Israeli technology company ICTV developed a method to optimize JPEG compression, JPEGmini, which can reduce the size of the file by 50%-80% without causing loss of visual recognition. In an interview with the developer of the technology, he mentioned that "sometimes you choose a higher image quality in Photoshop, but in essence it causes a loss of quality."
In an article entitled "JPEG Compression Quality from Quantization Tables," we saw this statement:
"A lot of people don't know that there is a trick in the way Photoshop defines image quality. Level 6 image quality is the last level to apply color sampling. At level 7 and higher, Photoshop does not perform color sampling. Because the color information from level 6 to level 7 is doubled, the file size will increase significantly.
“However, Adobe seems to be trying to establish a relationship between the image quality level and the file size. Therefore, Adobe has a higher level of compression for image quality of level 7. This means that: 7-level quality images are actually better than 6 Quality quality is worse (at least for brightness details).
"The only advantage of a quality image at level 7 is that the chrominance details are not sampled. Therefore, there may be some advantages in quality level 7 from the color information, but this advantage is offset by a higher compression ratio."
In the field of digital image processing, chroma sampling refers to expressing color (chrominance) information using a resolution with lower luminance information when representing an image. Chromaticity sampling is typically used when digitally sampling analog component video or YUV signals. Since the human eye is less sensitive to chrominance than illuminance, the chroma component of the image does not need to have the same resolution as the luminance component, so many video systems perform lower (relative luminance channel) resolution on the chroma channel. (eg, sampling frequency) sampling. This reduces the overall bandwidth of the video signal without significantly degrading the picture quality. Chromaticity values ​​lost due to sampling are replaced with interpolated values, or previous chroma values. --Wikipedia
Regarding the chromaticity sampling in Photoshop, the original explanation is:
"In Photoshop, the software decides whether or not to perform color sampling based on the quality level of the image selected when saving the image. Photoshop CS2 uses 1x1 sampling (no sampling) for grades 7-12; use 2x2 for levels 0-6. - Chroma sampling in both horizontal and vertical directions.Note that such chroma sampling will lose a lot of information compared to the chroma sampling of most digital cameras (except some low-end cameras) .
"Photoshop only provides 2x2 or 1x1 (no sampling) chroma sampling. Most digital cameras use 2x1 sampling. Therefore, you must choose or double the horizontal color resolution (7+) , Or lose the vertical color resolution (level 0-6)."
What conclusions can we draw from? Simply put, do not use 7-level image quality when saving JPEG files. If you need a smaller image size, choose 6 levels; if you choose a better image quality, use 8 or higher levels (even if 12 levels still compress the image).
In an article entitled "JPEG Compression Quality from Quantization Tables," we saw this statement:
"A lot of people don't know that there is a trick in the way Photoshop defines image quality. Level 6 image quality is the last level to apply color sampling. At level 7 and higher, Photoshop does not perform color sampling. Because the color information from level 6 to level 7 is doubled, the file size will increase significantly.
“However, Adobe seems to be trying to establish a relationship between the image quality level and the file size. Therefore, Adobe has a higher level of compression for image quality of level 7. This means that: 7-level quality images are actually better than 6 Quality quality is worse (at least for brightness details).
"The only advantage of a quality image at level 7 is that the chrominance details are not sampled. Therefore, there may be some advantages in quality level 7 from the color information, but this advantage is offset by a higher compression ratio."
In the field of digital image processing, chroma sampling refers to expressing color (chrominance) information using a resolution with lower luminance information when representing an image. Chromaticity sampling is typically used when digitally sampling analog component video or YUV signals. Since the human eye is less sensitive to chrominance than illuminance, the chroma component of the image does not need to have the same resolution as the luminance component, so many video systems perform lower (relative luminance channel) resolution on the chroma channel. (eg, sampling frequency) sampling. This reduces the overall bandwidth of the video signal without significantly degrading the picture quality. Chromaticity values ​​lost due to sampling are replaced with interpolated values, or previous chroma values. --Wikipedia
Regarding the chromaticity sampling in Photoshop, the original explanation is:
"In Photoshop, the software decides whether or not to perform color sampling based on the quality level of the image selected when saving the image. Photoshop CS2 uses 1x1 sampling (no sampling) for grades 7-12; use 2x2 for levels 0-6. - Chroma sampling in both horizontal and vertical directions.Note that such chroma sampling will lose a lot of information compared to the chroma sampling of most digital cameras (except some low-end cameras) .
"Photoshop only provides 2x2 or 1x1 (no sampling) chroma sampling. Most digital cameras use 2x1 sampling. Therefore, you must choose or double the horizontal color resolution (7+) , Or lose the vertical color resolution (level 0-6)."
What conclusions can we draw from? Simply put, do not use 7-level image quality when saving JPEG files. If you need a smaller image size, choose 6 levels; if you choose a better image quality, use 8 or higher levels (even if 12 levels still compress the image).
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