The computer can work only with digital media. To process by means of the computer the image, it needs to be expressed in a digital form. There are two main methods of numeral representation of images: raster and vectorial. The present chapter will be entirely devoted the first of them. Program Photoshop is intended for operation with bitmap graphics. Despite strict specialization, Photoshop allocates some "vectorial" tools, and their dial-up extends from the version to the version. Nevertheless, vectorial tools in Photoshop fulfill auxiliary functions and are used as additional means in the main task decision: correction and mounting of bitmap images.

Authors consider this chapter very important in spite of the fact that in it there are not enough exercises. If the beginner not quite represents, as the image which it edits is arranged, and that actually happens to the document the positive result will be perceived as a miracle, and process of operation with the program will remind magic ritual. On the contrary, the professional is clear in the mind about all process of handling of the image, understands, how tools operate and knows, what characteristics the image for the different purposes (review on the screen should have, the press on various devices, a paper, etc.). If you want to receive good and stable results, we advise to you to understand concepts which you meet in this chapter

Bitmap images are very similar to a mosaic in which the picture is formed of small monochromatic elements, glasses. If to depart from the mosaic panel it is far enough, separate glasses become is indiscernible are small, and the image seems homogeneous (continuoustone). By this principle the bitmap image in computer graphics is encoded. All it breaks into small cells, each of which receives the color averaged on the occupied area. And processing rate splitting is produced for simplicity as in the table: on horizontal lines and vertical columns (for this reason bitmap images always square-topped). By operation with the image the computer "remembers" this table and color of its each cell. Thus, in bitmap images there are no objects as those.

Such simple method of coding provides also ease of its automation by means of scanners. The main node of the flat bed scanner is the ruler from photosensitive elements. It is located on the original (a picture, a photo, a slide and etc.), and color measured by its everyone element is brought in an appropriate column of the table. At relocation of a ruler along the image color samplings through equal distances become and register in lines of the table. As a result in the table there is an exact picture of the original in the numeral form. Each cell of the table is called as a point, and all table — the bitmap image.

The monitor too is the raster device. Its screen is covered by a square-topped grid from phosphor points. At image demonstration the encoded information is visualized by means of operation, reverse to scanning. To each point of the image the point of the phosphor named пикселом is put in correspondence. Therefore a point of the numeral image often identify with пикселом and say that the bitmap image consists from пикселов though it and is not absolutely true. The monitor — not the unique output device. It is possible to print the image on the printer, the typographical machine, to deduce on a film or photographic paper. The majority of output devices, as well as monitors, too are raster devices, and to image points are put in correspondence of a point of these devices.

Let's underline that the numeral image which is in storage of the computer.не has the physical embodiment, it only a dial-up of digits. To see it it is possible only by means of any output device. For this reason appearance of the image (the size, quality, a color rendition and etc.) strongly depend on monitor or printer characteristics.

The more glasses make a mosaic, the more details the artist can transfer. Bitmap images too are characterized by an amount of points making them. Monitor resolution

Owing to a frequent identification of points and пикселов the sizes of images measure in пикселах. It is represented convenient if the image is intended only for demonstration on the monitor (web pages and other documents for electronic propagation). Convenience is caused by the standardized amount пикселов which can display monitors. For the majority of monitors of IBM-compatible computers this value makes 640х480, 800х600 and 1024х768 пикселов across and verticals, accordingly. Professional monitors are capable to display and большее an amount пикселов.

To imagine, how many places on the monitor screen will be occupied with the image of the known size, it is necessary to know, how many пикселов the monitor it is necessary on unit of length. Such value has own title, resolution, and is measured in пикселах on inch (pixelperinch, ppi). In each specific case it depends on the physical size of the screen and the installed size of a raster grid, i.e. an amount пикселов on a vertical and a horizontal. The number of possible combinations of these parameters is rather great, but more often resolution of monitors install in a range from 72 ppi to 96 ppi. At more high resolution elements of the interface of programs (the text in the menu and dialog boxes, toolbars and etc.) become too small, eyes quickly get tired. The low resolution, on the contrary, leaves on the screen not enough place for the most edited image or the text.

As it was already told, to express image size in пикселах conveniently by preparation of a drawing for electronic propagation. If aim consists in obtaining of a printing copy it is better to operate with metric units.

The resulted calculation is fulfilled proceeding from resolution of the monitor 96 ppi. For resolution 72 ppi the size of the same image appears other: 1,4х0,7 inch =35х18 mm (100 pix/72 ppi = 1,39 inch; 50 pix/72 ppi = 0,69 inch).

To produce similar calculations each time as soon as it is required to estimate image size on the specific output device, rather it is not convenient. Therefore the size of bitmap images characterize the same as also raster devices more often: resolution. Instead of the size in пикселах at creation or image scanning thus specify resolution and the geometrical size in centimeters or inches. The numeral image from it does not acquire the physical sizes. Such method is equivalent to the assumption: "if the image was deduced on the device with the given resolution it would have the given size".

Certainly, (with an identical amount пикселов) it is possible to present the same image uncountable set of ratios the size/resolution. The example to it has been resulted above: the image in the size 100х50 пикселов is characterized as "25х13 sm, 96 dpi" or "35х18 sm, 72 dpi". Such pairs the size/resolution can be invented as much as necessary. The more you set image resolution, the there will be its size less. This output is obvious enough. After all the above resolution of the prospective output device, the more small its raster points and less geometrical image size.

