RESPONSIZER

A command-line tool for batch generating multiple image sizes for responsive web design. If you're building websites with srcset or <picture> elements, you know the pain: for every image, you need 4, 6, sometimes 10+ size variants — each correctly sized, named, and optimized. Responsizer does this in a single command.

Drop your source images into a folder, tell Responsizer what widths (or heights) you need, and it generates all variants with consistent naming ready for your HTML. It can also produce WebP and AVIF copies alongside the originals for modern browsers.

Responsizer is a Python script that runs in the terminal. It requires Python and the Pillow library. It should work on macOS, Windows, and Linux, but has only been tested on macOS so far.

Input: your-image.png (2400×1600 px)

Command:

responsizer 1920,1280,960,640,320

Output (in output/ folder):

your-image-1920w.png
your-image-1280w.png
your-image-960w.png
your-image-640w.png
your-image-320w.png

All images in the folder, all sizes, aspect ratio preserved, done.

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Usage

Basic command

responsizer 1920,1280,960,640,320

Processes all images in the current directory (PNG, JPG, WebP, AVIF, GIF, TIFF, BMP), creates variants at the specified widths. Height is calculated automatically (preserves aspect ratio). Output goes to ./output/.

Specify source and output directories

responsizer 1920,1280,640,320 --source ./my-photos --output ./web-images

Force output format

responsizer 1920,1280,640 --format webp:85

The number after the colon is quality (0–100). Works with:

• webp:85 — WebP at quality 85
• jpg:90 — JPEG at quality 90
• png — PNG (lossless, no quality setting)
• avif:75 — AVIF at quality 75

Generate WebP / AVIF copies alongside originals

The most practical option for modern web — you get the original format AND WebP and/or AVIF copies:

responsizer 1920,1280,640,320 --also-webp 85

Output:

your-image-1920w.png
your-image-1920w.webp
your-image-1280w.png
your-image-1280w.webp
your-image-640w.png
your-image-640w.webp
your-image-320w.png
your-image-320w.webp

Both WebP and AVIF at once:

responsizer 1920,1280,640 --also-webp 85 --also-avif 75

Resize by height

Default is resize by width. If you need height-based sizing:

responsizer 1080,720,480 --by height

Output: your-image-1080h.png, your-image-720h.png, etc.

Change naming style

Default style is 1280w (number + letter). If you prefer w1280 (letter + number):

responsizer 1920,1280,640 --label before

Output: your-image-w1280.png instead of your-image-1280w.png.

Dry run (preview)

Want to see what would happen without creating any files?

responsizer 1920,1280,960,640,320 --dry-run

Allow upscaling

Default behavior: if a source image is smaller than the requested size, that size is skipped (to avoid quality loss from upscaling). To override:

responsizer 1920,1280,640 --allow-upscale

Keep EXIF metadata

Default: EXIF is stripped (smaller files, no GPS coordinates on the web). To keep it:

responsizer 1920,1280,640 --keep-exif

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All parameters

Parameter	What it does	Default
1920,1280,640	Sizes in pixels (required, or use --preset)	—
--source ./dir	Source directory	. (current)
--output ./dir	Output directory	./output
--by width/height	Resize by width or height	width
--label after/before	1280w or w1280 style	after
--format webp:85	Force output format + quality	keep original
--also-webp 85	Also generate WebP copies	—
--also-avif 75	Also generate AVIF copies	—
--allow-upscale	Allow upscaling	skip
--keep-exif	Keep EXIF metadata	strip
--preset hero	Load sizes from config	—
--config ./file	Path to config file	.responsizer.json
--dry-run	Preview only, no files created	—
--version	Show version	—

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Presets

Save your commonly used size sets so you don't have to type them every time.

Create a .responsizer.json file in your home directory or project folder:

{
  "presets": {
    "hero": [1920, 1280, 960, 640, 320],
    "card": [960, 640, 480, 320],
    "thumb": [300, 150]
  }
}

Then just:

responsizer --preset hero
responsizer --preset card --also-webp 85

Responsizer looks for the config file in this order:

1. --config /path/to/file.json (if specified)
2. .responsizer.json in the current directory
3. .responsizer.json in the home directory (~)

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Smart renaming

Responsizer automatically detects if a filename already contains a size label.

Input: banner-1920w.png

responsizer 1280,960,640

Output:

banner-1280w.png  (NOT banner-1920w-1280w.png)
banner-960w.png
banner-640w.png

Works with both styles — detects 1920w, w1920, 1080h, and h1080.

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WebP and AVIF — better compression

Responsizer can use external tools for better compression if they are installed on your system. If not, it falls back to the built-in Pillow library (works fine, just slightly larger files).

