gimp

GIMP — Python Manipulation Guide

This skill teaches setup-gen (create/modify images) and reward-gen (read/verify image properties) how to work with image files using pure Python code.

• Libraries: Pillow, numpy, opencv-python, scikit-image
• Install: pip3 install Pillow numpy opencv-python scikit-image
• File formats: .png, .jpg, .bmp, .xcf (GIMP native, read-only via special tools)

---

0. GUI Startup on VM (for setup-gen)

After generating image assets, setup-gen should open GIMP with the target initial file for the GUI agent.

CRITICAL VM LIMIT: GUI launches must set DISPLAY=:0.

import os
import shlex
import subprocess
import time

def launch_gui(command: str, delay_sec: float = 1.0):
    env = os.environ.copy()
    env["DISPLAY"] = ":0"
    subprocess.Popen(
        shlex.split(command),
        stdout=subprocess.DEVNULL,
        stderr=subprocess.DEVNULL,
        env=env,
    )
    time.sleep(delay_sec)

# Open target image in GIMP
launch_gui('gimp "/home/user/<task_id>_initial.png"', delay_sec=2.0)

Guidelines:

• Open initial artifact(s), never golden artifact(s).
• Use non-blocking launch (Popen) and short delays.

---

0.5. Media Asset Library (setup-gen)

Instead of generating synthetic images (color blocks, testsrc patterns), use the real photo library at assets/media/gimp/. These are high-resolution photos from Pexels (free commercial use).

Picking Assets from Manifest

import json, os, random

ROOT = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))  # project root
MANIFEST = os.path.join(ROOT, "assets", "media", "manifest.json")

def pick_asset(domain: str, category: str = None, tags: list = None) -> dict:
    """Pick a random asset matching domain/category/tags from manifest."""
    with open(MANIFEST) as f:
        manifest = json.load(f)
    candidates = [a for a in manifest["assets"] if a["domain"] == domain]
    if category:
        candidates = [a for a in candidates if a["category"] == category]
    if tags:
        candidates = [a for a in candidates if any(t in a.get("tags", []) for t in tags)]
    return random.choice(candidates) if candidates else None

# Pick a landscape photo for a brightness adjustment task
asset = pick_asset("gimp", "photos", tags=["landscape", "nature"])
local_path = os.path.join(ROOT, asset["path"])  # e.g., assets/media/gimp/photos/landscape_mountain_001.jpg

Uploading to VM

# Upload a photo to the VM before running initial_setup.py
python3 scripts/env_cli.py -c "<workdir>/env_config_initial.json" upload \
    "assets/media/gimp/photos/landscape_mountain_001.jpg" "/home/user/<task_id>.jpg"
python3 scripts/env_cli.py -c "<workdir>/env_config_golden.json" upload \
    "assets/media/gimp/photos/landscape_mountain_001.jpg" "/home/user/<task_id>.jpg"

Available Categories

Pattern: Upload + Reference Copy

# Setup-gen should upload the asset to BOTH VMs, then initial_setup.py saves the reference copy
python3 scripts/env_cli.py -c "<workdir>/env_config_initial.json" upload "assets/media/gimp/photos/xxx.jpg" "/home/user/<task_id>.jpg"
python3 scripts/env_cli.py -c "<workdir>/env_config_golden.json"  upload "assets/media/gimp/photos/xxx.jpg" "/home/user/<task_id>.jpg"

Then in initial_setup.py:

import shutil
shutil.copy(f'{WORKDIR}/{TASK_ID}.jpg', f'{WORKDIR}/{TASK_ID}_initial_reference.jpg')
# ... launch GIMP with the file

---

1. Creating & Modifying Images (setup-gen)

Basic Image Creation

from PIL import Image, ImageDraw, ImageFont, ImageFilter, ImageChops, ImageStat, ImageEnhance
from PIL.Image import Resampling
import numpy as np
import shutil, os

# Create blank image
img = Image.new("RGB", (800, 600), color=(255, 255, 255))
img.save("/home/user/Desktop/blank.png")

# Create RGBA image with transparency
img = Image.new("RGBA", (800, 600), color=(0, 0, 0, 0))
img.save("/home/user/Desktop/transparent.png")

