Resize image to fit canvas - Gimp

This is easy to do, but among the hundreds of possible options to be put on the program UI, it was not "elected" to be there.

The way out is to use the program's scripting capabilities to perform the action: what have to be determined programatcially is whether the ratio of the image/layer is larger on the width or height, and use this ratio to scale tha layer, and then center the layer.

For your convenience, I wrote some Python code for this in a single line, in a way you can just copy and paste on the python console (filters->python->console) to apply the effect on the top layer of the most recent open image.

img = gimp.image_list()[0]; layer = img.layers[0]; factor = min (float(img.width) / layer.width, float(img.height) / layer.height); layer.scale(int(layer.width * factor), int(layer.height * factor)); layer.set_offsets((img.width - layer.width) / 2, (img.height - layer.height) / 2)

Since this can be done, but is not practical, even more because it does not allow you to pick the image or layer to resize, I formated it as a python-script for GIMP as well. Just check your edit->preferences->folders->plug-ins for your plug-in directory, paste the contents bellow as a file there (if on Windows, the file must have the ".py" extension. On Linux and Mac OS, any extension would work, but you have to give the file the "exectuable" property" ).

After restarting GIMP, you will have the new command conveniently located on your Layer menu:

#! /usr/bin/env python
# coding: utf-8

from gimpfu import *

def scale_layer_to_canvas_size(img, layer):
    pdb.gimp_image_undo_group_start(img)
    factor = min (float(img.width) / layer.width,
                 float(img.height) / layer.height)

    layer.scale(int(layer.width * factor), int(layer.height * factor))
    layer.set_offsets((img.width - layer.width) / 2,
        (img.height - layer.height) / 2)
    pdb.gimp_image_undo_group_end(img)

register("scale-layer-to-canvas-size",
    "Scale layer to canvas size",
    "Scales the layer to canvas size, keeping the aspect ratio",
    "João S. O. Bueno", "Public domain", "2014",
    N_("Scale layer to canvas size..."),
    "*",
    [(PF_IMAGE, "image",       "Input image", None),
     (PF_DRAWABLE, "layer", "Input drawable", None), ], [],
    scale_layer_to_canvas_size,  menu="<Image>/Layer/",
    )

main()

Note it is the same code than above, but "img" and "layer" are now suplied by GIMP when picking the action from the menu, and there are two extra calls so that both scalignand centering are "undone" as a single action - the remaining code is justtheneeded boiler plate to register the function with GIMP

After shrinking my canvas using Image -> Canvas (and centering the layers as desired), the Layer -> Layer to Image Size did the trick (without scaling the image). This is with gimp 2.8.16

Layer > Scale Layer worked beautifully for me.

That feature is not in gimp for some reason. An alternative without any scripts is:

Layer -> Scale Layer