Image resolution refers to the number of dots per unit length and reflects the dimensional accuracy of the input pixels, expressed in dots per inch of length. In digitized images, the size of the resolution directly affects the quality of the image. In the process of color image processing, resolution is a very important issue. How to produce high-quality images on the level of existing equipment is a factor that cannot be underestimated. When we face an image with pixels as the basic unit, the first thing to know is that the image we process is digital.
To understand the setting of image resolution, first of all we need to know about computer image files.
Computer images exist in the form of computer files, which are essentially just some digital codes. The biggest difference between it and the so-called (traditional) image is that the computer image is digital, not analog (digital image) It is the result of analog image data), which is a discrete sampling of general images, so it has some concepts that are not in traditional images (such as pixels: on a computer, it is composed of more basic units, which are called "Pixels". Pixels are described point by point, with a clear position and color value. True color images use 24bit (RGB file) or 32bit (CMYK file) data to describe color for each point. There is also the concept of resolution , As already mentioned above, it will not be repeated here).
In addition, when we describe the size of a traditional image, we only need to say its size. Such as 8K, 16K, etc., but when we describe computer images, in addition to describing the size, we also care about the amount of data, usually expressed in bytes. The number of bytes of a computer image file is related to the size, pixel value, resolution and other factors of the image.
The quality of an image is closely related to the image resolution and size. For images of the same size, the higher the resolution, the clearer the image. The main factor that determines the resolution is the number of pixels per unit size, so the number of pixels and the resolution are also related.
Whether it is a printed image, a multimedia image or a webpage image, the resolution and size of the image must be determined before production, so that the image can be edited and processed more efficiently.
In the image processing software Photoshop, whether you change the image resolution, size, or increase or decrease the number of pixels, you need to use the Image Size dialog box (shown in Figure 1) to complete. In the dialog box, you can set the following:
Pixel Dimensions: Pixel values ​​used to display the width (Width) and height (Height) of the image.
Document size (Documet Size): Used to set the width (Width), height (Height) and resolution (Resolution) of the image.
Constrain Proportions: Select this check box to control the ratio of the width and height of the image, that is, the height will also change when the width is changed. When the check box is canceled, it means that the height is independent of the width, that is, changing the value of any item will not affect the other item.
Resample Image; when this check box is not selected, the image pixels are fixed and the image resolution and size can be changed; when this check box is selected, the image resolution and size are changed, the number of image pixels will vary To change this, you need to select an interpolation pixel selection method in the pixel list box of the reset image, that is, the algorithm of inserting pixels in the image when increasing or decreasing the number of pixels.
There is a close relationship between the resolution, size, and number of pixels of an image. For an image with the same resolution, if the size is different, the number of pixels is also different. The larger the size, the larger the saved file. Similarly, increasing the resolution of an image will also make the image file larger. Therefore, modifying the parameters of the first two directly determines the parameters of the third party. When the number of pixels is fixed and the image resolution changes, the image size must change accordingly. Similarly, when the image size changes, the resolution will automatically change in the opposite direction, and there is no sampling problem. Note: The image resolution affects the display size of the image on the screen. If the length and width of the image are unchanged, the resolution is doubled, and the original image will be displayed on the screen twice the actual image. In actual work, it is usually necessary to adjust the image size without changing the resolution, or increase or decrease the resolution by fixing the size. In this case, the number of pixels will change accordingly. When the resolution is fixed and the resolution is increased, the number of pixels in the image must be increased. At this time, the image will be re-sampled to increase or decrease the number of pixels in the image with minimum distortion.
To understand the setting of image resolution, first of all we need to know about computer image files.
Computer images exist in the form of computer files, which are essentially just some digital codes. The biggest difference between it and the so-called (traditional) image is that the computer image is digital, not analog (digital image) It is the result of analog image data), which is a discrete sampling of general images, so it has some concepts that are not in traditional images (such as pixels: on a computer, it is composed of more basic units, which are called "Pixels". Pixels are described point by point, with a clear position and color value. True color images use 24bit (RGB file) or 32bit (CMYK file) data to describe color for each point. There is also the concept of resolution , As already mentioned above, it will not be repeated here).
In addition, when we describe the size of a traditional image, we only need to say its size. Such as 8K, 16K, etc., but when we describe computer images, in addition to describing the size, we also care about the amount of data, usually expressed in bytes. The number of bytes of a computer image file is related to the size, pixel value, resolution and other factors of the image.
The quality of an image is closely related to the image resolution and size. For images of the same size, the higher the resolution, the clearer the image. The main factor that determines the resolution is the number of pixels per unit size, so the number of pixels and the resolution are also related.
Whether it is a printed image, a multimedia image or a webpage image, the resolution and size of the image must be determined before production, so that the image can be edited and processed more efficiently.
In the image processing software Photoshop, whether you change the image resolution, size, or increase or decrease the number of pixels, you need to use the Image Size dialog box (shown in Figure 1) to complete. In the dialog box, you can set the following:
Pixel Dimensions: Pixel values ​​used to display the width (Width) and height (Height) of the image.
Document size (Documet Size): Used to set the width (Width), height (Height) and resolution (Resolution) of the image.
