Gimp

GIMP Control

GIMP (/Applications/GIMP.app) embeds Python 3 and exposes its whole document model through libgimp over GObject Introspection (gi.repository.Gimp), with GEGL for pixels and filters. Drive it through GIMP's own Script-Fu server — a TCP endpoint GIMP ships with, so there is nothing to install — which hands each script to GIMP's python-fu-eval. The script runs against the live GIMP: the canvas updates, the layer stack is the one the user is looking at, and nothing happens headless. Every call in this skill was run against GIMP 3.2.4.

• scripts/gimp-send.sh — send a .py file (or -c 'inline code') and print its captured output; --ping checks the connection, --scheme evaluates raw Script-Fu.
• scripts/gimp-start.sh — launch the GIMP GUI with the Script-Fu server listening, and wait until it answers.
• scripts/gimp_client.py — the wire protocol, used by both.
• scripts/example-poster.py — a complete build → verify → export script to copy the shape from.
• references/api-reference.md — the version-checked libgimp API (images, layers, selections, drawing, text, GEGL filters, PDB calls, pixel readback, export). Read it before writing anything past the cheatsheet below.
• references/recipes.md — working on the user's own image, batch folders, per-layer export, masks, animation, and what the API cannot do.

The control loop

1. Check the connection: bash scripts/gimp-send.sh --ping → {"ok": true, "bridge": "gimp", "port": 10008, "reply": "3.2.4 | open images: 2"}.

   If nothing answers, there are two different situations and they need different moves:

   • GIMP is not running → bash scripts/gimp-start.sh starts the GUI with the server switched on and waits for it. This opens a window on the user's screen, so say that you are doing it.
   • GIMP is running without the server → you cannot switch it on remotely, and you must not start a second instance to get around that: two GIMPs share one config directory and the user's unsaved work is in the one already open. Ask them for one click: Filters ▸ Development ▸ Script-Fu ▸ Start Server…, listening on 127.0.0.1, port 10008. (Development stays English on a localised GIMP; on a Hungarian one the path reads Szűrők ▸ Development ▸ Script-Fu ▸ Kiszolgáló indítása….)

2. Write a build script to the scratchpad and send it: OUT=/path/to/outdir bash scripts/gimp-send.sh /path/build.py. OUT arrives both as a global and as os.environ["OUT"]; GIMP_SEND_TIMEOUT=600 (seconds) covers batches and big exports.

3. Read the feedback. The sender returns the script's captured stdout/stderr, and on failure the Python traceback, exiting non-zero — so print(...) is your channel back. Nothing else reports: GIMP's own error console stays inside the app.

4. Look at the result. image.get_thumbnail(w, h, Gimp.PixbufTransparency.SMALL_CHECKS) renders the composited image straight to a PNG you can Read. Do this at the end of every build script — one line, and it catches what no assertion will.

5. Iterate, then deliver. Fix the script and re-send. Scripts must be re-runnable: create a fresh image per version rather than mutating the one already open, so a re-send cannot stack layers onto the previous attempt.

Each send is a fresh python-fu-eval process, so nothing persists between sends except what is in GIMP (open images) or on disk. Round-trip is roughly a second — cheap enough to probe with a throwaway send rather than guess an API.

Scripting cheatsheet

import gi
gi.require_version("Babl", "0.1"); gi.require_version("Gegl", "0.4")
gi.require_version("Gimp", "3.0")
from gi.repository import Babl, Gegl, Gimp, Gio
Gegl.init(None)

Image and layer stack

image = Gimp.Image.new(1000, 1400, Gimp.ImageBaseType.RGB)
layer = Gimp.Layer.new(image, "background", 1000, 1400,
                       Gimp.ImageType.RGBA_IMAGE, 100.0, Gimp.LayerMode.NORMAL)
image.insert_layer(layer, None, 0)      # parent=None -> top level; 0 = topmost
layer.set_opacity(70.0)                 # 0–100 float, not 0–255
layer.set_mode(Gimp.LayerMode.SCREEN)
Gimp.Display.new(image)                 # show it — the user watches the build happen
Gimp.displays_flush()                   # nothing repaints without this

To work on what the user already has: Gimp.get_images()[0], most recently active first. Duplicate it before destructive work — see recipes.md.

