converting 32 bit images to 8 bit using batch script

littlenellie · **Joined:** Jan 24, 2021 **Posts:** 4

Hi,

I'd like to convert a large number of 32 bit png images (single layer) to 8 bit gray png (single layer).

I've not used Script-Fu or Python with GIMP before, but can't see anything in the Procedure Browser that has a parameter for specifying bit depth. Is that possible with scripting?

I'm using GIMP 2.10.20.

Many thanks

rich2005 · **Joined:** Mar 04, 2011 **Posts:** 2435

The easiest way for a large number of images is use an existing plug-in BIMP see: https://alessandrofrancesconi.it/projects/bimp/ where you can add various procedures including a convert to grayscale.

Attachment:

bimp.jpg [ 126.86 KiB | Viewed 4398 times ]

Edit: This issue came up one time. Using BIMP and png format an alpha channel is added. No way to avoid this with BIMP as the export is the final function.

If the alpha channel is a problem (larger file size etc) then this is a batch plugin for removing the alpha channel following on from BIMP.
https://github.com/samvanemelen/GIMP-pl ... ansparency

Edit again

maybe the gimp procedure if you want to write your own batch file is
pdb.gimp_image_convert_grayscale(image)
..but as you say not bitdepth. As an example attached a simple python script that does that for an open image.

ofnuts · **Joined:** Oct 25, 2010 **Posts:** 4750

littlenellie wrote:

Hi,

I'd like to convert a large number of 32 bit png images (single layer) to 8 bit gray png (single layer).

I've not used Script-Fu or Python with GIMP before, but can't see anything in the Procedure Browser that has a parameter for specifying bit depth. Is that possible with scripting?

I'm using GIMP 2.10.20.

Many thanks

Very likely much easier to do in ImageMagick (designed to be used in shell scripts). See

https://legacy.imagemagick.org/Usage/fo ... ng_formats

littlenellie · **Joined:** Jan 24, 2021 **Posts:** 4

rich2005 wrote:

The easiest way for a large number of images is use an existing plug-in BIMP see: (snip, snip) where you can add various procedures including a convert to grayscale.

....(snip, snip)

Edit: This issue came up one time. Using BIMP and png format an alpha channel is added. No way to avoid this with BIMP as the export is the final function.

If the alpha channel is a problem (larger file size etc) then this is a batch plugin for removing the alpha channel following on from BIMP.
(snip, snip)

Edit again

maybe the gimp procedure if you want to write your own batch file is
pdb.gimp_image_convert_grayscale(image)
..but as you say not bitdepth. As an example attached a simple python script that does that for an open image.

Great stuff, that's saved me a lot of time poking around trying to find something that didn't exist

I might come back to the python option, so thanks for the example script - very useful. Cheers

littlenellie · **Joined:** Jan 24, 2021 **Posts:** 4

ofnuts wrote:

Very likely much easier to do in ImageMagick (designed to be used in shell scripts). (snip, snip)

*facepalm* it hadn't even occurred to me to look for an alternative bit of software :oops:

That is an excellent suggestion, and good to know that about ImageMagick as I've never used it. Thank you! :bigthup

littlenellie · **Joined:** Jan 24, 2021 **Posts:** 4

Hey,

So for anyone looking to do this, Ofnuts's ImageMagick solution worked brilliantly.

The steps that worked for me:
* I downloaded the Windows version (I'm running Win10) of ImageMagick
* copied the *.png files that I wanted to convert into a separate folder
* opened a DOS Command Prompt window and navigated to the folder with the images in
* ran the following code (which creates a *.bat file in the same folder):

Code: Select all

for %f in (*.png) do echo magick %f -depth 8 %f >>set_files_to_8_bit.bat

* run the set_files_to_8_bit.bat file.
* Files converted, job done. :yes

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