How to correctly choose the compression value of JPEG for Greeting Cards
Aug 02, 2021
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How to correctly choose the compression value of JPEG for Greeting Cards
The Israeli technology company ICTV has developed a method for optimizing JPEG compression-JPEGmini, which can reduce the file size by 50%-80% without causing visually identifiable loss. In an interview with the developer of the technology, he mentioned that "sometimes you choose a higher image quality in Photoshop, but it actually causes a loss of image quality."
In an article called "JPEG Compression Quality from Quantization Tables", we saw this statement:
"A lot of people don’t know that there is a trick in Photoshop’s method of defining image quality. Image quality level 6 is the last level to apply chroma sampling. At level 7 and higher, Photoshop does not perform chroma sampling. Since the color information is doubled from level 6 to level 7, the file size will increase significantly.
"However, Adobe seems to be trying to establish a certain relationship between the image quality level and the file volume. Therefore, Adobe compresses the 7-quality images higher. This means that the 7-quality image is actually better than the 6-quality image. The picture quality of the first-class quality is worse (at least for the brightness details).
"The only advantage of 7-level quality images is that the chroma details are not sampled. Therefore, 7-level quality may have some advantages in terms of color information, but this advantage will be offset by a higher compression ratio."
In the field of digital image processing, chroma sampling refers to the use of lower resolution than brightness information to represent color (chroma) information when representing images. When performing digital sampling on analog component video or YUV signals, chrominance sampling is generally used. Since the human eye is less sensitive to chroma than to brightness, the chroma component of the image does not need to have the same clarity as the brightness component, so many video systems perform lower (relative brightness channel) clarity on the color difference channel (For example, sampling frequency) sampling. This reduces the total bandwidth of the video signal without significantly reducing the picture quality. The chrominance value lost due to sampling is replaced by the interpolation value or the previous chrominance value. --Wikipedia
Regarding chroma sampling in Photoshop, the original text explains:
"In Photoshop, the software will decide whether to perform chroma sampling according to the image quality level selected when saving the image. Photoshop CS2 uses 1×1 sampling (no sampling) for levels 7-12; 2×2 for levels 0-6 ——Perform chroma sampling in both the horizontal and vertical directions. It should be noted that such chroma sampling will lose a lot of information compared with most digital cameras (except for some low-end cameras). .
"Photoshop only provides 2×2 or 1×1 (no sampling) chroma sampling. Most digital cameras use 2×1 sampling. Therefore, you must choose or double the horizontal color resolution (level 7 or more) , Or lose the color resolution in the vertical direction (level 0-6)."
So what conclusion can we draw from it? Simply put, when saving JPEG format files, do not use 7-level image quality. If you need a smaller image volume, choose level 6; if you choose better image quality, use level 8 or higher (even level 12 will still compress the image).