Question: What exactly does the quality options (to control quality) do? If I put quality:50, does that result in an image that is 50% of the quality of the original, or an absolute level of 50%? It appears that this flag does the former, but I need the latter. Is there any way to convert all pictures, regardless of their original quality level, to the same uniform lower quality level?
For example: if I have 3 files whose original quality is 100, 75, and 50, if I use quality:50, will I get all three files with quality of 50, or will I get files with 50, 37.5, and 25, respectively? What I need is convert all 3 to quality level 50.
Answer: The "quality" option defines the degree of compression for JPEG-compressor. The less is "quality" value, the higher is compression rate, i.e. less file size and worse image image quality. When the file is being opened it is impossible to find out with what compression degree it was created.
The original image (for example image.bmp) and JPEG file (image.jpg) created from it with quality 100 contain images with different quality. Differences will be very slight, but they will be, because JPEG compression always leads to loss in quality.
File JPEG (for example image1.jpg) created with quality 75 can be converted back to bitmap (image1.bmp). If we convert (image1.bmp) to JPEG again with quality 75 (image2.jpg), the images image1.jpg and image2.jpg will be of different quality. image2.jpg will be of worse quality, because it has been compressed twice.
Whenever you convert compressed files (such as JPEG, GIF, PNG),
they are always at first unpacked and then converted. If JPEG (image2.jpg),
created with quality 75 unpack, and then convert it to JPEG with
quality 100 (image3.jpg), quality of image3.jpg won't any better
then quality of image2.jpg, but it will be larger in size, because
compression degree of
image3.jpg will be higher then that of image2.jpg.
In your case I recommend you to convert images with quality 50.
Example:
ICPCL.exe –convertto jpg –source " C:\Images\*.*" –dest "C:\out\" quality
:50
Quality of converted images will be as much as possible close to 50.