Image Annotations

Image Annotations

Add visual callouts to any image — screenshots, diagrams, architecture docs, demo frames — using PIL/Pillow. Highlights what changed or what to look at, so reviewers don't have to guess.

When to Use This Skill

Use this skill when you need to:

• Highlight a specific area in a screenshot for a PR description
• Annotate before/after images to show what changed
• Add labels and callouts to diagrams or architecture images
• Create annotated frames for animated GIF demos

Prerequisites

pip install Pillow -q

Color Rules

• Red (#E63946) — only for "bad" / "removed" things (e.g., circling a bug being fixed)
• Yellowish-orange (#FF9F1C) — for neutral highlights ("look here", "new feature", etc.)
• Never use red just because it's eye-catching — red = bad/removed

Font

• Use Ink Free (C:/Windows/Fonts/Inkfree.ttf) for a handwritten look on Windows
• On Linux/macOS, fall back to ImageFont.load_default()
• Size 36 for annotations on ~1400px-wide images
• stroke_width=1 with stroke_fill=<same color as fill> — gives body without being too thick
• Do NOT use white stroke — looks like a bad glow effect

Shapes

• Prefer rounded rectangles over circles/ellipses — less pixelation at edges
• draw.rounded_rectangle([x1, y1, x2, y2], radius=14, outline=color, width=5)
• Padding 18px around the target content

Reference Snippet

from PIL import Image, ImageDraw, ImageFont

# Setup
font = ImageFont.truetype('C:/Windows/Fonts/Inkfree.ttf', 36)  # or load_default()
color = '#FF9F1C'  # orange for highlights
stroke = 5
pad = 18

img = Image.open('screenshot.png')
draw = ImageDraw.Draw(img)

# Rounded rect with padding
draw.rounded_rectangle(
    [x1 - pad, y1 - pad, x2 + pad, y2 + pad],
    radius=14, outline=color, width=stroke
)

# Leader line (same thickness as rect)
draw.line([x2 + pad, cy, x2 + pad + 40, cy - 30], fill=color, width=stroke)

# Label — same-color stroke for body, NO white stroke
draw.text(
    (x2 + pad + 45, cy - 60), 'label text',
    fill=color, font=font, stroke_width=1, stroke_fill=color
)

img.save('annotated.png')

Algorithmic Annotation — annotate.py

For images with multiple elements to annotate, use the annotate.py module below. Save it next to your script and import from it. It handles automatic label placement without overlapping.

Quick start

from annotate import annotate_image

result = annotate_image(
    'screenshot.png',
    [
        {'elem': (560, 275, 635, 390), 'label': 'button', 'draw_box': True},
        {'elem': (105, 453, 236, 470), 'label': 'status text'},
    ],
    debug=True,
)
result.save('annotated.png')

• elem: (x1, y1, x2, y2) tight bounding box — must be exact pixel coordinates
• label: text label (supports \n for multi-line)
• draw_box: if True, draws a rounded rectangle around the element. If False (default), draws a V-arrowhead pointing at the element
• debug: shows targeting rectangles and candidate heatmap for placement validation

Coordinate grid helper

Always use grid_image() before annotating an unfamiliar image. Scaled-down previews display images smaller than actual pixel dimensions — the error compounds as you move away from (0,0).

from annotate import grid_image

grid = grid_image('screenshot.png', step=100)
grid.save('grid.png')

Then verify with small crops:

from PIL import Image
img = Image.open('screenshot.png')
crop = img.crop((x1 - 20, y1 - 20, x2 + 20, y2 + 20))
crop.save('verify.png')

Algorithm overview

1. Ring search: candidates between MIN_ARROW (25px) and MAX_ARROW (120px) from element edge
2. Contrast scoring: prefers placements where label text is readable — abs(avg_brightness - 147) - std * 0.3 - dist * 0.02
3. Joint resolution: candidates computed independently, placed greedily (best score first)
4. Hard blocks: labels cannot overlap any other annotation's element or breathing box
5. Proximity penalty: labels within 40px of other placed boxes get a score penalty
6. Arrow crossing penalty: -50 for arrows crossing already-placed arrows

Debug mode colors

Arrow styles

• draw_box=True: rounded rectangle + straight line to label, no arrowhead
• draw_box=False: V-shaped arrowhead with rounded line caps

annotate.py — full module

Save this as annotate.py and import from it:

