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Python-Fu script to change only selected area

Mar 6, 2011, 21:03
Post: #1
Python-Fu script to change only selected area

I'm struggling here to climb the learning curve in GIMP scripting. The Script-Fu syntax seems just crazy, so I immediately opted for Python. Got the basics of plug-in registration working, so now it's time to make something actually useful.

I have to admit that there are almost no decent tutorials on this topic. Perhaps that means that it's time to make one. When I master something, that is.

I have successfully written a script that reads pixel values and performs some transformation. But it is applied on the active layer, regardless what is the selected area. What I am now looking for, is how to apply the effect only to the selected area in the active layer, so pretty standard stuff. And, perhaps a fallback routine - apply to the active layer, when there is no selection. Could someone suggest either good online reading resource to learn some ropes, or perhaps paste a code here in forum?

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Apr 22, 2011, 22:42
Post: #2
RE: Python-Fu script to change only selected area
Thanks for the advice. It was quite a while ago I asked this, and now I've got back to tinkering with Gimp scripting.

This is how far I've got with my experimental plugin. It now respects the selection and it's alpha channel. But I don't know how to merge that dynamically created layer to the base image. At least it seems to be the good style that typical plugin affects the active layer.
I also don't understand should I use those img.disable_undo() and img.enable_undo()? My script works without those just fine. Or perhaps I should use those related to undo groups? I have no idea what's the difference and generally suggested practice.

I think I'd be not asking here some very specific dumb questions, if I could find any decent reference online. I really tried to find some, but failed. Or perhaps I don't know how to search for it? I mean something like API documentation, where all the objects and their methods are described.

PHP Code:
from gimpfu import *
import random

def col
(xya) :
= (random.randint(0,(x+y)%50+200), random.randint(0,(x+y)%50+200), random.randint(0,(x+y)%50+200), a)

def randompix
(imglayer) :
# Fill the selection with random-color pixels

    # Now: spoils the main layerm not undoable.
    # Next todo: create new layer, render the effect on that, then merge to main layer, respecting the selection. Limit the processing
    # to the selection bounding box only

is_selectionx1y1x2y2 pdb.gimp_selection_bounds(img)
sel img.selection
x1 0

#pdb.gimp_message(str(x1) + "; " + str(y1) + "; " + str(x2) + "; " + str(y2) + "; " + ("yes" if is_selection else "no"))

drawLayer gimp.Layer(img"tempLayer_1"img.widthimg.heightRGBA_IMAGE100NORMAL_MODE)
y in range(y1y2):
x in range(x1x2):
pdb.gimp_drawable_get_pixel(selxy)[1][0] if is_selection else 255
if 0:

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Apr 23, 2011, 22:47
Post: #3
RE: Python-Fu script to change only selected area
(Apr 23, 2011 16:20)ofnuts Wrote:  You don't merge a layer to the image. You merge it in another layer... Typically you would position you new layer just above the layer you are supposed to transform (the current layer, usually) and when done merge down you new layer in that current layer.

The only command I see there in te procedure browser is this:
pdb.gimp_image_merge_down(image, merge_layer, merge_type)
The description says that the layer will be merged to the next visible layer below. However, in some of the plugin scripts what I have downloaded from the plugin registry, I see this prodedure call: imageObj.merge_down(layerObj, 0)
Such merge_down method is not found in the procedure browser at all, so I assume this is one of those undocumented "surprise" methods. But anyway, I still don't know of any method that lets you merge layer A down to layer B, instead of just "to the next layer below", what fells like it might cause some harm, when I try to act on other than topmost layer.

Thanks for explaining about the undo control methods. I now implemented img.undo_group_start() and img.undo_group_end(). However, I read that for more complex operations it is wiser to create a new image object, disable undo on it, do all the operations, paste the result back to the active image, and discard the dynamically created image.

Regarding the API: how about establishing some community-supported wiki for this? At least plain list of modules, objects and method signatures would be a good start. While I can browse the procedure browser, it still is very clear that there are many aliases for most of the "pdb" methods, at least some of them are available as methods of the respective objects (as seen in, but that hardly feels like exhaustive list). Of course, performing dir() on those objects sheds some little light, but man, this is pretty wild way to explore the terrain indeed.
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Apr 24, 2011, 09:58
Post: #4
RE: Python-Fu script to change only selected area
(Apr 24, 2011 07:41)ofnuts Wrote:  PS: just had a look at the source code for the interface... finding the existing methods is quite easy. There are indeed two variants for the merge: merge_down and merge_visible.

Where in the source files you would look for the Python objects interface? Do you mean the /usr/lib/gimp/2.0/python/ and related modules?

I could not agree that wiki would be overkill. I mean, look at the present state of documentation available online, that's ridiculous. And that's completely understood that coders enjoy adding another useful feature rather than writing docs. But that's what community is for. If the Gimp scripting would be more documented, we could definitely see wider adoption of Gimp, as community would embrace the expansion possibilities.
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