the PS filter menu.

B After using USM sharpening tool, PS software will first produce a slightly blurred picture.

C, the software will use the original image with higher definition and subtract the fuzzy image with lower definition to get the difference between them.

Then this formula is: original image A- fuzzy image B= difference C.

Subtract the blurred picture b from the original picture a, and then get the difference c, which is the edge part needed for sharpening. We have been emphasizing before: "The feeling of sharpening comes from the contrast of edges." Then the difference obtained will be very useful.

After the previous discussion, let's look at a very simple mathematical operation:

The original image A minus the fuzzy image B equals the difference C, right?

Then think about it from another angle: does the original picture A- difference C necessarily equal to the fuzzy picture B? Is this a fact?

A-B=C, or A-C=B, there is no problem at all.

The original image A- difference C= blurred image B, but this is only the first step in the sharpening process. Next, let's look at the calculation method of the second step. Let's look at the question in the picture first: what is the difference between the original picture A and the difference C?

Please think about it. The difference C is the result of subtracting the fuzzy image B from the original image A, and it is also the edge part of the two images. Then, on the basis of the original image, adding this difference is not equal to sharpening?

This algorithm is difficult to understand. Next, I will connect the two steps together to make it easy for everyone to understand:

The software first generates a slightly blurred picture in the original picture, and then subtracts the blurred picture from the original picture to get the difference. Adding the difference to the original image is equal to the sharpening effect.

Then the sharpening formula is: original image+(original image-blurred image) = sharpening.

Let's illustrate all the steps of sharpening with pictures:

Look, this is the whole process and steps of USM sharpening.

Some friends may be curious: in the previous demonstration of sharpening process, a slightly blurred picture was used. Let's make a hypothesis here. What would happen if I changed the picture to a more blurred one?

As mentioned earlier, sharpening actually uses a fuzzy algorithm to achieve the sharpening effect. So back to the question just now, how about using more blurred pictures?

The answer is: the heavier edges will have differences, and adding this difference to the original image will be equivalent to a greater degree of sharpening:

Then the real question arises: what does the blur of this picture mean?

What is the use of the three sliders on the USM sharpening panel? Explained in written language, the difference of ambiguity lies in "radius".

Please note that at this time, we talked about the term "radius". When you open your own PS software and select USM sharpening tool, there are three parameters on the tool panel with sliders set, and the term "radius" is among them, right? Please remember!

As can be seen from the above legend, the final sharpening effect is a little too much, that is, distortion, due to the increase of photo blur. This leads to a very important concept: what factors produce different sharpening feelings?

This requires the emergence of the next term: "granularity".

Don't doubt, there is no typo, not strength. What is "granularity"? In fact, granularity is the scale of picture details.

When sharpening the effect, you must know which detail scale to emphasize in the picture. With this concept, it is easier to understand the three parameters on the USM sharpening panel:

Quantity: Represents the intensity of sharpening.

Radius: it is the granularity, and I hope to emphasize the scale of the details of the picture.

Threshold: specified in PS, when the contrast is less than a certain number, sharpening will not be performed.

Of the above three parameters, "radius" is the most important. When you operate, you must first find the appropriate radius, set the scale to decide which detail you want to emphasize in the picture, and then adjust the number and threshold to complete sharpening.

Well, after such a big circle, I finally finished writing the working principle of sharpening the knife. I hope this article can help some friends who don't know much about the principle of sharpening knives. I am "lascivious map 1", thanks for watching!