File Conversion Quality Guide
Every file conversion involves a trade-off between file size, visual or audio quality, compatibility, and processing time. This guide helps you choose the right settings for every scenario — whether you are compressing a video for WhatsApp, converting images for a website, or extracting text from a scanned document.
Video compression
Video compression uses CRF (Constant Rate Factor) encoding via FFmpeg. Lower CRF values mean higher quality and larger files. The quality slider maps to three presets:
| Setting | CRF | Best for |
|---|---|---|
| Best quality | 18 | Archival, professional delivery |
| Good quality | 23 | General sharing, YouTube pre-upload |
| Smallest file | 28 | Messaging apps, email attachments |
Codec choice
H.264: Universal compatibility — plays on every device made since 2008. Larger files than H.265 at the same quality.
H.265 (HEVC): 30–50% smaller files at the same quality. Requires modern hardware (2016+) for smooth playback. Firefox only added support in 2023.
Speed preset
Fast: Quick compression, slightly larger output. Standard: Balanced default. Slow: Best compression efficiency — can be 10–20% smaller than Fast at the same quality, but takes longer.
Recommended starting points
- WhatsApp / Telegram: H.264, CRF 23–28, Standard. Target under 16 MB for WhatsApp.
- YouTube pre-upload: H.264, CRF 18–23, Slow. YouTube re-encodes anyway — give it the best source.
- Email attachment: H.264, CRF 23–28, Standard. Most providers cap attachments at 20–25 MB.
- Personal archive: H.265, CRF 18, Slow. Best quality-to-size ratio for long-term storage.
Image conversion
Image conversion uses MiniMagick with proper color space handling. Format choice has the biggest impact on output size and quality.
Format recommendations
| Format | Best for | Avoid for |
|---|---|---|
| JPG | Photos, complex images | Graphics with text, transparency |
| PNG | Logos, screenshots, transparency | Photos (5–10× larger than JPG) |
| WebP | Web delivery (25–35% smaller) | Very old software |
Resizing
Resizing is the single biggest lever for file size reduction. A 4000px photo resized to 1200px (the widest most websites display) shrinks from ~4 MB to ~200 KB — a 95% reduction — with no visible quality change at normal viewing distances. Always resize to the actual display dimensions needed.
OCR text extraction
OCR accuracy depends primarily on the quality of the input image. Tesseract, the engine Tooler uses, achieves 95–99% accuracy on clean, high-contrast printed text at 300+ DPI.
Critical quality factors
- Resolution: 300 DPI minimum for standard text. 400–600 DPI for small fonts (< 10pt).
- Contrast: Dark text on light background is essential. Pre-process by converting to grayscale and increasing contrast.
- Alignment: Text must be horizontal. Even a 5° rotation significantly reduces accuracy.
- Format: Use PNG or TIFF for document scans. Avoid JPG — its compression artifacts around text edges confuse the OCR engine.
- Clean background: Remove speckles, stains, and background patterns. Crop tightly to the text area.
Always proofread OCR output. Even at 98% accuracy, a 500-word document has about 10 errors. Pay extra attention to names, numbers, dates, and punctuation.
Audio extraction
Audio extraction copies the audio stream from a video file and encodes it as MP3 at 192 kbps — a sweet spot balancing quality and file size. The extracted audio quality matches the original video's audio track exactly — extraction does not improve or degrade quality. If the source video has poor audio (background noise, low volume), the extracted MP3 will have the same issues.
Document conversion
Document conversion handles DOCX, HTML, and ODT files, converting to PDF or Markdown. PDF output preserves exact layout and fonts — ideal for distribution. Markdown output preserves heading hierarchy, lists, formatting, and basic tables but removes Word-specific features like headers, footers, tracked changes, and multi-column layouts. Complex tables with merged cells may need manual cleanup after conversion to Markdown.
General quality tips
- Start conservative. Begin with balanced/default settings and only increase compression if file size targets are not met. You cannot recover quality lost to aggressive compression.
- Test before bulk processing. Run one file through your chosen settings and verify the output meets your needs before processing many files.
- Keep lossless masters. Always retain the original, uncompressed file. Re-compressing an already-compressed file compounds quality loss.
- Match settings to destination. Different platforms (WhatsApp, YouTube, email, web) have different size limits and quality expectations. Tailor your settings accordingly.