Tools: encoders & metrics

Image Encoders

JPEG — cjpeg (libjpeg-turbo)

cjpeg -quality 85 input.ppm > output.jpg

• Quality range: 0–100 (typical: 75–95)
• Input: PPM format (use convert or Pillow to prepare)
• Sampling factor: -sample 2x2 (4:2:0) or -sample 1x1 (4:4:4)

WebP — cwebp

cwebp -q 85 input.png -o output.webp

• Quality range: 0–100 (typical: 75–95)
• Method (-m): 0–6 (higher = slower, better compression)
• Sharp YUV conversion: -sharp_yuv
• Input: PNG or JPEG

AVIF — avifenc (libaom backend)

avifenc -s 4 -q 60 input.png output.avif

• Quality (-q): 0–100 (typical: 50–85, very efficient at lower values)
• Speed (-s): 0–10 (higher = faster, lower quality; default: 6)
• YUV format: -y 420, -y 444

JPEG XL — cjxl

cjxl input.png output.jxl -q 85

• Quality range: 0–100 (typical: 75–95)
• Effort (-e): 1–10 (higher = slower, better compression)
• Also supports distance mode (-d)

Image Decoders

AVIF — avifdec

avifdec input.avif output.png

Used by the quality measurement pipeline to decode AVIF files back to PNG for metric computation.

JPEG XL — djxl

djxl input.jxl output.png

Used by the quality measurement pipeline to decode JXL files back to PNG for metric computation.

JPEG and WebP are decoded via Pillow (no separate CLI decoder needed).

Quality Measurement Tools

SSIMULACRA2

Perceptual quality metric designed specifically for lossy image compression. Higher is better.

ssimulacra2 original.png compressed.png

Score	Quality
90+	Visually lossless
70–90	Excellent
50–70	Good
30–50	Medium
< 30	Poor

Butteraugli — butteraugli_main

Perceptual distance metric that models the human visual system. Lower is better.

butteraugli_main reference.png distorted.png

Score	Quality
< 1.0	Excellent (imperceptible)
1.0–2.0	Good (minor artifacts)
2.0–3.0	Medium (noticeable)
> 3.0	Poor

Included in the JPEG XL static binaries.

PSNR (via FFmpeg)

Peak Signal-to-Noise Ratio in dB. Higher is better.

ffmpeg -i original.png -i compressed.png -lavfi psnr -f null -

Score	Quality
40+ dB	Excellent
30–40 dB	Good
20–30 dB	Fair
< 20 dB	Poor

SSIM (via FFmpeg)

Structural Similarity Index (0–1). Higher is better.

ffmpeg -i original.png -i compressed.png -lavfi ssim -f null -

Score	Quality
0.95–1.0	Excellent
0.90–0.95	Good
0.80–0.90	Fair
< 0.80	Poor

Tool availability

All encoders, decoders and metric tools are pre-installed in the dev container and available on PATH. Check versions with:

cjpeg -version
cwebp -version
avifenc --version
cjxl --version
ffmpeg -version
ssimulacra2  # prints usage if no args