"""
Algorithmic screenshot annotation with automatic label placement.

pip install Pillow numpy
Optional for diff_images: pip install scipy
"""
import math
import numpy as np
from PIL import Image, ImageDraw, ImageFont

# --- Defaults ---
DEFAULT_FONT = 'C:/Windows/Fonts/Inkfree.ttf'
DEFAULT_FONT_SIZE = 32
DEFAULT_COLOR = '#FF9F1C'
DEFAULT_STROKE = 5
MIN_ARROW = 25
MAX_ARROW = 120
TEXT_PAD = 6
BREATH = 18
CROSSING_PENALTY = 50
PROXIMITY_MARGIN = 40
PROXIMITY_PENALTY = 50


def _rect_intersects(a, b):
    return a[0] < b[2] and a[2] > b[0] and a[1] < b[3] and a[3] > b[1]


def _segments_intersect(p1, p2, p3, p4):
    def cross(o, a, b):
        return (a[0] - o[0]) * (b[1] - o[1]) - (a[1] - o[1]) * (b[0] - o[0])
    d1, d2 = cross(p3, p4, p1), cross(p3, p4, p2)
    d3, d4 = cross(p1, p2, p3), cross(p1, p2, p4)
    return ((d1 > 0 and d2 < 0) or (d1 < 0 and d2 > 0)) and \
           ((d3 > 0 and d4 < 0) or (d3 < 0 and d4 > 0))


def _line_rect_exit(cx, cy, tx, ty, rect):
    x1, y1, x2, y2 = rect
    dx, dy = tx - cx, ty - cy
    tmin, tmax = 0.0, 1.0
    for lo, hi, p, d in [(x1, x2, cx, dx), (y1, y2, cy, dy)]:
        if abs(d) < 1e-9:
            continue
        t0, t1 = (lo - p) / d, (hi - p) / d
        if t0 > t1:
            t0, t1 = t1, t0
        tmin, tmax = max(tmin, t0), min(tmax, t1)
    return (cx + dx * tmax, cy + dy * tmax)


def _rect_gap(a, b):
    dx = max(a[0] - b[2], b[0] - a[2], 0)
    dy = max(a[1] - b[3], b[1] - a[3], 0)
    if dx == 0 and dy == 0:
        return 0
    return math.sqrt(dx**2 + dy**2)


def _find_candidates(pixels, W, H, cyan, pw, ph, font):
    cx, cy = (cyan[0] + cyan[2]) / 2, (cyan[1] + cyan[3]) / 2
    excl_zone = (cyan[0] - MIN_ARROW, cyan[1] - MIN_ARROW,
                 cyan[2] + MIN_ARROW, cyan[3] + MIN_ARROW)
    sx1 = max(0, cyan[0] - MAX_ARROW - pw)
    sy1 = max(0, cyan[1] - MAX_ARROW - ph)
    sx2 = min(W - pw, cyan[2] + MAX_ARROW)
    sy2 = min(H - ph, cyan[3] + MAX_ARROW)
    step_x = max(8, min(pw // 2, MAX_ARROW // 3))
    step_y = max(8, min(ph // 2, MAX_ARROW // 3))
    cands = []
    for px in range(sx1, sx2, step_x):
        for py in range(sy1, sy2, step_y):
            pink = (px, py, px + pw, py + ph)
            if _rect_intersects(pink, excl_zone):
                continue
            gl, gr = cyan[0] - pink[2], pink[0] - cyan[2]
            gt, gb = cyan[1] - pink[3], pink[1] - cyan[3]
            hd, vd = max(gl, gr, 0), max(gt, gb, 0)
            ed = math.sqrt(hd**2 + vd**2) if (hd > 0 and vd > 0) else max(hd, vd)
            if ed > MAX_ARROW:
                continue
            region = pixels[py:py + ph, px:px + pw, :3].astype(float)
            score = abs(np.mean(region) - 147) - np.std(region) * 0.3
            dist = math.sqrt((px + pw/2 - cx)**2 + (py + ph/2 - cy)**2)
            score -= dist * 0.02
            cands.append(((px, py), score))
    return cands


