Teach you to use the channel mask to solve the problem of complex color bias

You often have to create a selection in a scene with two or more light sources. Figure 9-17 This professional photograph was taken for an advertisement, but the photographer could not compensate for the tungsten on one side of the image and the other side of daylight. Therefore, the left side of the image is naturally correct, but the right side has a blue color bias. Naturally, customers want to eliminate color bias.

Figure 9-17

Because of different types of illumination, this image has a blue bias on the right side, but the right balance on the left. In this image the traditional Chinese medicine eliminates unwanted tones that usually require a mask.

If there is a color bias on the bright part of the image, and there is a different color bias in the dark part, we may use a single set of RGB or CMYK curves to fix it. However, if there is a color bias on the right, and there is another color bias on the left, then no matter what color space we are in, we need to use a selection.

Because this file is sent to me in CMYK format, so we will keep this format. If the file is in RGB format, the required steps are exactly the same. Because the only goal now is to eliminate the color bias and not use the lab curve, but we still need to figure out how to select the air on the right side of the file.

If you insist on creating the mask in some other color space, it will take a long time to get into it, not only to try to exclude all parts that are not blue, but also to create a credible transition between the color-biased end area and the normal light start area. Realizing that the initial mask is already in a B channel that does not exist, the whole exercise can be done in less than a minute.

The first step is to put a copy of this hypothetical B-channel into the CMYK file. So, we execute the image > Copy command, and then execute the pattern >lab color command. Because the B channel is the third channel, the keyboard shortcut ctrl-alt-3 can be loaded directly into the lab file as a selection.

Now, because the selection is a B channel, we choose to select > Save the selection, which means we want to save a separate channel for future use. Photoshop asks where we're going to put it: the current document, or as a separate grayscale file, or save it in another open document with the same size as the current document. The last option is what we want: The original CMYK file is the same size as the lab copy, so we can put this B channel into the CMYK file as the nonprinting v Channel, as shown in Figure 9-18.

Figure 9-18

The active selection can be stored in the future as a mask, either as a separate document or as any open alpha channel with the same size file.

A and B channels are generally quite gloomy, but this channel is an exception. The gray value of 50% represents the neutral value, the image is basically a neutral photo. The slightly increased blue on the right side of the image is there, but it's hard to see. The contrast of this new channel must be enhanced before the mask can be used.

Ctrl-5 Opens the B-channel copy, which is now the fifth channel of the CMYK document. The contrast needs to be greatly enhanced, which cannot be done in one step. It can be done in several different ways, but, usually after a little blur, I start to apply the automatic level command and get the graph 9-19a. Remember, because this is a nonprinting channel, the alpha channel, so no matter what we do with it, it will have no effect on the final effect.

Figure 9-19a

Before you use a or B channel copy as a mask, you usually increase the contrast. The original B channel in Figure 9-18 is too gray to display here, and the image is blurred and then applied to the effect of the automatic color order.

Although the added blue on the right is now visible as an extra darkness in the mask channel, we need to go further. The left side should be pure white, which is not the case in Figure 9-19a. The most blue place on the right should be pure black. There should be a transition zone between the two. All of this can be done by using very steep curves.

In addition, the area masks that are allowed to be modified need to be lit, while the masked version of the locked file area is dimmed. Figure 9-19a is the opposite. When you load a mask, Photoshop provides us with an option to reverse check boxes. I find this option confusing, so I always use the image > Adjust > Reverse-phase command to reverse the mask channel instead of using this option. After this operation, add a curve and a slight blur to create the graph 9-19b.

Figure 9-19b using the curve above to increase the contrast, and then reverse the channel to create a final mask

We now go back to the original CMYK image and select > load the selection. The modified B-channel is displayed as an option and we load it. So far, we have created the ambient light constituency. After the operation is complete, at least 6 methods can be used to eliminate the color bias. I select the image > Adjust > Chroma/Saturation command, and I reduce the saturation control of the entire graph by 50 points, creating the effect shown in Figure 9-20.

Figure 9-20 Final image

Reduces the color bias by loading the graph 9-19b into a selection and decreasing the saturation with the hue/saturation command.

It takes longer to explain how to eliminate the color bias in figure 9-17 than to eliminate the color bias, even if it's a very experienced decorator that typically takes a long time to get bad results. The reason I know this is because I used this special photo for practice in the Advanced seminar.

The ability to visualize the visual effects of a channel is the key to creating any complex selection. The A and B channels look completely unrelated, which makes many people mistakenly ignore them. As we have seen in this chapter, they can often provide a better selection than other methods. Let's start with objects that can be easily selected in any color space, but when we continue to choose, it becomes more and more difficult to create constituencies without the help of a or B channels.

One last advantage: knowing when to use a or B channel to create a mask is the same technique as knowing when to use masks in some amazing channel mixes. After this, please remember the flowers in this chapter, perhaps your life will not become a hotbed of roses. But it's hard to say that image processing is like a trick makeup, and you'll find the edges obvious.

Summary:

A and/or B channels are often a good starting point when you need a selection or a mask. After the appropriate processing, they create a mask, such a mask effect with the weakening of the color, and RGB, RGB mask effect with the color of the darker and weaker.

Before loading a mask based on a or B channels, you typically need to enhance the contrast and blur it. You can use such a mask even if other works are not processed under lab. There is no rule to limit the copying of an image, convert it to lab, and pass one of its channel copies to the original file.

Category:

• PS Image Processing
• PS Mask Tutorial