Drawing Shapes and Text

draw = ImageDraw.Draw(img)
# Rectangle
draw.rectangle([100, 100, 300, 200], fill="red", outline="black", width=2)
# Circle / ellipse
draw.ellipse([400, 100, 550, 250], fill="blue")
# Triangle (polygon)
draw.polygon([(400, 300), (300, 500), (500, 500)], fill="yellow", outline="black")
# Line
draw.line([(0, 0), (800, 600)], fill="green", width=3)
# Text
font = ImageFont.truetype("/usr/share/fonts/truetype/dejavu/DejaVuSans.ttf", 36)
draw.text((100, 50), "Hello World", fill="black", font=font)
img.save("/home/user/Desktop/shapes.png")

Image Transformations

img = Image.open("/home/user/Desktop/photo.png")

# Resize (maintain aspect ratio)
target_height = 512
ratio = target_height / img.height
new_size = (int(img.width * ratio), target_height)
img_resized = img.resize(new_size, Resampling.LANCZOS)

# Crop
img_cropped = img.crop((left, top, right, bottom))  # box = (left, upper, right, lower)

# Rotate
img_rotated = img.rotate(90, expand=True)  # expand=True to fit full rotated image

# Mirror / flip
img_mirror = img.transpose(Image.FLIP_LEFT_RIGHT)     # horizontal mirror
img_flip = img.transpose(Image.FLIP_TOP_BOTTOM)        # vertical flip

# Convert color mode
img_gray = img.convert("L")         # grayscale
img_rgb = img.convert("RGB")        # RGB (drops alpha)
img_rgba = img.convert("RGBA")      # add alpha channel
img_palette = img.convert("P")      # indexed/palette mode
img_hsv = img.convert("HSV")        # hue-saturation-value

Brightness, Contrast, Saturation Adjustments

# Brightness
enhancer = ImageEnhance.Brightness(img)
img_darker = enhancer.enhance(0.7)    # < 1 = darker, > 1 = brighter

# Contrast
enhancer = ImageEnhance.Contrast(img)
img_contrast = enhancer.enhance(1.5)  # > 1 = more contrast

# Saturation / Color
enhancer = ImageEnhance.Color(img)
img_vivid = enhancer.enhance(1.5)     # > 1 = more saturated

# Sharpness
enhancer = ImageEnhance.Sharpness(img)
img_sharp = enhancer.enhance(2.0)     # > 1 = sharper

Transparency & Background Removal

# Make white background transparent
img = img.convert("RGBA")
data = np.array(img)
# Replace white pixels with transparent
white_mask = (data[:, :, 0] > 240) & (data[:, :, 1] > 240) & (data[:, :, 2] > 240)
data[white_mask, 3] = 0
img = Image.fromarray(data)

# Fill background layer with a color (keeping object intact)
bg = Image.new("RGB", img.size, (0, 255, 0))  # green background
if img.mode == "RGBA":
    bg.paste(img, mask=img.split()[3])  # paste using alpha as mask

Filters

img_blur = img.filter(ImageFilter.GaussianBlur(radius=5))
img_sharp = img.filter(ImageFilter.SHARPEN)
img_edges = img.filter(ImageFilter.FIND_EDGES)
img_emboss = img.filter(ImageFilter.EMBOSS)

Golden File Pattern

# Always copy-then-modify for golden files
shutil.copy("/home/user/Desktop/original.png", "/home/user/Desktop/golden.png")
img = Image.open("/home/user/Desktop/golden.png")
img = img.transpose(Image.FLIP_LEFT_RIGHT)  # example transformation
img.save("/home/user/Desktop/golden.png")

Compositing Multiple Images

bg = Image.open("/home/user/Desktop/background.png").convert("RGBA")
fg = Image.open("/home/user/Desktop/foreground.png").convert("RGBA")
# Paste foreground onto background at position
bg.paste(fg, (100, 100), mask=fg)  # use alpha as mask
bg.save("/home/user/Desktop/composite.png")

---

2. Reading & Verifying Images (reward-gen)

Structure Similarity (SSIM)

from skimage.metrics import structural_similarity as ssim

def check_ssim(src_path: str, tgt_path: str, threshold: float = 0.9) -> bool:
    """Compare two same-sized images by SSIM. Returns True if similar."""
    img1 = Image.open(src_path).convert("RGB")
    img2 = Image.open(tgt_path).convert("RGB")
    if img1.size != img2.size:
        return False
    arr1, arr2 = np.array(img1), np.array(img2)
    min_dim = min(arr1.shape[0], arr1.shape[1])
    win_size = min(7, min_dim if min_dim % 2 == 1 else min_dim - 1)
    if win_size < 1:
        return False
    try:
        score = ssim(arr1, arr2, win_size=win_size, channel_axis=2)
    except TypeError:
        score = ssim(arr1, arr2, win_size=win_size, multichannel=True)
    return score >= threshold