def _resolve_placements(annots, font):
    placed = []
    all_elem_zones = []
    for ann in annots:
        all_elem_zones.append(ann['cyan'])
        if ann.get('draw_box', False):
            c = ann['cyan']
            all_elem_zones.append((c[0]-BREATH, c[1]-BREATH, c[2]+BREATH, c[3]+BREATH))
    for ann in sorted(annots, key=lambda a: -a['best_score']):
        pw, ph = ann['pw'], ann['ph']
        cyan = ann['cyan']
        cx, cy = ann['cyan_center']
        draw_box = ann.get('draw_box', False)
        best_pos, best_score = None, -999
        valid = []
        for (px, py), score in ann['candidates']:
            pink = (px, py, px + pw, py + ph)
            ok = True
            for ez in all_elem_zones:
                if ez == cyan:
                    continue
                if ann.get('draw_box', False):
                    own_viz = (cyan[0]-BREATH, cyan[1]-BREATH, cyan[2]+BREATH, cyan[3]+BREATH)
                    if ez == own_viz:
                        continue
                if _rect_intersects(pink, ez):
                    ok = False; break
            if not ok:
                continue
            for p_pink, p_excl, p_viz, _ in placed:
                if _rect_intersects(pink, p_pink) or _rect_intersects(pink, p_excl):
                    ok = False; break
                if p_viz and _rect_intersects(pink, p_viz):
                    ok = False; break
            if not ok:
                continue
            for p_pink, p_excl, p_viz, _ in placed:
                for rect in [p_pink, p_excl, p_viz]:
                    if rect is None:
                        continue
                    gap = _rect_gap(pink, rect)
                    if gap < PROXIMITY_MARGIN:
                        score -= PROXIMITY_PENALTY * (1 - gap / PROXIMITY_MARGIN)
            for ez in all_elem_zones:
                if ez == cyan:
                    continue
                gap = _rect_gap(pink, ez)
                if gap < PROXIMITY_MARGIN:
                    score -= PROXIMITY_PENALTY * (1 - gap / PROXIMITY_MARGIN)
            tcx, tcy = px + pw/2, py + ph/2
            cand_start = _line_rect_exit(tcx, tcy, cx, cy, pink)
            if draw_box:
                viz = (cyan[0]-BREATH, cyan[1]-BREATH, cyan[2]+BREATH, cyan[3]+BREATH)
                cand_end = _line_rect_exit(cx, cy, tcx, tcy, viz)
            else:
                cand_end = _line_rect_exit(cx, cy, tcx, tcy, cyan)
            for _, _, _, pa in placed:
                if pa and _segments_intersect(cand_start, cand_end, pa[0], pa[1]):
                    score -= CROSSING_PENALTY; break
            valid.append(((px, py), score))
            if score > best_score:
                best_score, best_pos = score, (px, py)
        ann['valid_candidates'] = valid
        if best_pos is None:
            ann['pink'] = ann['tpos'] = ann['astart'] = ann['aend'] = ann['viz'] = None
            continue
        px, py = best_pos
        pink = (px, py, px + pw, py + ph)
        ann['pink'] = pink
        ann['tpos'] = (px + TEXT_PAD, py + TEXT_PAD)
        tcx, tcy = px + pw/2, py + ph/2
        ann['astart'] = _line_rect_exit(tcx, tcy, cx, cy, pink)
        if draw_box:
            viz = (cyan[0]-BREATH, cyan[1]-BREATH, cyan[2]+BREATH, cyan[3]+BREATH)
            ann['viz'] = viz
            ann['aend'] = _line_rect_exit(cx, cy, tcx, tcy, viz)
        else:
            ann['viz'] = None
            ann['aend'] = _line_rect_exit(cx, cy, tcx, tcy, cyan)
        placed.append((pink, ann['excl_zone'], ann['viz'], (ann['astart'], ann['aend'])))


