Pixel principle

Several other concepts, such as voxel, texture element and surface element, are all derived from the concept of pixel and used in other computer graphics and image processing applications.

Dots are sometimes used to represent pixels, especially for computer marketers, and most of the time they are represented by DPI (dots per inch).

We can say that pixels in a visible image (such as a printed page) are pixels represented by electronic signals, pixels represented by numbers, pixels on a display, or pixels in a digital camera (photosensitive element). Many other examples can be added to this list, and there will be some more accurate synonyms according to the context, such as pixel, sampling point, byte, bit, point, point, superset, triple, edge set, window and so on.

We can also discuss pixels in the abstract, especially when we use pixels as a measure of resolution (also called resolution, the same below), such as 2400 pixels per inch or 640 pixels per line. Although there is a more specific definition of resolution, the number of pixels in an image is sometimes called image resolution. The more pixels used to represent an image, the closer the result is to the original image.

Pixels can be represented by a number, such as a "0.3 megapixel" digital camera with a rating of 300,000 pixels; It can also be represented by a pair of numbers, such as "640x480 display", which means 640 pixels horizontally and 480 pixels vertically (just like VGA display), so its total number is 640x480 = 307,200 pixels.

The color sampling points of digital images (such as JPG files commonly used in web pages) are also called pixels. Due to the different types of computer monitors, these may not correspond to some areas of screen pixels one by one. In this obviously different area, the points in the image file are closer to the texture elements.

In computer programming, an image composed of pixels is called a bitmap or raster image. The word raster comes from analog TV technology, and bitmap images can be used to encode digital images and some types of computer-generated art. Simply put, a pixel is the value of a point in an image. Points are drawn into lines, and lines are drawn into surfaces. Of course, the clarity of a picture is not only determined by pixels. The pixel mentioned by the camera actually refers to the maximum pixel, which is the unit of resolution. This pixel value is only the effective maximum resolution supported by the camera.

300,000 640×480

500,000 800×600

806,5438+0024× 768 5 inches (3.5× 5 inches)

1.3 million 1.280× 960 6 inches (4× 6 inches)

2 million 1600× 12008 inches (6× 8 inches) 5 inches (3.5× 5 inches)

365438+ million 2048×153610 "(8×10 inch) 7" (5× 7 inch)

4.3 million 2400× 1800 12 inch (10× 12 inch) 8 inch (6× 8 inch)

5 million 2560× 1920 12 inch (10× 12 inch) 8 inch (6× 8 inch)

6 million 3,000× 200014 "(114 inch)10" (8×10 inch).

8 million 3264× 248816 "(12×16 inch)10" (8×10 inch).

1 1 4080× 2720 20 inches (16× 20 inches) 12 inches (10× 12 inches).

144,536× 3024 24 inches (18× 24 inches) 14 inches (1 1× 14 inches).

All the above are approximate sizes. When the picture size is in pixels, we need to specify its fixed resolution in order to convert the picture size into the actual size in reality. For example, the resolution of the most commonly used pictures in web pages is 72, that is, 7265438 pixels per inch +0 inch equals 2.54 cm, then it can be concluded that each cm equals 28 pixels through conversion; Another example is a picture with the length of 15x 15cm, which is equal to the length of 420*420 pixels.

Dots per inch DPI

LPI line per inch

PPI pixels per inch Because the resolution of most computer monitors can be adjusted by the computer's operating system, the pixel resolution of the display may not be an absolute measure. Modern liquid crystal displays have original resolution by design, which represents the perfect match between pixels and triplets. Cathode-ray tubes also use red, green and blue fluorescent triplets, but they do not coincide with image pixels, so they cannot be compared with pixels.

For this display, the original resolution can produce the finest image. But because the user can adjust the resolution, the display must be able to display other resolutions. Non-original resolution must be achieved by fitting resampling on the LCD screen, and interpolation algorithm should be used. This often makes the screen look broken or blurred.

For example, the display with the original resolution of1280x1024 looks the best. A pixel represented by several physical triplets can also display 800×600, but it may not be able to display 1600× 65434 completely. Pixels can be rectangular or square. There is a number called aspect ratio, which means there are many pixels.