def check_ssim_resized(src_path: str, tgt_path: str, threshold: float = 0.9) -> bool:
    """SSIM with auto-resize and transparency handling."""
    img1 = Image.open(src_path)
    img2 = Image.open(tgt_path)
    # Handle transparency: crop to content bounding box
    if img1.mode in ("RGBA", "LA"):
        alpha = img1.split()[-1]
        bbox = alpha.getbbox()
        if bbox:
            img1 = img1.crop(bbox)
    img1 = img1.convert("RGB").resize(img2.size if img2.mode == "RGB" else img2.convert("RGB").size, Resampling.LANCZOS)
    img2 = img2.convert("RGB")
    arr1, arr2 = np.array(img1), np.array(img2)
    try:
        score = ssim(arr1, arr2, win_size=7, channel_axis=2)
    except TypeError:
        score = ssim(arr1, arr2, win_size=7, multichannel=True)
    return score >= threshold

Pixel-Perfect Comparison

def compare_exact(src_path: str, tgt_path: str) -> bool:
    """Binary comparison: True only if all pixels match."""
    img1 = Image.open(src_path)
    img2 = Image.open(tgt_path)
    if img1.size != img2.size:
        img1 = img1.resize(img2.size, Resampling.LANCZOS)
    if img1.mode != img2.mode:
        img1 = img1.convert(img2.mode)
    diff = ImageChops.difference(img1, img2)
    return diff.getbbox() is None

Brightness Verification

def calculate_brightness(img: Image.Image) -> float:
    gray = img.convert("L")
    return ImageStat.Stat(gray).mean[0]

def verify_brightness_decreased(src_path: str, tgt_path: str) -> bool:
    """Verify src is darker than tgt AND structure preserved."""
    img_src = Image.open(src_path)
    img_tgt = Image.open(tgt_path)
    if calculate_brightness(img_src) >= calculate_brightness(img_tgt):
        return False
    # Normalize both to same brightness, then compare structure
    def normalize(img, target=128):
        factor = target / max(calculate_brightness(img), 1)
        return img.point(lambda x: min(255, max(0, int(x * factor))))
    mse = np.mean((np.array(normalize(img_src), dtype=np.float32) / 255 -
                    np.array(normalize(img_tgt), dtype=np.float32) / 255) ** 2)
    return mse < 0.03

Contrast Verification

def calculate_contrast(img: Image.Image) -> float:
    return float(np.std(np.asarray(img, dtype=np.float32)))

def verify_contrast_increased(src_path: str, tgt_path: str) -> bool:
    src, tgt = Image.open(src_path), Image.open(tgt_path)
    return calculate_contrast(src) > calculate_contrast(tgt) and check_ssim(src_path, tgt_path, threshold=0.65)

Saturation Verification

def verify_saturation_increased(src_path: str, tgt_path: str) -> bool:
    src_hsv = Image.open(src_path).convert("HSV")
    tgt_hsv = Image.open(tgt_path).convert("HSV")
    src_sat = np.mean(np.array(src_hsv.split()[1]))
    tgt_sat = np.mean(np.array(tgt_hsv.split()[1]))
    if src_sat <= tgt_sat:
        return False  # saturation did not increase
    # H and V channels must match
    h1, _, v1 = src_hsv.split()
    h2, _, v2 = tgt_hsv.split()
    return check_ssim_channel(h1, h2) and check_ssim_channel(v1, v2)

def check_ssim_channel(ch1, ch2, threshold=0.9):
    arr1, arr2 = np.array(ch1.convert("RGB")), np.array(ch2.convert("RGB"))
    if arr1.shape != arr2.shape:
        return False
    try:
        return ssim(arr1, arr2, win_size=7, channel_axis=2) >= threshold
    except TypeError:
        return ssim(arr1, arr2, win_size=7, multichannel=True) >= threshold

Image Size Verification

def verify_image_size(src_path: str, width: int = None, height: int = None,
                      ignore_transparent: bool = False) -> bool:
    img = Image.open(src_path)
    if ignore_transparent and img.mode in ("RGBA", "LA"):
        alpha = img.split()[-1]
        bbox = alpha.getbbox()
        if bbox is None:
            return False
        w, h = bbox[2] - bbox[0], bbox[3] - bbox[1]
    else:
        w, h = img.size
    if width is not None and w != width:
        return False
    if height is not None and h != height:
        return False
    return True