When an external tool is used, you'll see [cwebp] or [avifenc] next to each file in the output.

cwebp (by Google, for WebP):

# macOS:
brew install webp

# Windows: download from https://developers.google.com/speed/webp/download

# Linux:
sudo apt install webp

avifenc (for AVIF):

# macOS:
brew install libavif

# Windows / Linux: download from https://github.com/AOMediaCodec/libavif/releases

Installing these tools is optional. Responsizer works without them.

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Supported formats

PNG, JPG/JPEG, WebP, AVIF, GIF, TIFF, BMP — as both input and output.

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Real-world examples

Website reference screenshots

cd www/img/references/project/
responsizer 1920,1280,960,640,320 --also-webp 85

E-commerce product photos

responsizer --preset product --also-webp 85 --also-avif 75 --source ./products

Bulk conversion to WebP

responsizer 1920,1280,640 --format webp:85

Preview before running

responsizer 1920,1280,960,640,320 --dry-run

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Installation

1. Install Python

macOS:

brew install python

(If you don't have Homebrew, install it from https://brew.sh)

Windows:

• Download from https://www.python.org/downloads/
• During installation, check "Add Python to PATH" (important!)
• Click Install

Linux:

sudo apt install python3 python3-pip

2. Install the Pillow library

macOS:

pip3 install Pillow --break-system-packages --user

This looks scary but it's safe — --break-system-packages simply bypasses a protection in newer Homebrew, and --user ensures Pillow is installed only in your user profile (~/Library/Python/), not system-wide. Nothing will break. To uninstall later: pip3 uninstall Pillow.

Windows:

pip install Pillow

Linux:

pip3 install Pillow

(Or pip3 install --user Pillow if you don't have root access.)

3. Download responsizer

Save responsizer.py wherever you like (e.g. ~/Tools/ or C:\Tools\).

4. Set up as a global command (so it works from any directory)

Without this step, you'd have to type python3 /path/to/responsizer.py ... every time. With this step, you just type responsizer ... from anywhere.

macOS

Option A — wrapper script (recommended):

# 1. Create a folder for your tools (if you don't have one):
mkdir -p ~/Tools

# 2. Move responsizer.py into it:
mv responsizer.py ~/Tools/responsizer.py

# 3. Create the global command (requires password once):
sudo bash -c 'echo "#!/bin/bash
python3 $HOME/Tools/responsizer.py \"\$@\"" > /usr/local/bin/responsizer'
sudo chmod +x /usr/local/bin/responsizer

Done. You type responsizer 1920,1280,640 from anywhere. The password is only needed this one time during setup.

Option B — alias in .zshrc (simpler, no sudo):

# 1. Move the file:
mkdir -p ~/Tools
mv responsizer.py ~/Tools/responsizer.py

# 2. Open your terminal config:
nano ~/.zshrc

# 3. Add this line at the end of the file:
alias responsizer='python3 ~/Tools/responsizer.py'

# 4. Save and close:
#    - press Ctrl+X
#    - confirm with Y
#    - press Enter

# 5. Reload (or restart the terminal):
source ~/.zshrc

The difference: Option A creates an actual system command. Option B creates a shortcut only for your terminal. Both work the same way.

Windows

# 1. Create a folder for tools:
mkdir C:\Tools

# 2. Move responsizer.py into it (via File Explorer or PowerShell):
Move-Item responsizer.py C:\Tools\responsizer.py

# 3. Open your PowerShell profile:
notepad $PROFILE

If prompted "File does not exist, create it?", click Yes.

# 4. Add this line to the file:
function responsizer { python C:\Tools\responsizer.py @args }

# 5. Save the file and close Notepad.

# 6. Restart PowerShell.

You can now type responsizer 1920,1280,640 from anywhere.

Note: If PowerShell shows an execution policy error, run this once:

Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser

Linux

Same as macOS, just edit ~/.bashrc instead of ~/.zshrc:

mkdir -p ~/Tools
mv responsizer.py ~/Tools/responsizer.py
echo "alias responsizer='python3 ~/Tools/responsizer.py'" >> ~/.bashrc
source ~/.bashrc

Verify

Open a new terminal and run:

responsizer --version

You should see responsizer 0.1.0. If so, everything works.

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Troubleshooting

"python: command not found"

On macOS/Linux try python3 instead of python. If that doesn't work either, Python is not installed — see the Installation section.

"No module named PIL"

Pillow is not installed. See Installation, step 2.

Images are skipped with "upscale protection"

The source image is smaller than the requested size. Either use smaller dimensions, or add --allow-upscale.

Colors don't work on Windows

Use Windows Terminal (recommended) instead of the old cmd.exe.

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Version history

• 0.1.0 — Initial release. Resize, WebP, AVIF, presets, smart rename.