For example, the aspect ratio of 1.25: 1 means that the width of each pixel is 1.25 times its height. Pixels on computer monitors are usually square, but pixels used for digital images have rectangular aspect ratios, such as those used in PAL and NTSC, CCIR 60 1 digital image standards and corresponding widescreen formats. Each pixel of a monochrome image has its own gray level. 0 usually means black, and the maximum value usually means white.

For example, in an 8-bit image, the maximum unsigned number is 255, so this is a white value. In a color image, each pixel can be represented by its hue, saturation and brightness, but it is usually represented by the intensity of red, green and blue (see RGB). The number of different colors that a pixel can express depends on the number of bits per pixel (BPP). This maximum number can be obtained by taking the square of color depth.

For example, common values are:

8 bpp[2^8=256； (256 colors)];

16 bpp[2^ 16=65536； (65536 colors, called high color)];

24 bpp[2^24= 167772 16； (16,777,216 color, called true color)];

48 bpp[2^48=28 14749767 10656； (28 1, 474, 976, 7 10, 656 colors are used by many professional scanners).

Graphics of 256 colors or less are usually stored in a block or plane format in a video memory, where each pixel in the video memory is an index value of a color array called a palette. Therefore, these patterns are sometimes called index patterns.

Although there are only 256 colors at a time, these 256 colors are selected from a much larger palette, usually 16 megacolors. You can get the animation effect by changing the color values in the palette. 95(windows95 and 98(windows98' s logos is probably the most famous example of this kind of animation.

For depths exceeding 8 bits, these numbers are the sum of three components (red, green and blue). The depth of 16 is usually divided into 5 bits of red, 5 bits of blue and 6 bits of green (eyes are more sensitive to green).

The depth of 24 bits is usually 8 bits per component. In some systems, 32-bit depth is also optional: this means that a 24-bit pixel has 8 extra numbers to describe transparency. In the old system, 4bpp( 16 color) is also very common. When an image file is displayed on the screen, the number of bits per pixel may be different for raster text and display. Some raster image file formats have greater color depth than others.

For example, the maximum depth of GIF format is 8 bits, while TIFF files can handle 48-bit pixels. No display can display 48-bit colors, so this depth is usually used for special professional applications, such as film scanners and printers. This file is drawn at a depth of 24 bits on the screen. Pixels are only one of the conditions that determine clarity. A camera can use a resolution of 2048× 1536 pixels, which is usually called "31million pixels" (2048×1536 = 3145728). For different reasons, many monitors and image acquisition systems cannot display or perceive different color channels at the same point. This problem is usually solved by multiple sub-pixels, and each sub-pixel handles one color channel.

For example, LCD displays usually divide each pixel into three sub-pixels in the horizontal direction. Most LED displays decompose each pixel into 4 sub-pixels; One is red, two are green and one is blue. Most digital camera sensors also use sub-pixels and are realized by color filters. (CRT monitors also use red, green and blue fluorescent dots, but they are not aligned with image pixels, so they cannot be called sub-pixels).

For systems with sub-pixels, there are two different processing methods: sub-pixels can be ignored and regarded as the smallest accessible image element, or sub-pixels can be included in rendering calculation, which requires more analysis and processing time, but in some cases, better images can be provided. The latter method is used to improve the appearance resolution of color display.

This technique, called sub-pixel rendering, uses pixel geometry to manipulate sub-pixels individually, which is most effective for flat panel displays with original resolution (because the pixel geometry of such displays is usually fixed and known). This is a form of anti-aliasing, mainly used to improve the display of text. Microsoft's ClearType, which can be used on Windows XP, is an example of this technology. Megapixels are one million pixels, which are usually used to represent the resolution of digital cameras.

Digital cameras use photosensitive electronic devices, or coupled charge devices (CCD) or CMOS sensors to record the gray level of each pixel. In most digital cameras, CCD adopts a certain arrangement of color filters, and there are three areas of red, green and blue in Bayer filter module, so that photosensitive pixels can record the gray level of a single primary color.