Mirror Verification

def verify_mirror(src_path: str, tgt_path: str) -> bool:
    src = Image.open(src_path)
    tgt = Image.open(tgt_path)
    flipped = src.transpose(Image.FLIP_LEFT_RIGHT)
    arr1, arr2 = np.array(flipped.convert("RGB")), np.array(tgt.convert("RGB"))
    if arr1.shape != arr2.shape:
        return False
    try:
        return ssim(arr1, arr2, win_size=7, channel_axis=2) >= 0.99
    except TypeError:
        return ssim(arr1, arr2, win_size=7, multichannel=True) >= 0.99

Palette Mode Verification

def verify_palette_mode(src_path: str, tgt_path: str) -> bool:
    """Verify image is palette-based and structure matches."""
    img = Image.open(src_path)
    if img.mode != "P":
        return False
    return check_ssim(src_path, tgt_path, threshold=0.9)  # compare after RGB conversion

Green Background Verification

def verify_green_background(src_path: str, tgt_path: str) -> bool:
    """All non-black target pixels must have green source pixels (g > r and g > b)."""
    src = np.array(Image.open(src_path))
    tgt = np.array(Image.open(tgt_path))
    # Vectorized: find non-black pixels in target
    mask = np.any(tgt[:, :, :3] != 0, axis=2)
    if not np.any(mask):
        return True
    r, g, b = src[mask, 0], src[mask, 1], src[mask, 2]
    return bool(np.all(g > r) and np.all(g > b))

Shape Position Verification

def verify_centered(tgt_path: str, tolerance: float = 0.05) -> bool:
    """Verify a colored shape is centered within tolerance of image center."""
    img = np.array(Image.open(tgt_path))
    unique_colors, counts = np.unique(img.reshape(-1, img.shape[2]), axis=0, return_counts=True)
    sorted_colors = unique_colors[np.argsort(counts)]
    shape_color = sorted_colors[1]  # second most common = shape (not background)
    mask = np.all(img == shape_color, axis=2)
    coords = np.argwhere(mask)
    centroid = coords.mean(axis=0)
    center = np.array(img.shape[:2]) / 2
    return bool(np.all(np.abs(centroid - center) < tolerance * np.array(img.shape[:2])))

Text Position Verification

def verify_text_on_left(src_path: str, width_threshold: float = 0.05) -> bool:
    """Verify dark text starts within the left edge of the image."""
    gray = np.array(Image.open(src_path).convert("L"))
    h, w = gray.shape
    dark_mask = gray < 128
    if not np.any(dark_mask):
        return False
    # Find the leftmost dark pixel across all rows
    cols = np.where(dark_mask.any(axis=0))[0]
    left_most = cols[0] if len(cols) > 0 else w
    return left_most < w * width_threshold

Sharpness Verification

import cv2

def verify_sharper(src_path: str, tgt_path: str) -> bool:
    src = cv2.imread(src_path, cv2.IMREAD_GRAYSCALE)
    tgt = cv2.imread(tgt_path, cv2.IMREAD_GRAYSCALE)
    return float(np.var(cv2.Laplacian(src, cv2.CV_64F))) > float(np.var(cv2.Laplacian(tgt, cv2.CV_64F)))

File Existence & Size Verification

def verify_file_exists_and_similar(src_path: str, tgt_path: str) -> bool:
    if not os.path.isfile(src_path):
        return False
    return check_ssim(src_path, tgt_path)

def verify_file_size(src_path: str, max_bytes: int) -> bool:
    return os.path.isfile(src_path) and os.path.getsize(src_path) < max_bytes

GIMP Config Verification

def verify_gimp_config(config_path: str, key, value: str) -> bool:
    """Check GIMP gimprc config file for key-value pair.
    key can be str or list of str (for multi-word keys)."""
    with open(config_path, "r") as f:
        for line in f:
            if line.startswith("#") or line.strip() == "":
                continue
            items = line.strip().lstrip("(").rstrip(")").split()
            if isinstance(key, str):
                if items[0] == key and items[-1] == value:
                    return True
            elif isinstance(key, list) and len(key) == 2:
                if items[0] == key[0] and items[1] == key[1] and items[-1] == value:
                    return True
    return False

---

3. XCF (GIMP Native Format) Reading

GIMP's native .xcf format stores layers, channels, and paths. Pillow cannot read XCF. Use these approaches:

