Why We Show Comparisons
Before/after comparison sliders let you judge conversion quality firsthand—slide between original and converted to spot compression artifacts, color shifts, or sharpness changes. We include these on many image converter pages so you know exactly what to expect before uploading your files.
The Browser Compatibility Challenge
Modern image formats like HEIC (High Efficiency Image Container) deliver excellent quality and small file sizes, but most browsers don't support them natively. Safari 11+ can display HEIC on macOS and iOS, but Chrome, Firefox, and Edge cannot. If we tried to load a raw HEIC file in an <img> tag, you'd see a broken image icon.
We can't show a side-by-side comparison if your browser refuses to render the "before" image. So we developed a workaround.
Our Solution
For formats browsers can't render, we losslessly convert to PNG for the comparison demo only. Here's the flow:
- We take a representative sample file (e.g., a 12MP HEIC photo from an iPhone)
- Losslessly convert to PNG (no quality loss)
- Store the PNG in
/img/examples/heic-to-jpg/before.png - Display that PNG as the "before" image in the comparison slider
- Show the actual converted JPG as the "after" image
Your uploaded file is never touched. When you drop a HEIC file to convert, we use your original HEIC—not the demo PNG—for the actual conversion. The demo images exist solely to show quality on the webpage.
Example: HEIC → JPG Conversion
1. You upload: vacation.heic (2.8 MB, 12 megapixels)
2. Demo slider shows: PNG version (left) vs JPG output (right)
3. We convert: Your original vacation.heic → vacation.jpg
4. You download: vacation.jpg (3.9 MB, quality 90)
Why This Matters
Transparency is a core value at Tools.FAST. We could skip the explanation and hope no one notices, but that's not how we operate. If you inspect the demo slider and see a PNG file, we want you to understand exactly why—and trust that we're being honest about the methodology.
This approach lets us show real quality comparisons without browser limitations getting in the way. It's a technical workaround, not a deception. Your actual conversion pipeline never touches the demo images.
What We Do NOT Do
- ❌ We do not convert your uploaded file to PNG before converting to the target format
- ❌ We do not use the demo PNG for your actual conversion
- ❌ We do not cherry-pick unrealistic best-case examples (samples are representative)
- ❌ We do not hide this methodology—you're reading about it right now
✅ We do preserve original quality by using lossless PNG for demos
✅ We do use your original uploaded file for the actual conversion
✅ We do explain our methodology openly
Which Formats Need This?
Not all formats require the PNG workaround. Here's a breakdown of browser support as of January 2025:
| Format | Browser Support | Demo Uses |
|---|---|---|
| HEIC / HEIF | Safari 11+ only | Lossless PNG |
| TIFF | Limited / Requires plugins | Lossless PNG |
| AVIF | Chrome 85+, Firefox 93+, Safari 16.4+ | Native (or PNG fallback) |
| WebP | Chrome, Edge, Firefox, Safari 14+ | Native (or JPG fallback) |
| JPG / JPEG | Universal | Native |
| PNG | Universal | Native |
| BMP | Universal | Native |
| GIF | Universal | Native |
Browser support data current as of January 2025. Check caniuse.com for live compatibility updates.
Related Topics
What happens after I drop a file?
Learn about the full conversion pipeline from upload to download
Why are conversions so fast?
Architecture decisions that make Convert.FAST live up to its name
How do you encrypt my files?
TLS 1.3 in transit, AES-256 at rest, EU servers only
What metadata do you keep?
What we log about jobs—and what we never store