In ancient times, people expressed their thoughts and things around them through painting and sculpture, and preserved them for future generations. Later, cameras appeared and people used this more convenient tool to record the real world. Although photos are easier to obtain than paintings and sculptures, they cannot be preserved for a long time. With the rapid development of computer technology, especially the development of multimedia technology since 1990s, the computer has become the most advanced image processing tool, and the image processing technology is becoming more and more mature.
Because all the information processed by a computer is digital, it can only process digital images. Users can shoot with a digital camera, scan with a scanner or download digital images through the network, and then edit them with image processing software.
At present, there are many image processing software, and Photoshop is one of the most popular software.
Second, the basic concept of image processing
The essence of using Photoshop is to make and process various image files, so understanding the characteristics of various image files and some basic concepts of images is the premise of learning Photoshop software well.
1. bitmap
In a computer, images are digitally recorded, processed and saved. According to the different description principles of processed images, they are generally divided into two categories, namely vector images and bitmaps.
Bitmap images, also known as dot matrix images, are composed of many points, which are called "pixels". Bitmap images can accurately record the subtle levels of image colors, so the changes of colors and tones are very rich. However, this image file has a large capacity and is easily distorted when the image is rotated and scaled.
2. Vector diagram
Vector diagram, also known as vector image, is some straight lines and curves defined by mathematical formulas, and images are described according to their geometric characteristics. Its file capacity is small, it is easy to scale and rotate at will, and the graphics will not be distorted. However, vector graphics can't describe the image details with rich colors, and the drawn graphics are not very realistic, and it is not easy to exchange files between different softwares.
3. Pixels
Pixel, also known as grid, is the most basic unit of image in Photoshop. An image consists of many pixels, each of which has a specific position and color value, and the pixels are arranged in rows and columns. The more pixels per unit area, the more details can be displayed, and the higher the image quality.
4. Image form
According to the different ways of recording image information and compressing image data, image files can be divided into many formats, and each format file has a corresponding extension. Photoshop can handle image files in most formats. Common image file formats are:
(1)PSD format and PDD format are the default image file formats of Photoshop software. Using PSD format, every tiny part of image data, such as layers, masks and channels, can be saved. Although compression technology is used to save PSD files, the file capacity is still very large. However, since PSD format and PDD format will not cause any data loss, it is best to save the image in this file format when editing.
(2)JPEG format JPEG format is a lossy compressed file format with high compression rate. JPEG format has a large compression ratio and a small file, but some image data will be lost when storing the file.
(3)GIF format GIF format images are relatively small, forming a compressed 8-bit image file, so it is much faster to transmit GIF format files on the network than other formats. However, this format can only support up to 256 colors.
(4)TIF format and TIFF format TIF or TIFF format are called label image format files, which are standard image formats in the printing industry and have strong versatility. TIF or TIFF, like PSD format, supports 24-bit channels, layers and paths, and is widely used for image data exchange between programs and computer platforms.
(5)BMP format BMP format is the standard image file format in Windows, and the image in BMP format is lossless compressed, so the data will not be lost after image compression.
(6)EPS format EPS format is the image storage format of Illustrator, and Photoshop also supports files in this format. The advantage of EPS format is that it can be previewed at low resolution in typesetting software, edited and typeset for inserted files, and output at high resolution when printing. This format supports RGB, index color, grayscale and bitmap color modes, but does not support the Alpha channel.
(7)PNG Format PNG format is a network image format developed by Netscape for the Internet. This format supports the Alpha channel in RGB and grayscale color modes. Unlike images in GIF format, it can save 24-bit true color images. However, because not all browsers support this format, it is not as widely used as GIF and JPEG formats.
Step 5 solve
Resolution is an important concept in images. Resolution refers to the number of pixels per unit length. Resolution is divided into image resolution, device resolution, screen resolution and output resolution.
(1) Image Resolution Image resolution refers to the number of pixels contained in an image per inch, and the unit is dpi (pixels/inch). Under the same printing size, the high-resolution image contains more pixels than the low-resolution image, so the image is clearer and more delicate, and can show more detailed and fine color changes.
The higher the image resolution, the larger the image file capacity, and the more memory and CPU time required for processing. Therefore, the final use of the image should be fully considered in order to set the appropriate resolution for the image. If the image is used for screen display, just set the resolution to the display resolution (72 dpi or 96 dpi); If the image is used for print output, it needs to meet the requirements of the printer or output device; If the image is used for printing, the resolution of the image cannot be lower than 300 dpi.
(2) Equipment resolution Equipment resolution refers to the number of points or pixels output per unit length. For example, the display screen is composed of tiny phosphor luminescent units, and the commonly used display resolution is 800×600 or 1024×768, that is, the number of pixels in the horizontal and vertical directions. Printer resolution refers to the number of ink dots per inch when the printer outputs images.
6. Color depth
Color depth is used to measure the number of colors in an image, and the unit is bits. Commonly used color depths are 1 bit, 8-bit, 24-bit and 32-bit. A larger color depth means that the image has more available colors and more accurate color representation. For example, an image with a color depth of 8 bits contains 28 colors or 256 grayscales, and the color of each pixel can be one of the 256 colors.
7. Color mode of image
In the computer, in order to express the original color of things, different color schemes are needed. These color schemes are often referred to as image color modes. In Photoshop, the commonly used color modes are bitmap mode, RGB mode, CMYK mode, Lab mode, index color mode and gray mode.
(1) Gray Mode This mode uses a gray value of 0 ~ 255 to represent the color of pixels in the image, which can make the transition of the image smooth and delicate. It is also a transitional mode to convert RGB mode into bitmap mode. However, when RGB mode is converted to gray mode, the color will disappear and cannot be recovered.
(2) In RGB mode, the colors in visible light can be composed of red, green and blue in a certain proportion, which is called RGB mode. You can adjust the color of the image by changing the mixing ratio of the three basic colors. Usually, the R, G and B values of a pixel are stored in one byte. Therefore, a pixel contains 24 bits, and the image in RGB mode can have more than 654.38+06.7 million colors. RGB mode is the most commonly used color mode in Photoshop.
(3)cmYK mode This mode is formed by mixing four basic colors, namely, C (cyan), M (magenta), Y (yellow) and K (black) in a certain proportion, and the color of the image is formed by the principle of subtraction. This mode is mainly suitable for printing and printer output.
(4)Lab mode This color mode is defined by the International Commission on Illumination. It uses a brightness component L and two color components A and B to represent the color of the image, where component A represents the component from green to red and component B represents the component from blue to yellow.
(5) When saving an image in RGB mode or CMYK mode in indexed color mode, all colors of the image can be displayed, but the images saved in these two modes occupy too much storage space. So people made a 256-color index table, so that the color of each pixel can be represented by one byte, and the occupied space is much smaller than that of RGB mode represented by three bytes. However, after converting RGB mode to index color mode, color fading is easy to occur.