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content/posts/2023-04.md: update image format scores 

After re-calculation with ssimulacra2 v2.1.
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Alan Orth 2023-05-04 14:44:51 +03:00
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@ -469,14 +469,14 @@ $ psql -d dspace -c "update bundle set primary_bitstream_id=NULL where primary_b
| | Q75 | Q80 | Q92 |
|------|-----|-----|-----|
| JPEG | 70 | 73 | 88 |
| WebP | 73 | 76 | 82 |
| AVIF | 82 | 83 | 92 |
| JPEG | 71 | 74 | 88 |
| WebP | 74 | 77 | 82 |
| AVIF | 82 | 83 | 86 |
- Then I checked the quality and file size (bytes) needed to hit an average ssimulacra2 score of 80 with each format:
- **JPEG**: Q89, 124596 bytes
- **WebP**: Q88, 84935 bytes (32% smaller than JPEG size)
- **AVIF**: Q62, 60347 bytes (52% smaller than JPEG size)
- **JPEG**: Q89, 124923 bytes
- **WebP**: Q86, 84662 bytes (33% smaller than JPEG size)
- **AVIF**: Q65, 67597 bytes (56% smaller than JPEG size)
- [Google's original WebP study](https://developers.google.com/speed/webp/docs/webp_study) uses this technique to compare WebP to JPEG too
- As the quality settings are not comparable between formats, we need to compare the formats at matching perceptual scores (ssimulacra2 in this case)
- I used a ssimulacra2 score of 80 because that's the about the highest score I see with WebP using my samples, though JPEG and AVIF do go higher