'How can I know the jpeg quality of the read image using graphicsmagick
When I read a jpeg image using Magick::readImages(...) function. How can I know the estimated jpeg quality of the image? I know how to set the quality when I wanna write the image, but it is not relevant to the quality of the original image, so for example: when I read a jpeg image that its quality is 80% and I write it using 90% quality I will get a bigger image than the original one, since the 90% is not 90% out of the original 80%. How can I know the jpeg quality of the read image?
Solution 1:[1]
It is neither impossible nor difficult with GraphicsMagick (or with ImageMagick). Type
gm convert -log %e -debug coder in.jpg junk.ppm
and look in the output for the line
Quality: nn
if the image was created by Independent JPEG Group software or
Quality: nn (approximate)
otherwise.
You can also use
gm identify -verbose in.png
and look at the
Quality: nn
line; however, this doesn't distinguish between exact and approximate qualities.
Solution 2:[2]
It's possible but difficult. The code has to examine the image just like people do. Here's a paper on the subject http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.4.4621&rep=rep1&type=pdf
Solution 3:[3]
If you want to read the JPEG metadata, you can use the following in command-line:
gm identify -format "%[JPEG-Quality]" your-photo.jpg
In C++, it might be using IdentifyImage : https://imagemagick.org/api/MagickCore/identify_8c.html#a91079e7db05c1bad53b2dbb2b01f41ba
That said, as other users said, it's just a number generated by the software that produced the image. As a matter of fact, I can see that GM 1.4 returns nothing on a JPEG without profile information, but GM 1.3.35 does a wild guess with jpeg.c/EstimateJPEGQuality/932/Coder (that returns 100%).
Sources
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Source: Stack Overflow
| Solution | Source |
|---|---|
| Solution 1 | |
| Solution 2 | BenMorel |
| Solution 3 |