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Problem

You need to add watermarks to hundreds of different images to protect copyright, add branding, or mark drafts.

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Solution

What’s in this recipe:
  • Create simple text watermarks
  • Test transformations before applying
  • Apply to multiple images automatically
You add watermarks to images using a custom UDF that wraps Pillow’s ImageDraw (relies on PIL/Pillow). This gives you full control over watermark placement, font, transparency, and color. You can iterate on transformations before adding them to your table. Use .select() with .collect() to preview results on sample images—nothing is stored in your table. If you want to collect only the first few rows, use .head(n) instead of .collect(). Once you’re satisfied, use .add_computed_column() to apply watermarks to all images in your table. For more on this workflow, see Get fast feedback on transformations.

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Setup

%pip install -qU pixeltable

import pixeltable as pxt
from PIL import Image, ImageDraw, ImageFont

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Load images

# Create a fresh directory (drop existing if present)
pxt.drop_dir('image_demo', force=True)
pxt.create_dir('image_demo')

Connected to Pixeltable database at: postgresql+psycopg://postgres:@/pixeltable?host=/Users/alison-pxt/.pixeltable/pgdata
Created directory ‘image_demo’.
<pixeltable.catalog.dir.Dir at 0x135490e90>
t = pxt.create_table('image_demo.watermarks', {'image': pxt.Image})

Created table ‘watermarks’.
t.insert([
    {'image': 'https://raw.githubusercontent.com/pixeltable/pixeltable/main/docs/resources/images/000000000049.jpg'},
    {'image': 'https://raw.githubusercontent.com/pixeltable/pixeltable/main/docs/resources/images/000000000036.jpg'},
    {'image': 'https://raw.githubusercontent.com/pixeltable/pixeltable/main/docs/resources/images/000000000090.jpg'},
])

Inserting rows into `watermarks`: 3 rows [00:00, 891.46 rows/s]
Inserted 3 rows with 0 errors.
3 rows inserted, 6 values computed.

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Iterate: Add watermarks to a few images first

@pxt.udf
def add_watermark(img: Image.Image, text: str) -> Image.Image:
    """Add a watermark to bottom-right corner."""
    img = img.copy().convert('RGBA')
    overlay = Image.new('RGBA', img.size, (0, 0, 0, 0))
    draw = ImageDraw.Draw(overlay)

    # Draw white text in bottom-right corner
    font = ImageFont.load_default(size=40)
    position = (img.width - 150, img.height - 60)
    draw.text(position, text, font=font, fill=(255, 255, 255, 200))

    result = Image.alpha_composite(img, overlay)
    return result.convert('RGB')

# Test on first image
t.select(t.image, add_watermark(t.image, '© 2024')).head(1)

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Add: Add watermarks to all images in your table

# Add watermark to all images
t.add_computed_column(watermarked=add_watermark(t.image, '© 2024'))

Added 3 column values with 0 errors.
3 rows updated, 3 values computed.
# View all results
t.collect()

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Explanation

How the watermark technique works: The UDF creates a transparent overlay on top of each image. The overlay is created with the same dimensions as the image (Image.new('RGBA', img.size, ...)), so watermarks adapt automatically whether you’re processing small thumbnails or large photos. The function draws white text with semi-transparent fill (alpha=200, where 255 is fully opaque), composites the overlay onto the original image using Image.alpha_composite(), and converts back to RGB since most image formats don’t support transparency. To customize the UDF: - Position: Change the (x, y) coordinates in the position variable - Color: Modify the (R, G, B, Alpha) fill value (0-255 for each) - Size: Adjust the font size parameter in ImageFont.load_default(size=40) - Font: Use ImageFont.truetype('path/to/font.ttf', size) for custom fonts The Pixeltable workflow: In traditional databases, .select() just picks which columns to view. In Pixeltable, .select() also lets you compute new transformations on the fly—define new columns without storing them. This makes .select() perfect for testing transformations before you commit them. When you use .select(), you’re creating a query that doesn’t execute until you call .collect(). You must use .collect() to execute the query and return results—nothing is stored in your table. If you want to collect only the first few rows, use .head(n) instead of .collect() to test on a subset before processing your full dataset. Once satisfied, use .add_computed_column() with the same expression to persist results permanently. For more on this workflow, see Get fast feedback on transformations.

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See also