One article understands two ways of storing digital image data

Digital image processing is a method and technique for removing noise, enhancing, restoring, segmenting, and extracting features from a computer through a computer. In the computer, according to the color and the gray level, the image can be regarded as four basic types of binary image, grayscale image, index image, and true color RGB image. At present, most image processing software supports these four types of images.

(1) Binary image:

The two-dimensional matrix of a binary image consists of only two values ​​of 0 and 1, with "0" representing black and "1" representing white. Because each pixel (each element in the matrix) has only two values ​​of 0 and 1, the data type of the binary image in the computer is usually 1 binary bit. Binary images are commonly used for the storage of scanned text (OCR) and mask images.

2) Grayscale image:

Grayscale image matrix elements typically have a range of [0,255]. So its data type is generally an 8-bit unsigned integer (int8), which is what people often refer to as 256-grayscale images. “0” means pure black, “255” means pure white, and the middle digit from small to large means a transition from black to white. In some software, the grayscale image can also be represented by a double data type with a range of pixels [0, 1]. 0 represents black, 1 represents white, and a decimal between 0 and 1 means different values. Gray scale. Binary images can be seen as a special case of grayscale images.

(3) Index image:

The file structure of the index image is relatively complex. In addition to the two-dimensional matrix that stores the image, it also includes a two-dimensional array called the color index matrix MAP. The size of the MAP is determined by the matrix element value field in which the image is stored. For example, if the value of the matrix element is [0, 255], the size of the MAP matrix is ​​256×3, which is expressed by MAP=[RGB]. The three elements of each row in the MAP specify the red, green, and blue color values ​​of the corresponding color of the row, and each row of the MAP corresponds to a gray value of the image matrix pixel. For example, if a certain pixel has a gray value of 64, then The pixel has a mapping relationship with the 64th row in the MAP, and the actual color of the pixel on the screen is determined by the [RGB] combination of the 64th row. That is, when the image is displayed on the screen, the color of each pixel is obtained by retrieving the color index matrix MAP from the gray value of the pixel stored in the matrix as an index. The data type of the index image is generally 8-bit unsigned integer (int8), and the size of the corresponding index matrix MAP is 256×3, so the general index image can only display 256 colors at the same time, but by changing the index matrix, the type of color can be adjusted. The data type of the index image can also be double-precision. Index images are generally used to store images that require relatively simple colors. For example, Windows uses more simple colors to store index images. If the colors of the images are more complex, RGB true color images are used.

(4) RGB color image:

Both RGB and indexed images can be used to represent color images. Like the index image, it uses the combination of the three primary colors red (R), green (G), and blue (B) to represent the color of each pixel. However, unlike the index image, the color value of each pixel of the RGB image (represented by the three primary colors of RGB) is directly stored in the image matrix. Since the color of each pixel needs to be represented by three components of R, G, and B, M, N denotes the number of rows and columns of the image, and three M x N two-dimensional matrices respectively represent the three color components R, G, and B of each pixel. The data type of an RGB image is generally an 8-bit unsigned integer. It is usually used to represent and store true color images. Of course, it can also store grayscale images.

There are two ways to store digital image data:

Bitmap storage

Vector Store (Vector)

We usually describe digital images in terms of image resolution (ie, pixel points) and number of colors. For example, a digital picture with a resolution of 640*480, 16-bit color consists of 307,200 (=640*480) prime points of 2^16=65536 colors.

Bitmap: The bitmap method is to convert each pixel of the image into a data.

When the image is monochrome (black and white only), the data of 8 pixels occupies only one byte (one byte is 8 binary digits, and 1 binary number stores pixel points); 16 colors (distinguishable from The image of the previous "16-bit color" is stored with one byte for every two pixel points; each pixel of the 256-color image is stored with one byte. In this way, the image planes of various color modes can be accurately described. The bitmap image compensates for the defects of the vector image. It can produce images with rich color and tonal changes. It can realistically represent the scene in the natural world. It can also easily exchange files between different softwares. This is the bitmap image. The disadvantages; and its disadvantage is that it can not create a true 3D image, and the image zoom and rotation will produce distortion, while the file is larger, the memory and hard disk space capacity is also higher. The bitmap method is to convert each pixel of the image into a data. If it is recorded with 1-bit data, it can only represent 2 colors (2^1=2); if it is recorded with 8 bits, it can show 256 colors or tones (2^8=256), so it is used The more bit elements there are, the more colors that can be represented. Usually we use 16 colors, 256 colors, 16-bit enhancement, and 24-bit color. The so-called true color means that the 24 bit (2^24) bitmap storage mode is suitable for complex images and real photos. However, as the resolution and number of colors increase, the disk space occupied by the image is quite large; in addition, since the image must be blurred and distorted in the process of enlarging the image, the pixels of the enlarged image actually change. Become a pixel "square". The images acquired with a digital camera and scanner are all bitmaps.

Vector (Vector): A vector image stores the outline of image information, not every pixel of the image. For example, a circular pattern only needs to store the coordinate position and radius length of the center of the circle, as well as the length of the edges and the radius of the circle, and the colors of the edges and the interior of the circle. The disadvantage of this storage method is that it often takes a lot of time to do some complex analysis and calculation work, the display speed of the image is slower; but the image scaling will not be distorted; the storage space of the image is also much smaller. Therefore, vector graphics are more suitable for storing various charts and projects.

RGB (0,0,0) --------> (255,255,255)