def _draw_debug(img, annots, color):
    overlay = Image.new('RGBA', img.size, (0, 0, 0, 0))
    od = ImageDraw.Draw(overlay)
    for ann in annots:
        cands = ann.get('valid_candidates', ann['candidates'])
        if not cands:
            continue
        pw, ph = ann['pw'], ann['ph']
        scores = [s for _, s in cands]
        smin, smax = min(scores), max(scores)
        rng = smax - smin if smax > smin else 1
        for (px, py), score in cands:
            t = (score - smin) / rng
            if t < 0.5:
                r_c, g_c, b_c = 220, int(180 * (t * 2)), 0
            else:
                r_c, g_c, b_c = int(220 * (1 - (t-0.5)*2)), 200, 0
            alpha_fill = int(40 + 70 * t)
            alpha_out = int(80 + 120 * t)
            od.rectangle((px, py, px + pw, py + ph),
                         fill=(r_c, g_c, b_c, alpha_fill), outline=(r_c, g_c, b_c, alpha_out), width=1)
    for ann in annots:
        ez = ann['excl_zone']
        od.rectangle(ez, fill=(120, 120, 120, 50), outline=(160, 160, 160, 160), width=1)
        od.rectangle(ann['cyan'], fill=(0, 255, 255, 30), outline=(0, 255, 255, 180), width=2)
        if ann.get('pink'):
            od.rectangle(ann['pink'], fill=(255, 0, 255, 50),
                         outline=(255, 0, 255, 180), width=2)
    return Image.alpha_composite(img, overlay)


def _draw_annotations(img, annots, font, color, stroke_width):
    draw = ImageDraw.Draw(img)
    for ann in annots:
        if ann.get('viz'):
            draw.rounded_rectangle(ann['viz'], radius=12, outline=color, width=stroke_width)
        tpos = ann.get('tpos')
        astart, aend = ann.get('astart'), ann.get('aend')
        if not (tpos and astart and aend):
            continue
        sx, sy = int(astart[0]), int(astart[1])
        ex, ey = int(aend[0]), int(aend[1])
        draw.line([(sx, sy), (ex, ey)], fill=color, width=4, joint='curve')
        r = 2
        draw.ellipse([(sx-r, sy-r), (sx+r, sy+r)], fill=color)
        draw.ellipse([(ex-r, ey-r), (ex+r, ey+r)], fill=color)
        if not ann.get('draw_box', False):
            angle = math.atan2(ey - sy, ex - sx)
            al, spread = 18, 0.45
            ax = ex - al * math.cos(angle - spread)
            ay = ey - al * math.sin(angle - spread)
            bx = ex - al * math.cos(angle + spread)
            by = ey - al * math.sin(angle + spread)
            draw.line([(int(ax), int(ay)), (ex, ey)], fill=color, width=4)
            draw.line([(int(bx), int(by)), (ex, ey)], fill=color, width=4)
            for px_, py_ in [(int(ax), int(ay)), (int(bx), int(by))]:
                draw.ellipse([(px_-r, py_-r), (px_+r, py_+r)], fill=color)
        draw.text(tpos, ann['label'], fill=color, font=font,
                  stroke_width=1, stroke_fill=color)
    return img


def annotate_image(image_path, annotations, *,
                   debug=False,
                   font_path=DEFAULT_FONT,
                   font_size=DEFAULT_FONT_SIZE,
                   color=DEFAULT_COLOR,
                   stroke_width=DEFAULT_STROKE):
    """
    Annotate a screenshot with automatic label placement.

    Args:
        image_path: path to the input image
        annotations: list of dicts with keys:
            - elem: (x1, y1, x2, y2) tight bounding box of element
            - label: text label string
            - draw_box: (optional, default False) draw rounded rect around element
        debug: if True, draw developer rectangles (cyan/pink/gray/heatmap)
        font_path: path to TTF font file
        font_size: font size in pixels
        color: hex color for annotations (default orange #FF9F1C)
        stroke_width: width of orange highlight box outline