Reading XCF with python-xcftools

import subprocess, json, re

def get_xcf_info(xcf_path: str) -> dict:
    """Extract XCF metadata: dimensions, layers, color mode."""
    result = subprocess.run(["xcfinfo", xcf_path], capture_output=True, text=True)
    info = {"layers": [], "width": 0, "height": 0}
    for line in result.stdout.strip().split("\n"):
        # First line: "Version X, WxH RGB color, N layers, ..."
        if line.startswith("Version"):
            m = re.search(r'(\d+)x(\d+)\s+(\w+)', line)
            if m:
                info["width"], info["height"] = int(m.group(1)), int(m.group(2))
                info["color_mode"] = m.group(3)
        # Layer lines: "+ WxH+X+Y RGB-alpha Normal name"
        elif line.startswith("+") or line.startswith("-"):
            parts = line.strip().split()
            visible = parts[0] == "+"
            # Layer name is last quoted or unquoted token
            name = " ".join(parts[4:]) if len(parts) > 4 else ""
            info["layers"].append({"name": name, "visible": visible})
    return info

def get_xcf_layer_count(xcf_path: str) -> int:
    info = get_xcf_info(xcf_path)
    return len(info["layers"])

def verify_xcf_has_layer(xcf_path: str, layer_name: str) -> bool:
    info = get_xcf_info(xcf_path)
    return any(layer_name.lower() in l["name"].lower() for l in info["layers"])

Flattening XCF to PNG for Comparison

def xcf_to_png(xcf_path: str, png_path: str) -> bool:
    """Flatten XCF to PNG using xcf2png (from xcftools package)."""
    result = subprocess.run(["xcf2png", xcf_path, "-o", png_path], capture_output=True)
    return result.returncode == 0

def xcf_extract_layer(xcf_path: str, layer_name: str, png_path: str) -> bool:
    """Extract a single layer from XCF to PNG."""
    result = subprocess.run(
        ["xcf2png", xcf_path, layer_name, "-o", png_path],
        capture_output=True
    )
    return result.returncode == 0

# Install on VM: sudo apt install xcftools

---

4. GIMP Script-Fu Batch Mode

For tasks that require GIMP-specific operations (filters, layer manipulation), use headless batch mode.

def gimp_batch(script_fu: str, timeout: int = 30) -> str:
    """Run a Script-Fu command in GIMP batch mode (headless)."""
    result = subprocess.run(
        ["gimp", "-i", "-b", script_fu, "-b", "(gimp-quit 0)"],
        capture_output=True, text=True, timeout=timeout,
        env={**os.environ, "DISPLAY": ":0"}
    )
    return result.stdout + result.stderr

# Flatten an XCF and export as PNG
gimp_batch('(let* ((image (car (gimp-file-load RUN-NONINTERACTIVE "/home/user/file.xcf" "file.xcf"))) '
           '(drawable (car (gimp-image-flatten image)))) '
           '(file-png-save RUN-NONINTERACTIVE image drawable "/home/user/flat.png" "flat.png" 0 9 1 1 1 1 1))')

# Apply Gaussian blur and export
gimp_batch('(let* ((image (car (gimp-file-load RUN-NONINTERACTIVE "/home/user/in.png" "in.png"))) '
           '(drawable (car (gimp-image-flatten image)))) '
           '(plug-in-gauss RUN-NONINTERACTIVE image drawable 10 10 0) '
           '(gimp-file-overwrite RUN-NONINTERACTIVE image drawable "/home/user/out.png" "out.png"))')

# Get image properties (layer count, dimensions)
gimp_batch('(let* ((image (car (gimp-file-load RUN-NONINTERACTIVE "/home/user/file.xcf" "file.xcf")))) '
           '(gimp-message (number->string (car (gimp-image-get-active-layer image)))) '
           '(gimp-message (number->string (car (gimp-image-width image)))))')

Guidelines:

• -i = no GUI, -b = batch command
• Always end with (gimp-quit 0) or GIMP hangs
• Script-Fu uses Scheme syntax with car to unwrap return lists
• Set timeout — GIMP batch can hang on missing fonts or corrupt files

