We offer a variety of filters to get images down the wire and rendered as fast as possible. All of these can be enabled individually, or all at once using 'IISpeed rewrite_images'. IISpeed will back down when it detects that optimizations did not result in an image reduction.
User agent specific optimization
For user agents that support it, IISpeed is able to convert images to lossy or lossless WebP format for even smaller and faster downloads. For mobile user agents, IISpeed is able to first send low-res placeholders of images down the wire to speed up the initial view. Afterwards, a higher res version will be displayed.
The recompression feature will perform a few passes using different compression settings to figure out the smallest possible size for the configured image quality settings.
The recompression feature can also be configured to convert png to jpeg. Both jpeg and png can be configured for automatic conversion to webp. WebP will only be served to browsers that support it, and offers an additional 30 to 35 percent reduction in image size at the same quality as jpeg.
By enabling 'convert_jpeg_to_progressive', recompression will trancode jpeg images that are not already progressive and larger then 10kb to progressive. Progressive jpegs allow better compression opportunities, as increase the speed at which browsers can show the user an initial image, resulting in a speedier experience.
Often, images still contain lots of metadata like icc profiles, previews, and copyrights. IISpeed can automatically detect and remove these.
Small images can be inlined, to reduce the amount of requests needed to render the page. The threshold for this can be configured.
Inserting image dimensions
To help the browser render the page, IISpeed can insert the original image dimensions into IMG tags. Doing so prevents page reflows.
Filters marked bold are in the core filter set, which is enabled by default.
- Convert gif and png to jpeg : Convert gif and png files to jpeg when possible (no transparancy)
- Convert gif to png : Recompress gif to png. Enabled by recompress_images.
- Convert jpeg to progressive : Re-encode large jpeg images to progressive format for a faster user experience
- Convert jpeg to WebP : Convert images to WebP for browsers that support the format
- Inline images : Inlines small images. Enabled by rewrite_images
- Inline preview images : Inline low quality images, which will be replaced with full versions once the page has loaded.
- Insert image dimensions : Enrich IMG tags with 'width' and 'height' attributes where needed
- Lazyload images : Only load images for the initial viewport, and defer loading of the rest untill they become visible.
- Optimize Images : Optimize images by trimming excess fat and optionally transcoding. Inlines small images into the HTML
- Recompress images : Trim excess data from images. Enabled by rewrite_images
- Recompress jpeg : Recompress jpeg images. Enabled by recompress_images.
- Recompress png : Recompress png images. Enabled by recompress_images.
- Recompress webp : Recompress webp images. Enabled by recompress_images.
- Reduce JPEG subsampling : Reduces color sampling of jpeg images to 4:2:0. Enabled by recompress_images.
- Resize images : Scales images down to their specified size in html img elements. Enabled by rewrite_images.
- Resize mobile images : Like Inline Preview Images, but with smaller images. Will only rewrite for mobile user agents.
- Strip color profiles : Strips color profiles from images. Enabled by recompress_images.
- Strip image meta data : Remove EXIF meta data from images. Enabled by recompress_images.