    Returns:
        PIL.Image with annotations drawn
    """
    font = ImageFont.truetype(font_path, font_size)
    img = Image.open(image_path).convert('RGBA')
    pixels = np.array(img)
    W, H = img.size
    annots = []
    for i, spec in enumerate(annotations):
        eb = spec['elem']
        em_pad = min(20, max(10, (eb[2] - eb[0]) // 10))
        cyan = (eb[0] - em_pad, eb[1] - em_pad, eb[2] + em_pad, eb[3] + em_pad)
        lines = spec['label'].split('\n')
        tw = max(font.getbbox(line)[2] - font.getbbox(line)[0] for line in lines)
        bbox = font.getbbox('Ay')
        line_h = bbox[3] - bbox[1]
        th = line_h * len(lines) + 4 * (len(lines) - 1)
        pw, ph = tw + 2 * TEXT_PAD, th + 2 * TEXT_PAD
        cands = _find_candidates(pixels, W, H, cyan, pw, ph, font)
        annots.append({
            'id': i,
            'label': spec['label'],
            'draw_box': spec.get('draw_box', False),
            'cyan': cyan,
            'cyan_center': ((cyan[0]+cyan[2])/2, (cyan[1]+cyan[3])/2),
            'excl_zone': (cyan[0]-MIN_ARROW, cyan[1]-MIN_ARROW,
                          cyan[2]+MIN_ARROW, cyan[3]+MIN_ARROW),
            'pw': pw, 'ph': ph,
            'candidates': cands,
            'best_score': max((s for _, s in cands), default=-999),
        })
    _resolve_placements(annots, font)
    annots.sort(key=lambda a: a['id'])
    if debug:
        img = _draw_debug(img, annots, color)
    img = _draw_annotations(img, annots, font, color, stroke_width)
    return img


def diff_images(before_path, after_path, *, threshold=30, min_pixels=300,
                dilate=5, debug=False):
    """Find changed regions between two screenshots and return cluster boxes.

    Returns (clusters, debug_img_or_None):
        clusters: list of (x1, y1, x2, y2, pixel_count) sorted largest-first
        debug_img: if debug=True, PIL Image with heatmap overlay and cluster boxes
    """
    from scipy import ndimage
    img_a = Image.open(before_path).convert('RGB')
    img_b = Image.open(after_path).convert('RGB')
    if img_a.size != img_b.size:
        raise ValueError(f"Image sizes differ: {img_a.size} vs {img_b.size}")
    arr_a = np.array(img_a, dtype=np.float32)
    arr_b = np.array(img_b, dtype=np.float32)
    W, H = img_a.size
    diff = np.abs(arr_b - arr_a).max(axis=2)
    mask = diff > threshold
    dilated = ndimage.binary_dilation(mask, iterations=dilate)
    labeled, n_clusters = ndimage.label(dilated)
    clusters = []
    for i in range(1, n_clusters + 1):
        ys, xs = np.where(labeled == i)
        if len(ys) < min_pixels:
            continue
        clusters.append((int(xs.min()), int(ys.min()),
                          int(xs.max()), int(ys.max()), len(ys)))
    clusters.sort(key=lambda c: -c[4])
    debug_img = None
    if debug:
        overlay = img_b.copy().convert('RGBA')
        norm = np.clip(diff / 255.0, 0, 1)
        show_mask = diff > 10
        r = np.clip((norm * 2) * 255, 0, 255).astype(np.uint8)
        g = np.clip((1 - np.abs(norm - 0.5) * 2) * 200, 0, 200).astype(np.uint8)
        b = np.clip((1 - norm) * 255, 0, 255).astype(np.uint8)
        a = np.where(show_mask, np.clip(norm * 200 + 40, 40, 220).astype(np.uint8), 0)
        heat = Image.fromarray(np.stack([r, g, b, a], axis=2), 'RGBA')
        overlay = Image.alpha_composite(overlay, heat)
        draw = ImageDraw.Draw(overlay)
        try:
            font = ImageFont.truetype('C:/Windows/Fonts/consola.ttf', 18)
        except OSError:
            font = ImageFont.load_default()
        for idx, (x1, y1, x2, y2, px_count) in enumerate(clusters):
            draw.rectangle([x1, y1, x2, y2], outline=(0, 255, 255, 200), width=3)
            label = f"#{idx+1}  {px_count:,}px"
            bbox = font.getbbox(label)
            tw, th = bbox[2] - bbox[0], bbox[3] - bbox[1]
            lx, ly = x1, max(0, y1 - th - 8)
            draw.rectangle([lx, ly, lx + tw + 8, ly + th + 4], fill=(0, 0, 0, 180))
            draw.text((lx + 4, ly + 2), label, fill=(0, 255, 255, 255), font=font)
        debug_img = overlay
    return clusters, debug_img


def grid_image(image_path, step=100):
    """Draw a coordinate grid on an image for precise element location."""
    img = Image.open(image_path).convert('RGBA')
    draw = ImageDraw.Draw(img)
    W, H = img.size
    try:
        font = ImageFont.truetype('C:/Windows/Fonts/consola.ttf', 14)
    except OSError:
        font = ImageFont.load_default()
    for x in range(0, W, step):
        draw.line([(x, 0), (x, H)], fill=(255, 0, 0, 120), width=1)
        draw.text((x + 2, 2), str(x), fill=(255, 0, 0, 200), font=font)
    for y in range(0, H, step):
        draw.line([(0, y), (W, y)], fill=(255, 0, 0, 120), width=1)
        draw.text((2, y + 2), str(y), fill=(255, 0, 0, 200), font=font)
    return img

