Comparing compression in different formats: Difference between revisions
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The graph shows that at high compression/low SSIM, the ECW format is easily the most effective. However, as SSIM increases, the three formats converge, and at around SSIM=0.9 the formats produce equally sized files. | The graph shows that at high compression/low SSIM, the ECW format is easily the most effective. However, as SSIM increases, the three formats converge, and at around SSIM=0.9 the formats produce equally sized files. | ||
[[File:All methods ssim comparison plot.png|frame|left|File size against SSIM. Quality parameter is labeled on each data point. ]] | [[File:All methods ssim comparison plot.png|frame|left|File size against SSIM. Quality parameter is labeled on each data point. ]] | ||
=== Comparing formats at SSIM=0.9 | |||
<gallery mode="slideshow"> | |||
Image:Baseline image.png|Baseline image | |||
Image:Ecw ssim09 q95.png|ECW quality 95 | |||
Image:Webp ssim09 q35.png|WEBP quality 35 | |||
Image:Jpeg ssim09 q25.png|JPEG quality 25 | |||
</gallery> | |||
Revision as of 14:29, 22 June 2022
This is a comparison of a small selection of compression methods used in georeferenced raster imagery. The intention is to find which method gives the best "bang for buck" - the best quality for the smallest file size. The measure for quality is SSIM - structural similarity index , which is a perception-based model to calculate the difference between two images.
Method
We compare a lossless baseline image to a compressed image and measure the SSIM using Python package scikit-image. This is repeated with a range of quality values for the compressed images. This should give an indication of which method is most efficient. In addition, it will indicate the quality value which will achieve the same SSIM across the compression methods.
Note that each of the file formats use a different scale for the input "quality" parameter. We include a wide range of the legal values from each of the formats, so this difference should not matter.
The baseline image has the following attributes:
| Attribute | Value |
|---|---|
| File size | 273 MB |
| Format | GeoTIFF |
| Compression | LZW |
| Dimensions | 10000 x 10000 px |
| Pixel size | 0.25 meters |
Result
The graph shows that at high compression/low SSIM, the ECW format is easily the most effective. However, as SSIM increases, the three formats converge, and at around SSIM=0.9 the formats produce equally sized files.
=== Comparing formats at SSIM=0.9
Baseline image
ECW quality 95
WEBP quality 35
JPEG quality 25
