[solved] Batch convert images so they are all JPG with minimal loss of data

[mhulse]

I have a big folder of images (jpg, png, tif, etc.)

I need to convert all of them so they are all JPGs.

I am writing a script that checks the image extension and converts to JPG only if not already a JPG format.

Questions:

1. If the file extension is .jpeg, .jif, .jfif, can I just change the file name extension to ".jpg" without converting (note that I will be skipping files that already end in ".jpg")?

2. When converting, I want to avoid loss of quality (at least, keep it to a minimum - I understand how jpg compression works); with that said, what would be the best command to do the conversion on a lot of unknown file types?

I was hoping that this would do the trick (because it's simple):

convert input.png output.jpg

Will that retain as much of the original image's settings as possible (i.e., resolution, quality, etc.)?

I did a quick test on a JPG:

https://www.diffchecker.com/ERYIXm64

Interestingly, the changes look minimal.

Basically, I want to convert to JPG without changing, or changing only a minimal amount, of the original image as possible.

Thanks in advance!!!

[snibgo]

In IM, all JPG compression is lossy. Compression is controlled by "-quality X" where 0 <= X <= 100. Small numbers are low quality, high compression. High numbers are high quality, low compression. So "-quality 100" gives the least change.

[mhulse]

Hi snibgo! Thanks for the quick reply, I really appreciate it!!!

So, looks like I should do:

convert input.png -quality 100 output.jpg

Do you know if that will retain things like resolution? The original images may have various PPI settings.

Basically, all I want to do is normalize the file extensions/types in the folder I'm working on. I plan on just converting non-JPGs ... Once converted, I will save the JPG to the same folder and throw away its old non-JPG file.

Thanks again for the help!

[fmw42]

1. If the file extension is .jpeg, .jif, .jfif, can I just change the file name extension to ".jpg" without converting (note that I will be skipping files that already end in ".jpg")?

IM will decompress and recompress and thus lose some quality. So you should do that processing from your OS.

Note that some jpg files do not have units set nor possibly density (resolution), so when processing with IM, be sure to set the units, if not specified.

Do you care if sRGB or CMYK or do you need to convert them all to sRGB and add a profile, so they all view properly and consistently on different devices?

[mhulse]

Hi Fred! Thank you so much for your reply, I really appreciate it!

Great questions and tips! Thank you!

IM will decompress and recompress and thus lose some quality. So you should do that processing from your OS.

Perfect! I will do that.

Note that some jpg files do not have units set nor possibly density (resolution), so when processing with IM, be sure to set the units, if not specified.

Great! Here's what I have so far:

Code: Select all

convert \
"${args.inputImage}" \
-quality 100 \
-units PixelsPerInch \
-density 300 \
-depth 8 \
"${args.outputImage}" 2> /dev/null

Though, I am curious, if density and depth is already set (before conversion), will IM retain those "settings" when converting to JPG if they are not set in conversion script?

Do you care if sRGB or CMYK or do you need to convert them all to sRGB and add a profile, so they all view properly and consistently on different devices?

In this case, I want to retain the profile and colorspace of the original images. I don't mind if CMYK or RGB. In the end, I will be printing these images and I am using Indesign to manage colors.

Thanks again for your help, I really appreciate it!

[fmw42]

If you specify a units and/or density, then IM will change the image to those you specify. If you do not want that, then you need to check the units and density before processing the image and skip if you like the results. See %x and %y for resolution and %U for units at http://www.imagemagick.org/script/escape.php. Likewise, you can check for profiles with %[profiles] and %[profile:XXX]. You can also check for quality with %[quality] or %Q.

You can also check the format using %m (JPEG, etc) rather than looking at the suffix.

So you would likely need to check and store each of these and then use a bunch of conditionals (depending upon your OS) to decide what IM convert command to call or given one convert command, what variables to insert to include changes.

[mhulse]

Wow, that's excellent info Fred!!!! Thank you so much for you help!

Thank you snibgo and Fred for your pro help and guidance, I greatly appreciate it!