The camera interpolates the color information of adjacent pixels, a process called demosaicing, and then creates the final image. In this way, the final color resolution of an x-megapixel image in a digital camera may be only a quarter of that of the same image in a scanner. In this way, the images of blue or red objects tend to be more blurred than those of gray objects. Green objects seem to be less blurred because green is assigned more pixels (because eyes are sensitive to green). See [1] for a detailed discussion. As a new development, Foveon X3 CCD uses a three-layer image sensor to detect the red, green and blue intensities of each pixel. This structure eliminates the need for spelling, thus eliminating related image aliasing, such as color blur at high contrast edges. First of all, it should be clear that the actual pixel values of digital photos are different from those of sensors. Take a general sensor as an example, each pixel has a photodiode, which represents a pixel in the photo.

For example, a 5-megapixel digital camera, whose sensor can output an image with a resolution of 2560 x 1920-in fact, this value is only equivalent to 4.9 million effective pixels. Other pixels around the effective pixel are responsible for other tasks, such as deciding "what is black". Many times, not all pixels on the sensor can be used.

Sony F505V is one of the classic cases. The sensor of Sony F505V has 3.34 million pixels, but at most it can only output 1, 856 x 1, 392, which is 2.6 million pixels. The reason is that Sony put a new sensor in the old digital camera, which is bigger than the old one, resulting in the sensor being too big, and the original lens can't completely cover every pixel in the sensor.

Therefore, the digital camera uses the principle that the pixel value of the sensor is greater than the effective pixel value to output digital pictures. In today's market, which is constantly pursuing high pixels, digital camera manufacturers often target sensor pixels with higher values in advertisements, rather than effective pixels that reflect the actual imaging clarity. Generally speaking, each pixel in different positions in the sensor constitutes each pixel in the picture.

For example, a photo with 5 million pixels is obtained by measuring and processing the light entering the shutter with 5 million pixels in the sensor (other pixels except the effective pixels are only responsible for calculation). But sometimes we can see such a digital camera: it only has 3 million pixels, but it can output 6 million pixels of photos! In fact, there is nothing false here, but the camera calculates and interpolates on the basis of the sensor's 3 million pixel measurement to increase the number of pixels in the photo.

When photographers take photos in JPEG format, the quality of this "built-in magnification" will be better than that of our computer, because the "built-in magnification" is completed before the pictures are compressed into JPEG format. Photographers who have experience in digital photo processing know that enlarging JPEG images in a computer will make the picture quick, delicate and smooth.

Although the image quality of digital camera interpolation is better than the normal output of sensor pixels, the file size of the interpolated image is much larger than the normal output image (for example, 3 million sensor pixels are interpolated into 6 million pixels, and the image finally input into the memory card is 6 million pixels). Therefore, the high pixels obtained by interpolation do not seem to have much merit. In fact, using interpolation is just like using digital zoom-it can't create details that the original pixels can't record. Pixel is the most important index to measure a digital camera.

Pixel refers to the resolution of a digital camera.

Depending on the number of photosensitive elements on the photoelectric sensor in the camera, one photosensitive element corresponds to one pixel. Therefore, the larger the pixel, the more photosensitive elements, and the greater the corresponding cost. The image quality of a digital camera is partly determined by pixels. When they are larger than a certain size, it is meaningless to simply compare them with pixels. The pixels of mainstream SLR digital cameras are around1000000, but 5 million pixels are enough for ordinary photography and home use, because the resolution of the display we use is limited, generally between 1024×768 and 1920× 10. The imaging quality mainly depends on the size and quality of camera lens and photosensitive element.

The larger the pixel, the higher the resolution of the photo and the larger the printable size. But the early digital cameras were all below 6.5438+0 million pixels. Since the second half of 1999, 2-megapixel products have gradually become the mainstream of the market. (Mobile phones are generally 2 million pixels, and ordinary digital cameras are generally above 3 million pixels. )

At present, the development trend of digital cameras, pixels are like the main frequency of a PC's CPU, and there is an increasing momentum. In fact, from the perspective of market classification, for popular products, considering the factor of cost performance, the bigger the pixel, the better. After all, a 2-megapixel product can meet most applications of ordinary consumers.

