The opacity property is one of the main characteristics of a layer in Adobe Photoshop. Moreover, historically, that the realization layer transparency the authors of the program and the program improved over 20 years – started several times and in different years was solved this problem in different ways, so ultimately, to date, the user has several ways to control the transparency of any fragments of the image.
The most easy to understand may be the idea that a layer of Photoshop such real rectangular sheet of paper – a page from a magazine or photograph. From these images you could do a collage or suitable cutting with scissors from these leaves unwanted edges, cutting out fragments, displaying these clippings on a background sheet, superimposing on each other, gluing together stationery glue, etc.
And digital layer, you can cut the desired fragment, just cut off the edges.
Try to test it in practice. For example:
- open the file with any digital image
- taking the Polygonal Lasso tool, draw around a certain area of the image is a closed polygonal curve (with this action you have assigned allocation, it now appears as a shimmering contours)
- press Ctrl+J. You will see in the layers list, there is a new layer containing just the portion you just cut out of the main picture. (By the way, if the layer list is not visible, press F7)
now, if you temporarily turn off the visibility of the previous layer – click on the icon of the eye next to the sink with layers-the original – you'll see that around the cutout objects in the working area appeared grey-white squares is a conventional technical designation transparency in Photoshop.
The task is done — the area around the selected fragment is transparent.
Now the created object you can move, deploy, repaint, apply filters, overlay on another background etc. of Course, from the received snippet and then you can cut the pieces and slotting it in the "hole" – selecting areas and pressing the Delete key or Backspace key, erasing it with the Eraser (Eraser), etc.
Thus, to make the picture background transparent, enough

- carefully cut from the original image necessary objects

and copy them into another layer.

In addition to Ctrl+J, this can be done through menu Layer> New> Layer via Copy (Layer>New>Layer by copy)
The disadvantage of this method is that as the real paper sheet, cutting the excess, you will not be cut off forever. If the path is simple, the chance to make a mistake when cutting small, but if we are talking about a complex object, then accidentally deleted the sections at the edges can spoil the whole impression. In the best case, you will have to refer back to the source and re-cut out the missing pieces, then "glue" them to the cut layer, that is, one way or another again to produce the same work. Agree, this is not a very bright prospect.
Fortunately, the authors of the program provided another way of creating transparency. This so-called transparency mask.
The principle of its operation is that for each layer can be created by individual map, each point of which is responsible for the transparency of the corresponding portion of a layer with the image. Changing the color map from black to white can achieve an absolute visibility, translucency or invisibility of a layer element. Thus, the image on the layer can be made invisible without clipping and destroying him completely, just "camouflaged". Then, to find in the course of further work poorly cut edge to restore its correct shape by editing the appropriate place on the mask.
To create a transparency mask of a layer by pressing the Add Layer Mask icon at the bottom of the layers panel. If you asked any selection on image – for example, around the outline of some details of the created mask will inherit this allocation, that is, according to the allocation will automatically become black and white transparency map. To recognize the generated mask can be introduced on the layers panel in the line next to the rectangular, embodying the contents of the layer, the additional rectangle with the schematic of the mask.
The edit mask is available through the corresponding panel display which is possible only through the main menu item Window>Masks (Window>Masks). To edit the mask is available in all the standard tools – brushes, stamp, brightness and contrast, filters, etc.
Thus, to create a transparent background in the picture, it's necessary

so it was created a transparency mask,

- and this mask was applied black-and-white contour image, which will do the required elements are visible, and the background is transparent.
You need to be aware of one "quirk" of the system of layers in Adobe Photoshop.
Again, for historical reason, there is a vestige of the separate type layers, the so-called Background, which can have a transparency mask. This layer is automatically created by default when you open an image of inferiority and misleads beginners. To eliminate the absurd restrictions and create the same on the layer transparency, you need to make a simple procedure is to convert the layer into an ordinary layer of Photoshop. This is done using menu item Layer>New>Layer from Backgound layer (Layer>New>Layer from Background)