Gimp Python-Fu how to copy a channel

Question

Suppose I want to copy the red channel of an image (thus creating a new channel). How do I do that with Python-Fu?

EDIT: basically, I need to create luminosity masks. So far, I tryed the following code:

# get active layer
layer = pdb.gimp_image_get_active_layer(image)
# copy the selected layer
layer_copy = pdb.gimp_layer_new_from_drawable(layer, image)
# insert the layer_copy on top of the layers stack
pdb.gimp_image_insert_layer(image, layer_copy, None, 0)
# desaturate the layer
pdb.gimp_drawable_desaturate(layer_copy, DESATURATE_LIGHTNESS)
# select the red channel
ch = pdb.gimp_channel_new_from_component(image, 0, "testchannel")
pdb.gimp_image_insert_channel(image, ch, 0, 1)

Unfortunately I got the error TypeError: wrong parameter type on the last command pdb.gimp_image_insert_channel(image, ch, 0, 1)

Answer 1

Your third argument should be None:

pdb.gimp_image_insert_channel(i, c, None,0)

The doc is mostly written with the script-fu point if view, where items are just integer ids, and "no item" is therefore 0. In python-fu, you pass object references, so "no item" is None.

You can do the same thing more simply (but with a bit less control) using:

image.insert_channel(channel)

Btw, you can also look at the python-fu object methods, using image.active_layer is a lot more readable than pdb.gimp_image_get_active_layer(image).

Answer 2

The R/G/B channels you see at the top of the Channels list are somewhat virtual since they are the result of the composition of the layers.

To get a layer with only the red component you can use the channel mixer:

pdb.plug_in_colors_channel_mixer(img, l, False, 1.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0)

(note that it changes the layer it is called with, so you may want to do a copy first).