Colour: sRGB in, sRGB out

Gegl.Color stores linear RGB, so the constructor and the setter are not the same thing and the difference is a visibly washed-out image:

Gegl.Color.new("#1a73e8")                 # parses sRGB — use this
colour.set_rgba(0.10, 0.45, 0.85, 1.0)    # LINEAR components — rarely what you meant
list(colour.get_bytes(Babl.format("R'G'B'A u8")).get_data())   # [26,115,232,255]
Gimp.context_set_foreground(Gegl.Color.new("#ffb703"))

Selections, fills, drawing

image.select_rectangle(Gimp.ChannelOps.REPLACE, x, y, w, h)   # also ellipse, polygon,
layer.edit_fill(Gimp.FillType.FOREGROUND)                      # round_rectangle, color
Gimp.Selection.none(image)               # every later edit respects the selection

layer.fill(Gimp.FillType.FOREGROUND)     # whole layer, ignores the selection
Gimp.context_set_gradient_fg_bg_rgb()
layer.edit_gradient_fill(Gimp.GradientType.LINEAR, 0.0, False, 1.0, 0.0, True,
                         x1, y1, x2, y2)

Gimp.context_set_brush(Gimp.Brush.get_by_name("2. Hardness 100"))
Gimp.context_set_brush_size(20.0)
Gimp.paintbrush_default(layer, [40.0, 40.0, 200.0, 200.0])   # ONE drawable, flat x/y list

Text

font = Gimp.context_get_font()                 # safer than naming one: get_by_name
text = Gimp.TextLayer.new(image, "GIMP", font, 150.0, Gimp.Unit.pixel())
image.insert_layer(text, None, 0)
text.set_color(Gegl.Color.new("#f8f9fa"))
text.set_justification(Gimp.TextJustification.CENTER)
text.set_offsets((image.get_width() - text.get_width()) // 2, 300)   # measure AFTER

Filters — prefer non-destructive

GIMP 3 filters stay live and editable, which is the right default on someone else's artwork:

blur = Gimp.DrawableFilter.new(layer, "gegl:gaussian-blur", "glow blur")
config = blur.get_config()
config.set_property("std-dev-x", 90.0); config.set_property("std-dev-y", 90.0)
blur.update()                            # push config into the filter
layer.append_filter(blur)                # or layer.merge_filter(blur) to bake it in

Discover ops and property names instead of guessing — Gegl.list_operations() and Gegl.Operation.list_properties("gegl:gaussian-blur"). The legacy plug-in-gauss family is gone in GIMP 3.

Anything without a binding: the PDB

proc = Gimp.get_pdb().lookup_procedure("script-fu-drop-shadow")   # None if absent
config = proc.create_config()
config.set_property("run-mode", Gimp.RunMode.NONINTERACTIVE)
config.set_property("image", image)
config.set_core_object_array("drawables", [layer])     # NOT set_property
result = proc.run(config)

Export

Gimp.file_save(Gimp.RunMode.NONINTERACTIVE, image,
               Gio.File.new_for_path(OUT + "/poster.xcf"), None)   # .png/.jpg/... too
flat = image.duplicate(); flat.flatten()                 # flatten a copy, not the original
Gimp.file_save(Gimp.RunMode.NONINTERACTIVE, flat,
               Gio.File.new_for_path(OUT + "/poster.png"), None)
flat.delete()

.xcf keeps every layer, mask and live filter editable for the user; ship it alongside the flat export unless they said otherwise. Format options (JPEG quality, GIF timing) come from calling the export plug-in through the PDB — see api-reference.md.

