Local Compression Options
Lossless vs. Lossy
When we use the term "Pixel Perfect", this is lossless compression. Lossless means that no pixels are changed and the images (before and after) will be visually identical. Lossy used to mean that you would lose significant amounts of quality, but also save lots of space in the process. EWWW IO's lossy tools achieve an average of 50% savings with no noticeable loss of quality. The average is a 0.15% change in the image pixels, and a study published in 2014 by Radware showed that users only begin to notice a difference when the change in pixel information exceeds 1.5%, which is 10x the amount of quality loss of our highest compression.
JPG Optimization Level
- Pixel Perfect - Uses the stock Jpegtran tool in lossless mode.
- Pixel Perfect Plus - Uses the Mozilla build of Jpegtran for extra compression, requires a Compress API key (also lossless).
- Premium - Saves an average of 45% with very little quality loss, requires a Compress API key (lossy).
- Premium Plus - Saves an average of 55% with minimal quality loss, requires a Compress API key (lossy).
PNG Optimization Level
- Pixel Perfect - Uses Optipng, and optionally Pngout, which have variable compression levels that can be configured via Overrides (lossless).
- Pixel Perfect Plus - Uses Oxipng, PngOptimizerCL, and Advpng to squeeze extra bytes out of your images using the Compress API (lossless).
- Premium - Uses Pngquant and is available in "local" mode without an API key. If an API key is entered, the plugin will automatically use the API for this compression level (lossy).
- Premium Plus - Compresses PNG images even better than Pngquant with minimal quality loss, requires an EWWW IO API key (lossy).
GIF Optimization Level
- There is only one level here, and it uses Gifsicle. While Gifsicle does various optimizations, it excels in removing pixels that are duplicated in animations. Thus to edit an optimized GIF, you need to use Gifsicle's --unoptimize option. If an API key is entered, then GIF files will be processed via the API.
PDF Optimization Level
- Pixel Perfect - Uses the cpdf tool from Coherent PDF, and requires an API key (lossless).
- High Compression - Uses Ghostscript to compress images within PDFs, but does poorly on PDFs which contain images of text (which is what you get when you scan a document and don't perform OCR on it). The cpdf tool is run afterwards to make sure the final PDF is fully optimized (lossy). Any PDF with the word 'compressed' in the filename will bypass lossy compression.
SVG Optimization Level
Be aware that SVG optimization is a trickier business than most. Instead of just re-encoding a raster image, this involves cleaning up the actual code of an SVG file, and can potentially break an image--it shouldn't, but just know that it IS possible and be sure you have solid backups.
If a Compress API key is available, all SVG optimization will be performed on the API servers to conserve resources.
- Minimal - Uses svgcleaner with the safest options possible.
- Default - Uses svgcleaner with the default options.
Ever need to undo an optimization? When this is enabled, the plugin stores a copy of the original image on the API servers for up to 30 days. If an image is re-optimized, the original is not overwritten, but the time stamp is updated, so that the original will be kept 30 days from the time of last optimization.