Gimp batch resize and change levels

Question

I have hundreds of images, of which I want to perform the following operations in Gimp:

1. Scale it using Sinc (Lanczos3)

2. Change the input levels to a certain value

3. Save it

My question is how do I perform these tasks in batch operation? If Gimp is not capable of doing such task, is there any alternative software?

Answer 1

To do all of this:

1. Copy this GIMP python-fu script:

#!/usr/bin/env python

from gimpfu import *

def scale_level_save(ignored1, ignored2, scale_x, scale_y, black, white):
    # Get the list of all opened images
    images = gimp.image_list()
    image_number = len(images)

    # Loop through each image
    for i in range(image_number):
        current_image = images[i]
        current_drawable = current_image.active_layer
        pdb.gimp_progress_update((i * 1.0) / image_number)
        pdb.gimp_progress_set_text("Processing image #" + str(i + 1) + "/" + str(image_number) + "...")

        # Save the image type
        mode = current_image.base_type

        if mode != RGB:
            pdb.gimp_convert_rgb(current_image)

        # 1. Scale the image using Sinc (Lanczos3) interpolation
        # Get the original dimensions of the image
        orig_width, orig_height = current_image.width, current_image.height

        # Calculate width and height from the scaling factors
        new_width, new_height = current_image.width * scale_x, current_image.height * scale_y

        # Scale the image using the Sinc (Lanczos3) interpolation method
        pdb.gimp_image_scale_full(current_image, new_width, new_height, INTERPOLATION_LANCZOS)

        # Scale the drawable (layer) to match the image dimensions
        pdb.gimp_layer_scale(current_drawable, new_width, new_height, True)

        # Resize the image canvas if the new dimensions are larger than the original
        if scale_x > 1 or scale_y > 1:
            pdb.gimp_image_resize(current_image, new_width, new_height, 0, 0)

        # 2. Change the input levels to a certain value
        pdb.gimp_levels(current_image.active_layer, 0, black, white, 1, 0, 255)

        # Restore the image type
        if mode == INDEXED:
            pdb.gimp_image_convert_indexed(current_image, NO_DITHER, MAKE_PALETTE, 256, FALSE, FALSE, '')
        elif mode == GRAY:
            pdb.gimp_image_convert_grayscale(current_image)

        # 3. Save the file
        filename = pdb.gimp_image_get_filename(current_image)
        pdb.gimp_file_save(current_image, current_image.active_layer, filename, '?')

    # Refresh the image to see the changes
    pdb.gimp_displays_flush()

# Define the parameters for the script
register(
    "python_fu_scale_level_save",
    "Scale image using Sinc (Lanczos3) interpolation, level and save",
    "Scale the image with the Sinc (Lanczos3) interpolation method",
    "Fabrice TIERCELIN",
    "Fabrice TIERCELIN",
    "2023",
    "<Image>/Filters/Scale Level and Save",
    "*",
    [
        (PF_FLOAT, "scale_x", "Scale X:", 2),
        (PF_FLOAT, "scale_y", "Scale Y:", 2),
        (PF_SPINNER, "black", "Black level (0-255)", 0, (0, 255, 1)),
        (PF_SPINNER, "white", "White level (0-255)", 255, (0, 255, 1))
    ],
    [],
    scale_level_save
)

# Main function
main()

2. Paste it into a text file
3. Save it with a .py extension
4. Put this file on your C:\Users\YourName\AppData\Roaming\GIMP\2.10\plug-ins folder (more information below if it doesn't work)
5. Start GIMP
6. Open all your image files by drag and drop
7. Go on Filter → Scale Level and Save
8. Choose your options
9. Launch the script → all the files are already saved with the 3 operations
10. Quit GIMP ignoring the prompts

For systems other than Windows, look at the settings in GIMP:

1. Click on Edit
2. Click on Preferences
3. Go on left pane; at bottom
4. Click on Folders
5. Click on Plug-ins
6. Click on Add a new folder

Successfully tested on GIMP 2.10.36 on Windows 10.