Image Diffing

Find what changed between two screenshots programmatically. Use as a safety net for subtle changes — when the difference is obvious, annotate directly instead.

from annotate import diff_images

clusters, debug_img = diff_images(
    'before.png', 'after.png',
    threshold=30,     # pixel difference floor (0-255)
    min_pixels=300,   # ignore tiny noise clusters
    dilate=5,         # merge nearby changed pixels
    debug=True,       # render heatmap overlay
)

# clusters = [(x1, y1, x2, y2, pixel_count), ...] sorted largest-first
if debug_img:
    debug_img.save('diff-debug.png')

# Feed clusters into annotate_image:
annotations = [
    {'elem': (x1, y1, x2, y2), 'label': f'Change #{i+1}', 'draw_box': True}
    for i, (x1, y1, x2, y2, _) in enumerate(clusters[:3])
]

Debug heatmap colors: Blue = small difference, Yellow = medium, Red = large, Cyan boxes = cluster bounding boxes.

When to use: subtle opacity changes, dashed lines, minor color shifts, anti-aliasing differences. When NOT to use: any change you can see by eye — annotate directly for better labels.

Animated GIF Annotations

Different from static images — animations have timing, transitions, and competing visual motion.

Element highlighting

1. Rects for big areas, arrows for small elements — 500x300px area = rect, 200x25px element = arrow
2. Labels go RIGHT NEXT to what they describe — short arrow (30-80px), label adjacent. Viewer's eye shouldn't travel more than ~100px
3. Arrow must not cross its own label — pick the edge closest to the target
4. No bottom bar / subtitle approach — eyes jump between content and bar. Contextual placement only
5. Hero message gets a bigger font — main takeaway 64pt+, detail annotations 38pt

Timing and pacing

6. Fade: 2-frame pop-in at 10fps — 50% → 100% opacity (0.2s total). Easing curves look bad at low FPS
7. Type → pause → annotate — during fast action, show NO annotation. Pause, then add it
8. Variable frame duration — fast during action (100ms), slow during pauses (600-800ms), long hold for hero (500ms)
9. Higher FPS for smooth motion — 10fps minimum for typing/interaction

Pop-in fade implementation

# 2-frame pop-in at 10fps
FADE_ALPHAS = [0.50, 1.00]

for frame_idx in range(total_frames):
    if annotation_just_changed and local_idx < len(FADE_ALPHAS):
        alpha = FADE_ALPHAS[local_idx]
    else:
        alpha = 1.0
    # Apply alpha to annotation elements:
    # - pill background: fill=(r, g, b, int(base_alpha * alpha))
    # - text: fill=(*color, int(255 * alpha))
    # - rect outline: outline=(*color, int(255 * alpha))

Guidelines

1. All elements same thickness — rect width, line width, and visual text weight should feel consistent (~5px)
2. Place labels close to the rect — short leader line (25-35px)
3. Labels can overlap content — the stroke gives enough contrast
4. Show locally first — verify before uploading to a PR
5. Take screenshots at native 1x, control display size in HTML — use <img width="300"> in markdown, never resize with PIL (creates artifacts)
6. Always check Image.open(path).size first — HiDPI screenshots are larger than they appear (150% scaling = 1.5x CSS pixel dimensions)
7. Short labels work better — wide labels have fewer valid placements. Use 1-3 words when possible
8. Verify with debug=True — always check the first annotation of a new image with debug mode

Limitations

• Ink Free font is Windows-only; other platforms need a fallback font
• PIL text rendering is basic — no rich text, no markdown
• Animated GIF annotations require frame-by-frame processing which can be slow for long recordings
• Algorithmic placement works best with 2-6 annotations; more than that may produce crowded results