Code:
Resulting PDF information in each file is below. Notice that the bitmap of the page has identical resolution, depth, and encoding method.
The data doesn't mean anything. An image and its copy can have identical specs but be very different in quality. The proof is in the seeing. Here's a simple example:
I used imagemagick to deliberately reduce the quality of an image. These are the specs of both images:
Code:
General
Complete name : 150729145544-03-trump-quotes-super-169.jpg
Format : JPEG
File size : 81.1 KiB
Image
Format : JPEG
Width : 1 100 pixels
Height : 619 pixels
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Stream size : 81.1 KiB (100%)
General
Complete name : 150729145544-03-trump-quotes-super-169_q=1.jpg
Format : JPEG
File size : 6.61 KiB
Image
Format : JPEG
Width : 1 100 pixels
Height : 619 pixels
Color space : YUV
Chroma subsampling : 4:2:0
Bit depth : 8 bits
Compression mode : Lossy
Stream size : 6.61 KiB (100%)
The only difference is in the name and size of the file.
Seeing:
I have Ghostscript 9.15 on OS X 10.8.5. It shouldn't really matter which version of gs or what operating system. gs has beed around for decades and doesn't change that much.
First I used your settings:
Code:
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET -dBATCH -sOutputFile="$o" "$i"
That gave inferior results so I had to try different parameters and settings based on prior experience. This is the combination that gave identical results:
Code:
gs -sDEVICE=pdfwrite -dPDFSETTINGS=/prepress -dNOPAUSE -dBATCH -sOutputFile="$o" "$i"
You can see increased mosquito noise in the top image for the default gs settings that you used. The bottom one, done using my settings, is identical to the middle one, the original.
The files you uploaded looked identical but that doesn't mean that your default settings will work in all cases. Here is just one example that shows that it doesn't.