Image mode selection and conversion points (on)

Image mode refers to the data type of the image, which is related to a specific color space and output device. Although the image mode can be freely converted in photoshop, due to the different color ranges, channel numbers, and image file sizes of the different modes, some data loss may occur more or less during the conversion. Therefore, in the mode conversion, it is necessary to avoid unnecessary losses to obtain high-quality images.

1 Issues to Consider When Selecting Image Mode

Selecting the image mode depends on the output requirements and design goals. Usually the following issues need to be considered.

1) Image output and input methods

If the image is output in printing mode, it must be converted to "CMYK color" mode; if it is only displayed on the screen, it is used in video, multimedia and web design, and output in "RGB color" or "index color" mode. More; To output single-tone images (usually black and white), select the "grayscale" mode. The input is usually used "RCB color" mode, because the mode has a wider color range and operating space.

2) Editing function

When selecting an image mode, it is necessary to consider the functions that can be used in photoshop. For example, images in the CMYK mode cannot use some filters, and you cannot use free rotation, layer functions, etc. in the bitmap mode. Therefore, RCB mode is generally selected to operate, and after editing is completed, it is converted to the desired image mode for storage.

3) Color range

Different modes have different color ranges. Therefore, when editing, try to select RGB and Lab modes with a wide range of colors to obtain the best image effects.

4) Memory and disk space occupied by files

The file size saved in different modes is different, the grayscale mode file is smaller, and the CMYK mode file is larger. The larger the file is, the more memory is used to process the image and the slower it is. In comparison, RGB mode is the best choice.

2 Image mode conversion

1) Convert to bitmap mode for black and white image output

Bitmaps are called black-and-white images in printing. Each pixel contains only one bit of data and uses the least amount of disk space. During the conversion, the midtone pixels are converted to black and white images in the specified manner, and therefore, a rich-tone image cannot be produced in the bitmap mode.

Only "grayscale" and "multi-channel" images can be converted to bitmap images. After the color image is converted to a grayscale image, the hue and saturation information of each pixel will be deleted, leaving only the luminance information.
The conversion from grayscale mode to bitmap mode requires specifying the output resolution and conversion method. Wherein, if the output resolution is higher than the original resolution of the image, a grayscale interpolation method is used to generate the desired pixel.
Note: When a grayscale image is converted to a bitmap image and then converted to a grayscale image, the effect of the original image cannot be displayed. This is because the information lost after the image conversion cannot be restored; When you create an image, you can create special output effects, such as copper engraving.

2) Convert to grayscale mode for single-tone image output

A grayscale image consists of a series of pixels of different brightness (ie grayscale) levels, each pixel being represented by 8 bits of data and having 256 grey levels. This image has a rich hierarchy of changes, similar to black and white photographs. The pixel value of the grayscale image is usually converted by the dot area rate. For example, when the dot area ratio is 25%, the grayscale value is 256×(1-25%)−1=191.

1 bitmap mode is converted to grayscale mode

Converting the bitmap mode to grayscale mode will bring up a dialog box prompting for the size ratio of the input image, with a range of 1-16. For example, if you enter 2 in the text box, the converted image size will be reduced by half and the number of pixels will also decrease.

After the bitmap mode is converted to the grayscale mode, it is often not seen that there is an effect change, but its nature has changed, and at this time, a halftone pixel can be generated.

2 from a color mode to grayscale mode

If the digitized color image is converted to grayscale mode, the color information in the file will be discarded. The channel palette shows only one black channel. When working with grayscale images, you can drag the grayscale slider labeled K to change the percentage of grayscale. You can use the Brightness/Contrast command to make simple tonal adjustments to the converted grayscale image.

When converting from the RGB mode to the gray mode, in order to avoid the parallelism of the image, the loss of brightness information should be minimized, and in particular, the loss of brightness and darkness should be overcome. In photoshop, if you convert RGB mode to grayscale mode, the conversion algorithm is:

Gray Level=0.30R+0.59G+0.11B

When converting from the CMYK mode to the gray mode, the image must first be converted into the RGB mode by converting the current color separation parameters in photoshop, and then converted into a grayscale image.

Convert from Lab mode to grayscale mode. A single color channel in an RGB or CMYK image simply represents the relative intensity of the color component, and no channel is used to represent the light and dark information of the image. In the process of converting to grayscale mode, the weighted average summation must be carried out to extract the grayscale components. This results in the darkening of the image in the dark part, which causes the blurring of multiple levels, and causes many subtle levels to be lost. This also darkens the overall image tone. This is especially true for manuscripts with many green components. The Lab pattern represents the gamut space composed of all colors in nature, which is the largest gamut space. We can extract the L (luminance) channel information as the grayscale component of the entire image. It not only saves complicated operations, but also maximizes the gamut space where the image is located. The tone is the most complete and truly represents the gray information of the image itself.

Note: Using photoshop's Channel Mixer (Image/Adjust/Channel Mixer), you can remove colors from RGB images and create images with a variety of different grayscale styles. You can also select "Image/Adjust/Decolor" to remove the colors in the RGB image. You can also select a grayscale color value in the "Save as Webpage Format" dialog box; the successful conversion of the grayscale mode depends on the image Large contrasts of darkness, not color contrast. If the image is strongly contrasted and the tone is relatively poor, there will be a lack of layering after conversion.

(to be continued)