How to use PS curve tool to increase local contrast degree

How to use PS curve tool to increase local contrast degree

Before starting this PS tutorial, first introduce the main points of the tutorial:

1. How tonal, contrast and curve work;

2. How to read tonal curves and histograms;

3. How to use control points;

4. Adjust the order of operation;

5. How to use the curve for local contrast adjustment.

Part 1. Dynamic distribution

After you create a curve adjustment layer, Adobe Photoshop provides a histogram and some settings. This is called the dynamic distribution. First, we need to introduce a concept: Each digital photo is made up of countless pixels, each pixel corresponding to the intensity of the gray value. Picture tonal dynamic distribution refers to the image from black to white, how many layers of gray tones. For example, in 8-bit grayscale digital images, each pixel is uniquely distributed in 256 levels of gray intensity.

The histogram is the pixel intensity distribution state in the form of image representation. Essentially histograms tell you from another angle that each gray value corresponds to the number of pixels that exist in the screen. This does not need to look directly at the photo itself to know the image pixel gray distribution, this visual separation of pixel information is very convenient and useful, the following will be further explained.

Part 2. How to read tonal curves and histograms

The top of the curve histogram is the high light, the middle is medium tone, the bottom is the dark part. The Curve tool operates on the histogram, the x axis represents the input value, the y-axis represents the output value, and the XY axis is set to 0 to 255.

1. The leftmost 0 points are black, the pixel strength is 0, the top right end is 255, and the pixel strength is the largest white.

2. The right side of the 0 point is the dark part, at both ends of the middle area is the gray, the rightmost is the high light area.

Figure 01

Curve Slope control Contrast degree

Contrast is to indicate the difference between tones, there is a whole picture of the overall contrast, there are specific areas of the local comparison. Each new curve is a diagonal 45° line, which means that the light dark output and input ratio is exactly 1:1, at which point the filter does not produce any effect.

Changing the slope of the curve is to change the relationship between input and output, the slope adjustment to more than 45 ° will increase the contrast, lower than 45° will reduce the contrast. The upper and lower displacements of the curves also adjust light and dark. The overall move down will reduce the output value, so that the image darkened, moving up will increase the output value, the image becomes brighter.

As can be seen in the histogram above, most of the pixels in the example image are concentrated in the middle region, and are evenly distributed between the high light and the dark part. But at the same time there are a considerable number of pixels are in the high light and shadow, which means that the image of the tonal distribution of relatively uniform, more fully exposed.

Figure 02

In fact, the contrast of photos directly derived from the camera is not as high as it is supposed to be, and this is normal. Because, in order to keep the highlights and shadows as much as possible, digital camera manufacturers generally use a relatively conservative approach, directly to the sensor intake of RAW image information displayed in the photo. Therefore, relative to the loss of important image details and even contrast disorders, from the retention of details, relatively small contrast of the image start is a good choice.

The effect of contrast degree

Our eyes are easily attracted by something that is very contrast-intensive. Imagine that polar bears are hard to spot because of their white fur, which is low contrast; Snow leopard's spotted pattern is easy to spot in the snow because of high contrast. Although a bit subjective, but the appropriate contrast is to ensure that white is not too taibai, black is not too dark.

In general, contrast is the difference between the light and dark parts. Enhanced this difference will make the image more prominent, lower will make the picture more mediocre. Global adjustment of contrast is useful, but not the most useful, and local adjustment works better when dealing with a particular scenario. Whether you increase or decrease the contrast, local adjustment of the image contrast may result in attracting attention. Then, the histogram is used to understand and adjust the dynamic distribution of the hue so as to properly guide the visual focus of the person looking at the graph. As for the specific how to adjust the local contrast, is based on the image content and creative designers to decide.