---

5. Color Space & Color Verification

Color Temperature / White Balance Verification

def calculate_color_temperature_shift(src_path: str, tgt_path: str) -> tuple:
    """Calculate average RGB shift between two images. Returns (dr, dg, db)."""
    src = np.array(Image.open(src_path).convert("RGB"), dtype=np.float32)
    tgt = np.array(Image.open(tgt_path).convert("RGB"), dtype=np.float32)
    if src.shape != tgt.shape:
        tgt = np.array(Image.open(tgt_path).convert("RGB").resize(
            Image.open(src_path).size, Resampling.LANCZOS), dtype=np.float32)
    diff = np.mean(src - tgt, axis=(0, 1))
    return tuple(diff)

def verify_warmer_tone(src_path: str, tgt_path: str) -> bool:
    """Verify src has warmer tone than tgt (more red/yellow, less blue)."""
    dr, dg, db = calculate_color_temperature_shift(src_path, tgt_path)
    return dr > 2 and db < -2  # red increased, blue decreased

def verify_cooler_tone(src_path: str, tgt_path: str) -> bool:
    """Verify src has cooler tone than tgt (more blue, less red)."""
    dr, dg, db = calculate_color_temperature_shift(src_path, tgt_path)
    return dr < -2 and db > 2

Histogram-Based Color Distribution

def get_color_histogram(img_path: str, bins: int = 64) -> np.ndarray:
    """Get normalized RGB histogram."""
    img = np.array(Image.open(img_path).convert("RGB"))
    hist_r = np.histogram(img[:,:,0], bins=bins, range=(0,256))[0]
    hist_g = np.histogram(img[:,:,1], bins=bins, range=(0,256))[0]
    hist_b = np.histogram(img[:,:,2], bins=bins, range=(0,256))[0]
    hist = np.concatenate([hist_r, hist_g, hist_b]).astype(np.float32)
    return hist / hist.sum()

def verify_histogram_similar(src_path: str, tgt_path: str, threshold: float = 0.85) -> bool:
    """Compare color distributions via histogram correlation."""
    h1 = get_color_histogram(src_path)
    h2 = get_color_histogram(tgt_path)
    correlation = float(np.corrcoef(h1, h2)[0, 1])
    return correlation >= threshold

def verify_grayscale(img_path: str) -> bool:
    """Verify image is grayscale (R == G == B for all pixels, or mode is L)."""
    img = Image.open(img_path)
    if img.mode == "L":
        return True
    if img.mode in ("RGB", "RGBA"):
        arr = np.array(img.convert("RGB"))
        return bool(np.all(arr[:,:,0] == arr[:,:,1]) and np.all(arr[:,:,1] == arr[:,:,2]))
    return False

def verify_inverted(src_path: str, tgt_path: str, threshold: float = 0.95) -> bool:
    """Verify src is the color-inverted version of tgt."""
    src = np.array(Image.open(src_path).convert("RGB"), dtype=np.float32)
    tgt = np.array(Image.open(tgt_path).convert("RGB"), dtype=np.float32)
    if src.shape != tgt.shape:
        return False
    inverted_tgt = 255.0 - tgt
    mse = np.mean((src - inverted_tgt) ** 2)
    return mse < (1.0 - threshold) * 255 * 255

---

6. Gradual Scoring Pattern (reward-gen)

def compute_reward(src_path: str, golden_path: str, task_type: str = "transform",
                   checks: list = None) -> float:
    """Compute a 0.0-1.0 reward score with multiple verification dimensions.

    task_type: "transform" (modify existing), "create" (new image), "config" (GIMP settings)
    checks: optional list of (check_fn, weight) tuples for custom verification
    """
    score = 0.0
    total_weight = 0.0

    # Dimension 1: File existence (weight: 0.1)
    w = 0.1
    total_weight += w
    if os.path.isfile(src_path):
        score += w
        try:
            img = Image.open(src_path)
            img.verify()
            score += 0  # already counted above
        except Exception:
            score -= w * 0.5  # file exists but corrupted — partial credit

    # Dimension 2: Image dimensions match (weight: 0.15)
    if task_type in ("transform", "create"):
        w = 0.15
        total_weight += w
        try:
            src_img = Image.open(src_path)
            golden_img = Image.open(golden_path)
            if src_img.size == golden_img.size:
                score += w
            elif abs(src_img.size[0] - golden_img.size[0]) < 10 and \
                 abs(src_img.size[1] - golden_img.size[1]) < 10:
                score += w * 0.5  # close but not exact
        except Exception:
            pass

    # Dimension 3: Structural similarity (weight: 0.4)
    if task_type in ("transform", "create"):
        w = 0.4
        total_weight += w
        try:
            ssim_score = _compute_ssim(src_path, golden_path)
            score += w * max(0, ssim_score)
        except Exception:
            pass