Constrain Proportions: Select this check box to control the ratio of the width and height of the image, that is, the height will also change when the width is changed. When the check box is canceled, it means that the height is independent of the width, that is, changing the value of any item will not affect the other item.
Resample Image; when this check box is not selected, the image pixels are fixed and the image resolution and size can be changed; when this check box is selected, the image resolution and size are changed, the number of image pixels will vary To change this, you need to select an interpolation pixel selection method in the pixel list box of the reset image, that is, the algorithm of inserting pixels in the image when increasing or decreasing the number of pixels.
There is a close relationship between the resolution, size, and number of pixels of an image. For an image with the same resolution, if the size is different, the number of pixels is also different. The larger the size, the larger the saved file. Similarly, increasing the resolution of an image will also make the image file larger. Therefore, modifying the parameters of the first two directly determines the parameters of the third party. When the number of pixels is fixed and the image resolution changes, the image size must change accordingly. Similarly, when the image size changes, the resolution will automatically change in the opposite direction, and there is no sampling problem. Note: The image resolution affects the display size of the image on the screen. If the length and width of the image are unchanged, the resolution is doubled, and the original image will be displayed on the screen twice the actual image. In actual work, it is usually necessary to adjust the image size without changing the resolution, or increase or decrease the resolution by fixing the size. In this case, the number of pixels will change accordingly. When the resolution is fixed and the resolution is increased, the number of pixels in the image must be increased. At this time, the image will be re-sampled to increase or decrease the number of pixels in the image with minimum distortion.
figure 1
Open the Image Size / Auto / Auto Resolution dialog box (as shown in Figure 2). In the Screen text box, the user can set the dot frequency of the output device. In the Quality (Quality) option group, you can set the quality of the printed matter: when set to Draft, the resulting resolution is the same as the dot frequency (not less than 72 pixels per inch); when set to Good, the resulting The resolution is 1.5 times the dot frequency; when set to Best, the resulting resolution is twice the dot frequency. The photoshop software will automatically calculate the resolution required by the user and return to the Image Size dialog box. At this time, the new size and resolution values ​​have changed. The value now displayed by the resolution is the number of nets entered by the user.
figure 2
In digital images, the higher the resolution, the clearer the image, but the larger the resulting file, the higher the memory and CPU processing time required in the work. Therefore, when making images, different resolutions should be used according to different needs. It is important to know that the resolution of the image used is not as high as possible, but the medium in which the image is finally released should be considered. If the produced image is used for computer screen display (such as multimedia or web page image), the image resolution only needs to meet the typical monitor resolution (72ppi or 96ppi). If the image is used for printout, high resolution (150ppi and 300ppi) must be used, and the low resolution image printout will have obvious grain and jagged edges. For the setting of the resolution of the line art scanned image, it is not necessary to make the image resolution higher than the output resolution; for the settings of grayscale and color bitmap resolution, it should not exceed twice the output dot frequency (1.0-2.0 times).
In addition, to obtain the best results when producing a smaller image, you can use the large original picture, and then zoom the image to the page, or use the USM Mask filter after changing the image size. If you want to change the image to a larger size and ensure the quality of the image, it is best to rescan the image at a higher resolution. The image processing often involves the problem of image resolution changes. It is generally unwise to reduce the resolution of the image and then increase it again. This is commonly referred to as sampling, that is, the increase and decrease of pixels. When reducing the resolution, the pixels in the image will be reduced due to deletion; when increasing the resolution, the intermediate interpolation method is used to add information that is not in the original image. In fractional image processing systems, it is often tried to buffer the inconsistency between the two initial pixels and the added pixels, which will result in a somewhat blurred image.
In Photshop software, the Image Size command is different from the Canvas Size command. Using the image size command can only change the image size, not the image appearance. But the canvas size command will not only change the image size but also the image appearance.
How to set an appropriate image resolution to produce a relatively high-quality image is a process of continuous accumulation of experience. Only when you are familiar with user requirements, existing equipment, and back-end printing process requirements can you achieve mastery.
In addition, to obtain the best results when producing a smaller image, you can use the large original picture, and then zoom the image to the page, or use the USM Mask filter after changing the image size. If you want to change the image to a larger size and ensure the quality of the image, it is best to rescan the image at a higher resolution. The image processing often involves the problem of image resolution changes. It is generally unwise to reduce the resolution of the image and then increase it again. This is commonly referred to as sampling, that is, the increase and decrease of pixels. When reducing the resolution, the pixels in the image will be reduced due to deletion; when increasing the resolution, the intermediate interpolation method is used to add information that is not in the original image. In fractional image processing systems, it is often tried to buffer the inconsistency between the two initial pixels and the added pixels, which will result in a somewhat blurred image.
In Photshop software, the Image Size command is different from the Canvas Size command. Using the image size command can only change the image size, not the image appearance. But the canvas size command will not only change the image size but also the image appearance.
How to set an appropriate image resolution to produce a relatively high-quality image is a process of continuous accumulation of experience. Only when you are familiar with user requirements, existing equipment, and back-end printing process requirements can you achieve mastery.
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