Image Format & PNG Compression Performance

indi-allsky supports saving primary capture images in either JPEG (.jpg) or PNG (.png) format. Choosing between these formats involves tradeoffs between image fidelity, file storage size, and processor encoding overhead.

Format and compression settings are configured under Configuration → Image Processing → Image File Settings.

---

1. PNG vs JPEG Format Tradeoffs

• JPEG (.jpg):
  • Advantages: Highly optimized hardware and SIMD-accelerated encoding (simplejpeg / libjpeg-turbo). Fast compression (typically $< 100$ milliseconds per frame). Compact storage footprint.
  • Disadvantages: Lossy compression. High-contrast star borders may exhibit minor 8x8 DCT compression ringing.
• PNG (.png):
  • Advantages: Lossless compression. Preserves pixel values and color depth without artifacting.
  • Disadvantages: Substantially higher CPU utilization during deflate/zlib compression. Uncompressed 16-bit PNG frames require significant disk space and transfer bandwidth.

---

2. PNG Compression Levels Benchmarks

The PNG specification defines compression levels from 0 (uncompressed) to 9 (maximum compression). On embedded ARM single-board computers (such as the Raspberry Pi 4 and 5), levels above 5 introduce severe latency that can exceed the capture interval:

PNG Level	Average Compression Time (1080p Frame)	CPU Load Impact	Recommendation
Level 0	$< 0.1$ seconds	Minimal	Uncompressed. Creates massive files; not recommended.
Level 1 – 3	$0.3$ – $0.8$ seconds	Low	Recommended for PNG. Rapid encoding with acceptable compression ratios.
Level 4 – 5	$1.2$ – $1.8$ seconds	Moderate	Balanced. Suitable if exposure interval $\ge 15$ seconds.
Level 6	$2.5$ – $3.5$ seconds	High	Noticeable queue latency on multi-frame sequences.
Level 7 – 8	$5.0$ – $18.0$ seconds	Severe	Can cause image processing queues to fall behind capture loops.
Level 9	$25.0$ – $38.0$ seconds	Extreme	Do not use. Compressing a single frame can take longer than the entire exposure interval.

---

3. Operational Recommendations

1. Default Recommendation: Utilize JPEG with quality set between 85 and 95.
2. If Lossless Output is Required: Select PNG and set compression level to 1, 2, or 3. Levels 7 through 9 yield minimal additional file size reduction (often $< 3%$) while increasing CPU encoding duration by several hundred percent.
3. Astrophotographic FITS Storage: If scientific pixel analysis is required, leave standard images as JPEG and enable Save FITS or Save RAW in camera settings to archive unprocessed calibration frames.