    # Dimension 4: Color / histogram similarity (weight: 0.2)
    if task_type in ("transform", "create"):
        w = 0.2
        total_weight += w
        try:
            h1 = get_color_histogram(src_path)
            h2 = get_color_histogram(golden_path)
            correlation = float(np.corrcoef(h1, h2)[0, 1])
            score += w * max(0, correlation)
        except Exception:
            pass

    # Dimension 5: Task-specific checks (weight: 0.15)
    if checks:
        w = 0.15
        total_weight += w
        passed = sum(1 for fn, _ in checks if fn())
        score += w * (passed / len(checks))
    else:
        w = 0.15
        total_weight += w
        # Default: pixel-level closeness
        try:
            src_arr = np.array(Image.open(src_path).convert("RGB"), dtype=np.float32)
            gld_arr = np.array(Image.open(golden_path).convert("RGB"), dtype=np.float32)
            if src_arr.shape == gld_arr.shape:
                mse = np.mean((src_arr - gld_arr) ** 2)
                pixel_score = float(np.exp(-mse / 1000))
                score += w * pixel_score
        except Exception:
            pass

    return score / total_weight if total_weight > 0 else 0.0

def _compute_ssim(src_path: str, tgt_path: str) -> float:
    img1 = np.array(Image.open(src_path).convert("RGB"))
    img2 = np.array(Image.open(tgt_path).convert("RGB"))
    if img1.shape != img2.shape:
        img2 = np.array(Image.open(tgt_path).convert("RGB").resize(
            Image.open(src_path).size, Resampling.LANCZOS))
    min_dim = min(img1.shape[0], img1.shape[1])
    win_size = min(7, min_dim if min_dim % 2 == 1 else min_dim - 1)
    if win_size < 3:
        return 0.0
    try:
        return ssim(img1, img2, win_size=win_size, channel_axis=2)
    except TypeError:
        return ssim(img1, img2, win_size=win_size, multichannel=True)

Vision-Based Scoring (for semantic / non-deterministic tasks)

For tasks where the result cannot be exactly predicted (background removal, artistic effects, retouching, compositing), use call_vision_judge from reward_judge.py instead of golden comparison. The LLM compares the BEFORE and AFTER images visually.

When to use: task involves subjective or multi-solution operations (remove background, enhance photo, apply artistic filter, composite images, retouch blemish, etc.)

When NOT to use: task has a deterministic outcome (rotate 90°, crop to 800x600, convert to grayscale). Use compute_reward with golden comparison for those.

CRITICAL — Initial image preservation: The agent often overwrites the initial file in-place. initial_setup.py MUST save a reference copy at {TASK_ID}_initial_reference.<ext> (see Rule 6 in setup-gen.md). The reward script reads the BEFORE image from this reference, not the canonical artifact path.

import sys
sys.path.insert(0, "/tmp")
from reward_judge import call_vision_judge

def compute_reward_semantic(
    initial_path: str,
    result_path: str,
    task_instruction: str,
    property_checks: list = None,
) -> float:
    """Compute reward for semantic image tasks using vision LLM + property checks.

    initial_path: the BEFORE image (from {TASK_ID}_initial_reference — guaranteed unmodified)
    result_path: the AFTER image (agent's or golden result at {TASK_ID}.<ext>)
    task_instruction: the original task description
    property_checks: optional list of (check_fn, weight) tuples for measurable properties
    """
    score = 0.0
    total_weight = 0.0

    # Dimension 1: File exists and is valid image (weight: 0.1)
    w = 0.1
    total_weight += w
    if os.path.isfile(result_path):
        try:
            img = Image.open(result_path)
            img.verify()
            score += w
        except Exception:
            pass

    # Dimension 2: Something actually changed (weight: 0.1)
    w = 0.1
    total_weight += w
    try:
        src_arr = np.array(Image.open(initial_path).convert("RGB"))
        res_arr = np.array(Image.open(result_path).convert("RGB"))
        if src_arr.shape == res_arr.shape:
            diff = np.mean(np.abs(src_arr.astype(float) - res_arr.astype(float)))
            if diff > 2.0:  # not identical — agent did something
                score += w
        else:
            score += w  # different size = definitely changed
    except Exception:
        pass

    # Dimension 3: Measurable property checks (weight: 0.3)
    if property_checks:
        w = 0.3
        total_weight += w
        passed = sum(1 for fn, _ in property_checks if fn())
        score += w * (passed / len(property_checks))