For this reason, while most manufacturers pursue high-end digital cameras with high pixels, the largest output at present is still the popular mega-pixel products. Top-level special cameras have products with more than 1 100 million pixels. In addition, it is worth consumers' attention that the current digital camera products are nominally divided into CCD pixels and software-optimized pixels, and the latter is much higher than the former. For example, a brand of Volkswagen digital camera has 2.3 million CCD pixels, while the pixels after software optimization can reach 3.3 million.

The pixels of a digital camera are divided into the largest pixel and the effective pixel.

maximum pixels

The English name is Maximum Pixels, and the so-called maximum pixel is obtained by interpolation. Interpolation operation through the DSP chip inside the digital camera, when the image needs to be enlarged, the pixels that need to be added after the image is enlarged are added by using the nearest neighbor interpolation, linear interpolation and other operation methods. The image quality obtained after interpolation operation can't be compared with the real photosensitive image.

Some merchants in the market will mark "XXX pixels can be realized by hardware interpolation", which is the same principle, but from the perspective of image quality and sensitivity, photos taken with the largest pixel are not as clear as those taken with effective pixels.

The largest pixel also refers directly to the pixel of CCD/CMOS photosensitive device. In order to increase sales, some businesses only promote the maximum pixel of digital cameras. When setting the picture resolution of digital cameras, they do take pictures with the highest pixel resolution. However, users should be aware that this is the value obtained through internal calculation of digital camera, and the loss of image quality will be very obvious when printing pictures.

effective pixels

Effective number of pixels is called effective number of pixels in English. Different from the largest pixel, the effective pixel number refers to the pixel value that really participates in photosensitive imaging. The value of the highest pixel is the real pixel of the photosensitive device, and this data usually includes the non-imaging part of the photosensitive device, while the effective pixel is the value converted under the zoom magnification of the lens. Take Minolta's DiMAGE7 as an example, its CCD pixel is 5.24 million (5.24 megapixels). Because some CCDs do not participate in imaging, the effective pixel is only 4.9 million.

Digital pictures are generally stored in pixels, and each pixel is the smallest unit in digital pictures. The larger the pixel, the larger the area of the picture. To increase the area of a picture, if no more light enters the photosensitive device, the only way is to increase the area of pixels, which may affect the sharpness and clarity of the picture. Therefore, under the condition of constant pixel area, the digital camera can get the largest picture pixel, which is the effective pixel.

When users buy digital cameras, they usually see the merchants promoting "the maximum pixel reaches XXX" and "the effective pixel reaches XXX", so how should users choose? When choosing a digital camera, we should pay attention to the effective pixel number of the digital camera, and the value of the effective pixel number is the key to determine the picture quality. The function of TV pixel in TV system is:

(1) determines the image clarity. The smaller the pixel division, the more total pixels in the picture, and the clearer the image.

(2) It is convenient for TV transmission of images. Image information can be taken out point by point by scanning and converted into transmittable electrical signals.

(3) It is convenient for TV imaging. No matter what form of imaging is adopted, the image points can be recovered point by point by scanning. Pixels are actually made up of many points. The "pixel painting" we are talking about here is not a dot matrix image corresponding to a vector diagram, but an icon-style image. This style of image emphasizes clear outline and bright color, and the shape of pixel map is often cartoon, so it is loved by many friends. The method of making pixel map hardly uses aliasing to draw smooth lines, so it is often used. Gif format, and pictures often appear in dynamic form. However, due to its special production process, it is difficult to guarantee the style if the size of the picture is changed at will.

Pixel painting is widely used, from the picture of FC home red and white machine played as a child to GBA handheld machine today; From black and white mobile phone pictures to today's full-color handheld computers; Even the computer we face every day is full of pixel icons of various software. Nowadays, pixel painting has become an art, which deeply shocks you and me.