Comparison
Apolo vs Caesium.
Both are free, open source and work on your computer. What changes is the formats, the systems and how the result is measured.
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In short
Caesium Image Compressor is a free, open-source app for Windows and Mac (on Linux you build it yourself) that compresses JPG, PNG, WebP and TIFF in batches, with a preview. Its latest release, 2.8.5, is from May 2025, and version 3 is in the works.
Apolo also does batches on the desktop, and adds AVIF and JPEG XL, the SSIMULACRA 2 quality score, ready-to-install ARM and Linux builds, and a server for AI agents.
Side by side
| What | Apolo | Caesium |
|---|---|---|
| Price | Free | Free |
| License | GPL-3.0 (open source) | GPL-3.0 (open source) |
| Where it runs | macOS, Windows and Linux, on x64 and ARM | Windows 10 and macOS 12 or later; on Linux, building it yourself |
| Your images | Never leave your computer | Never leave your computer (it also has a web version) |
| Formats it writes | WebP, AVIF, JPEG XL, JPEG (MozJPEG), PNG (OxiPNG) and QOI | JPG, PNG, WebP and TIFF |
| Whole folders | Yes, in parallel and with subfolders | Yes, several at once |
| Command line | Yes, with the options of cwebp, cjpeg, oxipng, avifenc and cjxl | Yes, separately (CaesiumCLT) |
| AI agents (MCP) | Yes, eleven tools | No |
| Measures the loss | Yes: SSIMULACRA 2, PSNR and SSIM, and quality by score | No; a preview of the result |
What each one does better
What Caesium does better
- It handles TIFF, which Apolo reads but doesn't write.
- It has a web version to compress a few images without installing anything.
What Apolo does better
- It writes AVIF and JPEG XL, which weigh considerably less than JPG and WebP at the same perceived quality.
- It measures the loss with SSIMULACRA 2 and finds the lowest quality that reaches a score.
- Packages for Linux (.deb, .rpm, AppImage and repositories), and for Windows and Linux on ARM.
- It opens iPhone HEIC and ships an MCP server for AI agents.
Which to choose
Caesium
JPG, PNG or TIFF with a simple tool, on Windows or Mac.
Apolo
Modern formats, Linux or ARM, or measuring the loss.
Keep reading
- Apolo vs SquooshThe same engines; Squoosh in the browser and one by one, Apolo on the desktop, by folders and measuring the loss.
- Apolo vs TinyPNGTinyPNG on its servers and with limits; Apolo on your computer, free and unlimited.
- Apolo vs ImageOptimImageOptim makes images lighter without changing format, on Mac only; Apolo converts to WebP, AVIF and JPEG XL on all three systems.