    # Dimension 4: LLM Vision Judge — compare BEFORE vs AFTER (weight: 0.5)
    w = 0.5
    total_weight += w
    try:
        vision_score = call_vision_judge(
            task_instruction=task_instruction,
            initial_image=initial_path,
            result_image=result_path,
        )
        score += w * vision_score
    except Exception as e:
        print(f"Vision judge failed: {e}")

    return score / total_weight if total_weight > 0 else 0.0

# --- Example: "Remove the background from the image" ---
# reward.py reads _initial_reference (untouched) as BEFORE, canonical artifact as AFTER
#
# def compute_reward():
#     initial = f"/home/user/{TASK_ID}_initial_reference.png"  # BEFORE — saved by initial_setup.py
#     result  = f"/home/user/{TASK_ID}.png"                     # AFTER — overwritten by agent or golden_patch
#
#     property_checks = [
#         (lambda: Image.open(result).mode == "RGBA", 1.0),
#         (lambda: np.mean(np.array(Image.open(result).split()[-1]) < 10) > 0.1, 1.0),
#     ]
#
#     return compute_reward_semantic(
#         initial_path=initial,
#         result_path=result,
#         task_instruction="Remove the background from the image",
#         property_checks=property_checks,
#     )

---

7. Bitter Lessons

1. SSIM requires same-size images. structure_check_by_ssim returns False if sizes differ. Always resize before comparing. Use Resampling.LANCZOS for quality downsampling.

2. SSIM window size must be odd and <= image dimension. Default win_size=7 fails for images smaller than 7px on any side. Adapt: win_size = min(7, min_dim if min_dim % 2 == 1 else min_dim - 1).

3. channel_axis vs multichannel in scikit-image. Newer versions use channel_axis=2, older use multichannel=True. Always try/except both to support all environments.

4. Image mode must match for comparison. Comparing RGB to RGBA fails silently or gives wrong results. Always .convert("RGB") both images before SSIM or pixel comparison.

5. Transparency crops use alpha.getbbox(). To measure content size ignoring transparent pixels, get the alpha channel bounding box. getbbox() returns None for fully transparent images — handle this case.

6. img.convert("P") creates palette mode, but SSIM needs RGB. After verifying mode == "P", convert back to RGB for structure comparison. The palette conversion is lossy.

7. Brightness normalization before structure check. When comparing brightness-adjusted images, normalize both to the same target brightness (128) before MSE comparison. Otherwise the brightness difference dominates the structural metric.

8. Saturation comparison inverts direction. The evaluator checks tgt_saturation < src_saturation because src is the enhanced image. In GIMP tasks, the "source" result has higher saturation than the "target" reference.

9. Contrast SSIM threshold is lenient (0.65). Contrast adjustments change many pixels significantly. The evaluator uses 0.65 instead of the default 0.9 to account for this.

10. ImageChops.difference().getbbox() is None for identical images. The bounding box of the difference is None when no pixels differ. This is your "exact match" check.

11. Truncated/corrupted images need retry logic. GIMP export can produce truncated files if the export dialog times out. Use a retry mechanism: try opening 3 times with 0.5s delay.

12. Shape detection uses color frequency, not edge detection. The evaluator finds shapes by sorting pixel colors by frequency. The second-most-common color is the shape (background is most common). This fails if shape color equals background color.

13. Mirror check uses 0.99 SSIM, not exact match. GIMP's export introduces minor compression artifacts. Pixel-perfect comparison fails; use SSIM >= 0.99 for mirror verification.

14. GIMP config (gimprc) uses S-expression format. Lines are (key value) with parentheses, not key=value. Parse by stripping parens and splitting on whitespace.

15. XCF files cannot be read by Pillow. Use xcftools (xcfinfo, xcf2png) or GIMP batch mode. Install with sudo apt install xcftools. Always flatten XCF to PNG before SSIM comparison.

16. GIMP batch mode hangs without (gimp-quit 0). Always pass two -b flags: one for your command, one for (gimp-quit 0). Set a subprocess timeout (30s) as safety net.

17. Script-Fu car is needed to unwrap return values. Most GIMP Script-Fu procedures return lists. (gimp-image-width image) returns (1920), not 1920. Use (car ...) to get the scalar.

18. Color temperature shifts need float arithmetic. When comparing warm/cool tone shifts, use dtype=np.float32 to avoid uint8 underflow in subtraction (200 - 210 wraps to 246 in uint8).