Part 3. How to use control points

The above description sounds like a color-level adjustment, but the main difference is that the curve can be accurately adjusted through critical control points, rather than in the broad range of the black and white tone. The control point is positioned on the curve, changing the input and output values of the tonal range of the control point by dragging up or down. As shown in the following illustration, there are three control points on the curve, located at the bottom, middle, and top, with three control points to adjust the curve to an "S" curve.

Figure 03

The "S" adjustment curve will enhance the contrast of the midtones, the contrast between the high light and the shadow is reduced, the light portion is more light and the shadow part is darker, so it can be said that the adjustment step is more general. Although the direct mobile control point is a bit less intuitive, it is fairly quick and easy to make adjustments simply by dragging.

Figure 04

Operation Tip: Select the control point, press the direction key can fine-tune; press TAB to convert between control points; Drag the control point off the histogram and release it to remove.

Part 4. Adjust the order of operation

Through the above introduction, already understand how the curve and control point is how to operate, below we see how to do local contrast adjustment.

Step 1: Complete global correction and adjustment

Many times we need to do batch operations, after the batch operation and then according to the characteristics of each photo to be specific optimization. Since the photos are basically derived from a camera, it can save a lot of effort to adjust the global optimization according to the camera's characteristics. Processing some pictures in advance will make the adjustment work out of sync and increase the difficulty of synchronizing the pictures. All in all, first make a global adjustment to all the photos.

Here you can consider using the "S" adjustment curve, which can basically meet the requirements before the details are adjusted.

Step 2: Complete the necessary local correction

Some of the photos require additional remediation. Complete the necessary partial corrections before starting the local adjustment. such as false light correction, barrel distortion, edge, color, etc. If the correction is done before the adjustment, local correction will be difficult to complete, so first do local correction.

Step 3: Evaluate the development plan

After the global correction, adjustment and partial correction, the photos will look more pleasing to the eye. At this time notice the contents of the photo itself, what are the highlights, how the feeling of ascension, write down these questions.

Figure 05

Then think about what elements the screen needs to emphasize, what parts of it need to be enhanced, or some of the parts that need to be reduced in contrast. These are the questions you need to ask yourself to adjust your photos, and write down your questions and your answers. This adjustment process becomes logical and intuitive, and the adjustment changes are connected, so it is necessary to look at the image and evaluate whether the image has reached the desired effect if the next step is not achieved.

Step 4: Complete the local adjustment

When you think about it, the operation becomes a little bit more pure. Add a new adjustment curve to the area that needs to be adjusted.

Step 5: Re-evaluation and comparison

After a round of adjustments, stop and see if the image is what you think it is, and perhaps your understanding of the content of the photo will change. Never mind, keep your thoughts down. At the same time with the same series of photos to see if it has been adjusted in place. If not, you will need to continue the adjustment.

Part 5. How to use curve to adjust local contrast

In the example of the photo, the "S" adjustment curve is a good way to complete the global adjustment, but as you can see, there are some local areas that need to be adjusted. In the picture, the sky appears too flat and needs some change. Coarse wood grain and fluffy clouds are good details, and their presence can increase the overall depth of the picture and need to be highlighted further. So how do we do that? Our goal is to darken the sky so that the clouds become more prominent, darkening the shadow of the ship and the shore.

Step 1: Increase the curve adjustment layer

Create a curve adjustment layer, and then increase the control point in the tonal range that you want to adjust. Select the image sampling, click on the Finger icon, choose the following figure to fill in the blanks, which will increase the control point.

Figure 06

This control point is the hue area of the sky, and the control point is turned down to darken the sky. In addition to the sky, other areas such as the beach are also changed in the same color range, so adjustments are needed to compensate for this change. The application characteristics of the curve can make the adjustment effect better than the color bands.

Step 2: Use Mask control curve adjustment Application scope

As with other adjustment layers, the curve adjustment layer does not directly change the pixel of the image, and the curve adjustment layer is paired with a layer mask, so you can use the scope of the Mask control curve adjustment. Use a black brush in the area where you don't want to adjust.

Figure 07

Classification:

• PS Getting Started Tutorial