At creation or scanning of images to you will be always known (at least approximately) the demanded geometrical image size and its resolution. The geometrical size is defined by design of the "paper" publication or the web page. Resolution is defined by the prospective output device. Setting these parameters even at creation or scanning of images, you not only relieve yourselves of calculations. The data about the geometrical sizes of images is used at the press from Photoshop and at layout of images in publishing systems and illustration programs. Correspondence thus resolutions of images to output device resolution will be on your conscience.

The information on the geometrical size and resolution of images is stored in image files. These values are used at an image location in the program of imposition or preparation of illustrations

Till now we considered a case when at an image output of one it пиксел it is transferred to one пикселом devices. Other cases when пиксел images it is transferred by several пикселами output devices are possible also or пиксел output devices transfers a little пикселов images. Thus the image size on the output device in the first case increases, and in the second decreases. There is an image scaling.

More often you face scaling of images at review on the monitor. As you know, tool Zoom (Scale) allows to regulate scale of demonstration of images freely. Pay attention that it only demonstration scale (an output of the image to the monitor). The image saves both the "absolute" sizes in пикселах, and the installed size and resolution. Considering a stamp through a magnifier, you see its increased image, but the size of the mark thus does not change. In the same way, installing больший display scale on the monitor, you do not change the sizes of the image but only increase review scale.

1. Open the image Pyramid.jpg with applied diskettes.

2. Activate on a toolbar tool Zoom (Scale).

3. Select in menu View (Type) command Actual Pixels (the Real size) or click on the button of a property bar with the same name. The image is presented on the monitor in scale 1:1. пикселу images one corresponds to one пиксел the monitor.

4. Click tool Zoom (Scale) on the image. The review scale increased, the image in a document window became more. Thus one пиксел images is represented two пикселами the monitor. Value of scale which Photoshop shows in status bar and in window title testifies to it.

5. Click tool Zoom (Scale) some more time. Now the bitmap image mosaic structure became absolutely obvious. One пиксел images is transferred by a considerable quantity пикселов the monitor.

6. Again install scale 1:1, pushing button Actual Pixels (the Real size).

7. Click on the image tool Zoom (Scale) at a pushed button. Now a situation reverse: three пиксела images are transferred by two пикселами the monitor (scale of 66,7 %). Colors пикселов are averaged, and fine details do not become visible.

The majority of devices of the press from printers to typographical machines are raster. Nevertheless, technologies of rasterization which in them are used, can differ rather essentially. We still explicitly consider rasterization mechanisms in various output devices in chapter 10. Resolution of the device of the press also plays it a key role. While only we note that for the press on the printer the image resolution should be, as a rule, in limits from 120 to 150 ppi, and for a qualitative letterpress printing — 250—300 ppi.

The insufficient image resolution on the press leads to the same results, as on the monitor

The review scale does not influence in any way image sizes. If needed their changes it is necessary to address to the special command Image Size (Image size) which are in menu Image (Image).

1. Open a file Pyramid.jpg.

2. Select command Image Size (Image size) of menu Image (Image).

3. In appeared dialog box Image Size (Image size) (fig. 2.1) in the field Pixel Dimensions (the Size in пикселах) image sizes in пикселах are by default displayed. In our case it will be 425х285 пикселов. Near to title of area Pixel Dimensions (the Size in пикселах) is specified volume disk and a random access memory, occupied with the image — 10 Kbytes.

4. In group of parameters Document Size (the Size of the document) you see geometrical image sizes which it will have at a print on the printer. More low, in the field Resolution (Resolution), is specified resolution in пикселах in inch.

5. Without touching while values in fields, close a window click on button Cancel to (Cancel).

Dialog box Image Size (Image size) allows not only to learn, but also to change document parameters. Two variants, defined by a state of flag Resample Image (to Scale the image) are thus possible

• If the flag is installed, the size and image resolution vary independently. If to you it is more convenient, can regulate "absolute" image size in пикселах. It forces Photoshop to delete from the image пикселы or to add to it new. Colors added пикселов are calculated by means of interpolation.
• If the flag is dropped, "the absolute" image size in пикселах is fixed. Change of the size leads to resolution change and on the contrary. In essence, thus with the image it is not produced any action but only new values of the sizes and resolution are remembered.

The list allocated to the right of flag Resample Image (to Scale the image), contains the list of accessible algorithms of interpolation. The slowest and exact algorithm - Bicubic (Bicubic) We recommend to use this method always. Superfluous fractions of a second of waiting completely justify high quality of the scaled image.

Flag Constrain Proportion (to Save proportions) fixes a ratio of the sides of the image. When it is installed, to enter length only one side of the image enough, and the second will be calculated automatically

Scaling changes an amount of the graphic information which contains the image. For experiment the document Pyramid.jpg, and its copy is used not.

1. Create a copy of the current document. It is easy for making by means of command Duplicate to (Double) menu Image (Image).

2. Open dialog box Image Size (Image size). In the field Resolution (Resolution) enter number 96. Pay attention that the field of resolution and a field of the sizes in пикселах are interconnected. At resolution reduction values in fields Width and Height (Height) (136х91) and value of a file of the document decreased (Width) also. Pay attention that geometrical image sizes thus do not change. Push the button APPRX. you received a copy of the document with the under resolution.

3. 3. Resolution reduction reduced number пикселов images, and accordingly, and an amount of the graphic information containing in it. These пикселы are lost for ever, them not to return any more resolution increase. Again open dialog box Image Size (Image size), increase resolution to initiating and close a window pushing the button APPRX.

4. Compare the original and an image copy (fig. 2.2). The scaled image looks unsharp, deprived of details. Пикселы which were imported to the image by interpolation, do not refine its quality. For this reason cases when increase of resolution or magnification of the sizes of images really is required, it is possible to enumerate on fingers.

5. Close open documents, without saving them.