Verification

• Returned stdout is the immediate signal — print(...) comes straight back and a failure returns the traceback with a non-zero exit.
• A thumbnail PNG is the visual check: save image.get_thumbnail(500, 700, Gimp.PixbufTransparency.SMALL_CHECKS) into $OUT and Read it. It is the composited image, so it is honest about blend modes and filters in a way a layer-by-layer description is not.
• Numbers: image.pick_color([...], x, y, True, False, 0.0) sampled through Babl.format("R'G'B'A u8") proves a colour landed exactly; layer.histogram(...) gives mean/median on a 0–255 scale. Dump size, layer names, offsets, modes and attached filters as metrics.json and Read that rather than eyeballing the stack.
• Hand-off: the exported PNG/JPEG opens anywhere; the .xcf keeps the work editable.

Gotchas

• Gegl.Color.set_rgba is linear, Gegl.Color.new("#hex") is sRGB. Mixing them up silently lightens every colour; read values back through get_bytes(Babl.format("R'G'B'A u8")), not get_rgba().
• set_core_object_array for arrays of GIMP objects. config.set_property( "drawables", [layer]) raises could not convert [...] to type 'GimpCoreObjectArray', and reading one back needs result.get_core_object_array(i) — result.index(i) returns a GBoxed that will not iterate.
• Paint calls take one drawable, not a list (Gimp.paintbrush_default(layer, pts)), while pick_color takes a list of drawables. The asymmetry is real.
• Gimp.Font.get_by_name returns None for a missing font, and the failure only surfaces inside TextLayer.new as Argument 2 does not allow None as a value. Use Gimp.context_get_font() unless the user named a font, and check the return.
• Text layers resize themselves — centre and position them after setting text, size and spacing, or you are measuring the previous layout.
• Gimp.displays_flush() after edits, or the user stares at a stale canvas and you both think the script did nothing.
• Success flags come first in get_offsets, pick_color, histogram and selection.bounds. Unpack the tuple; don't index into it blindly.
• Resource names are localised — on a Hungarian GIMP the gradients are Előtérből háttérbe (RGB), not FG to BG (RGB). List them at runtime (gimp-*-get-list via the PDB) instead of hard-coding English names.
• image.delete() discards unsaved changes without asking. Call it only on images your own script created; never on the user's.
• Group edits with image.undo_group_start() / undo_group_end() so one Ctrl+Z undoes the whole build rather than four hundred individual operations.
• No state survives a send — each one is a new python-fu-eval process. Version your images ("poster_v2") so re-runs don't fight the previous attempt.
• --batch needs --batch-interpreter in GIMP 3; without it GIMP prints a list of interpreters and quits. gimp-start.sh already passes it.
• A dead GIMP can leave the port bound. The script-fu-server plug-in is its own process and outlives a quitting GIMP for a while, so a bare port check says "up" when nothing is behind it. --ping sees through this (the connection closes mid-reply, and the orphan then exits) — if it reports that, just start GIMP again.
• GIMP 2.10 tutorials do not port. gimpfu, pdb.gimp_*, RGB tuples and the plug-in-gauss filters are all gone; check lookup_procedure for None before trusting a procedure name you read somewhere.
• Tools are not scriptable — no transform widget, no free select, no menu-action trigger. Fills, paint calls, selections and GEGL filters are the whole vocabulary; see recipes.md.

Security

There is nothing to install here, but starting the Script-Fu server still turns GIMP into a code-execution server. Say so before asking for that click.

• The server listens on 127.0.0.1:10008 and hands whatever arrives to python-fu-eval — arbitrary Python with the user's privileges, over the user's open images. It carries no authentication: every local process, and every other user on a shared machine, can drive GIMP through it.
• Always 127.0.0.1. The Start Server dialog accepts any listen address, and a server bound to 0.0.0.0 or a LAN address is remote code execution for the whole network. gimp-start.sh pins loopback; if the user starts it by hand, tell them the address.
• It is not HTTP — the protocol is length-prefixed with a G magic byte, so a browser's request is rejected at the first byte. That is a side effect of the wire format, not an origin check.
• To stop it, quit GIMP. The port can stay bound briefly after GIMP exits while the plug-in process winds down, so a port check is not proof anything is live — --ping is (gimp-send.sh --ping).