8-2 First introduction to masks – Beginner’s tutorial

Before continuing, please make sure you fully understand the knowledge in Chapter #04, that is, the meaning and role of black, white and gray in the channel. Otherwise the following will be difficult to understand.

Now imagine that we want to remove the edges of a painting and hang it on the wall. The best way to do this is to use scissors to cut off the unnecessary parts. Cut along the red line as shown in the picture below. It is obvious that this cropping has caused permanent damage to the painting and cannot be restored.
This situation is just like the operation we did on the image in Photoshop before, it is a destructive operation. Although the operation can be undone to achieve the purpose of restoration. But we also know that undoing is actually the return of the historical record, and since the operations in the historical record are linear, other operations must be undone even if they are correct. For example, if we crop first and then make color adjustments, then to undo the cropping, we must first undo the color adjustments.

Now imagine that we want to remove the edges of a painting and hang it on the wall. This time, instead of using scissors, I stuck the painting behind a piece of glass, and then painted the edges of the glass with black paint. Finally, the painting and glass were hung on the wall. As shown below on the left. It is obvious that although the effect seen this time is the same as the previous cut, it does not actually cause damage to the painting. By removing the glass, the original picture can be restored. You can even scrape off part of the black paint as shown in the picture on the right below to reveal the original contents of the place.
This situation is what we will learn in this lesson, using masks to hide certain parts of the layer to avoid directly deleting the layer content. And if you delete it, the history record will not be returned.

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The glass can be understood as a transparent “plate”, a glass plate. The action of sticking glass onto the painting can be called “mask”. Pasting the glass means covering the glass plate, which is a mask. From this we get our first impression of masking, which uses a layer of “mask” on the layer to achieve the purpose of shielding (i.e. hiding) part of the content of the layer.
It should be noted that the glass itself is transparent. To hide the content of the layer, it needs to be “blackened”. The blackened area is naturally invisible. If you want this area to be visible again, you need to scrape off the black paint that has been applied to the glass. In this way, the shielding effect disappears.

If the above metaphor is difficult to understand, let’s give another analogy. Everyone should know the idiom “frog at the bottom of the well”, which means that the frog living in the well can only see the sky as large as the mouth of the well. In fact, the sky is infinite, but the size of the wellhead affects the observer’s visual range. Here, the real sky is equivalent to the layer, the wellhead is equivalent to the mask of the sky, and we are the frogs. Due to the restriction of the mask, we can only see a fixed size of the sky. It is not difficult to imagine that if the wellhead is changed to a larger size, the frog can see more of the sky. And if the frog walks outside the well, the wellhead will no longer have a restrictive effect, which is equivalent to no mask, and you can see the complete sky.

Now let’s make a summary of what we have learned so far, and introduce some basic knowledge about masks:
Masks are used to shield (i.e. hide) the contents of a layer. When you need to delete an image in a certain area of ​​a layer, you can use a mask to do so. Masks do not destroy the image and provide more room for modification later. It can be of any shape.
Masks are applied to a single layer, and different layers can use different masks. A layer can only have one mask (you will learn more about paths later).

What exactly is a mask? What does it do? Why use it? How to use it? These 4 questions trouble many beginners.
The first two questions have actually been answered and are used to mask layers. But pay attention to the two words here: one is shielding, which is different from the operation of hiding layers through the layer palette; the other is…�For layers, not for the entire image. So why use masks? In fact, it has already been answered, because the mask is not destructive. This is actually the principle of “maintaining maximum editability” that we have always advocated. As for how to use masks, that’s a big topic. We will gradually accumulate experience in using masks in future studies.

Now let’s repeat the selection selection operation we did before. Select the sky part as shown in the left picture below. Then convert the background layer to a normal layer (hold down ALT and double-click the background layer in the layers palette). This step is necessary because masks cannot be used on the background layer.
0803After the selection is established, what we have to do is to hide the sky part of this selection, so through [Layer_Layer Mask_Hide Selection], at this time we have obtained an image with a partially transparent sky, as shown in the middle picture below. The layers palette is as shown on the right (using a medium thumbnail).
The content in the Layers palette is now very different from before. A mask thumbnail is added to the right of the original layer thumbnail. And this thumbnail is black and white like a channel. For now, everyone forgets about the mask. Press [CTRL_ALT_Z] to undo the operation and go to the step of creating the selection, which is the effect of the picture on the left below.

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We already know that storing a selection is actually creating a new Alpha channel in the channel, and in this Alpha channel, white and black are used to represent the inside and outside of the selection respectively. So if you save the selection now, you should see a new Alpha channel with the sky part white and the rest black in the Channels palette, as shown by the red arrow in the left picture below. This is not difficult to understand.
In fact, channels and masks are similar, that is, both use black and white to represent valid or invalid areas. The white color in the channel is the effective area, indicating that the area is a selection. Black means it is not a selection, but an invalid area. The same is true for the mask. The white and black in the mask also represent whether it is valid or not, but this validity is specific to the layer: the white in the mask means valid, which means the layer is valid, that is, it can be seen. See the layer’s contents. The black color in the mask indicates that the layer is invalid, that is, the area in the layer is blocked and the content cannot be seen.
If we must discuss the mask, the black area in the mask means that the mask has a shielding effect, and the white area means that there is no shielding effect. At this level, the definition of whether a mask is valid or not is the opposite of a layer. In future tutorials, any application involving masking will take the layer as the main object, such as “shield this part with a mask”, which means that the corresponding part of the mask should be shielded from the layer. black.

I have spent so much effort to let everyone understand the role of the mask and how it works. In the earliest courses, we mentioned that grayscale color mode is often used to represent places other than color, such as channels to represent selections, and now masks are used to represent masked areas. If it was difficult to understand this concept at the beginning, you should have a complete understanding by now. The most important purpose of this tutorial is to let everyone understand these basic concepts so that they can have the ability to expand and analyze in the future. For example, when you see an image effect, you can analyze how it was made. When you want to achieve an effect, you can also know the general direction of production. Therefore, if you still know a little about it at this time, you should review the previous content and be sure to fully master it.

<!–Collectio