Image optimization

Image optimization is essential for improving the performance of your website or application. Resizing your images and compressing them without sacrificing quality are the critical factors to consider:

Scaling operations, such as various methods of resize and crop, have the biggest effect.
Automating images with -/format/auto/ delivers the most efficient format that the client's browser supports.
Further optimization of the image quality of the resulting image.

It's important to use all three types of operations to achieve the best results. Try -/quality/smart/ operation that adjusts compression and format automatically to preserve visual quality while minimizing the file size.

Check out our responsive images guide for more details.

How it works

Every URL transformation generates a modified image version on the fly, while the original file stays intact. The transformed image version will be cached on CDN nodes to optimize delivery.

https://ucarecdn.com/:uuid/-/format/auto/-/quality/smart/-/preview/

Also, you can fetch and deliver images to apply operations without explicitly uploading them:

https://yoursite.ucr.io/-/resize/500x/https://yoursite.com/assets/image.jpg

When applying any image operation, a new file is created (cached on our CDN, and not counted towards your storage limits). If not set, this file is encoded with the default quality and original format.

Image processing operation may be applied only to the appropriate file. On uploading, the file is analyzed by Uploadcare: file type, metadata, and so on; the file is assigned an attribute is_image. If this attribute is true, then an operation can be applied to the file. See the Limitations section.

Without any image processing operation in the URL, CDN instructs browsers to show images (Content-Disposition: inline) and download other file types (Content-Disposition: attachment). Browsers may not show all image formats, such as TIFF and HEIC formats. If you need to display an image, add any image processing operation, -/preview/ for instance.

Limitations

Image processing operations have limits that you should consider:

Input image formats
Mandatory usage of core operations
File size
Output image dimensions
SVG files
Image resolution
Rotation
Animated images

Learn more about image processing limitations.

Scaling operations

Rather than delivering large, full-sized images and relying on the browser to resize them, you can do it programmatically:

The advanced scaling operations allows you to resize images more precisely:

Format

-/format/:format/

Converts an image to one of the following formats:

jpeg is a lossy image format (good compression for photos). JPEG doesn’t support an alpha channel; hence you can use the setfill operation that sets a background color. All browsers support JPEG.
png is a lossless format (good compression only for graphics) with alpha channel support. All browsers support PNG.
webp is a modern format with more efficient compression than JPEG and with alpha channel support. It works well for all images, yet not all browsers and OSs support it.
auto is an automatic image format selection based on alpha channel presence and a client's device and browser.

Note: We partially support AVIF. See the next chapter for details.

How `auto` works and prioritize image formats

First, the algorithm checks the Accept header with MIME types to figure out what image format a client browser supports.

AVIF is used as output when all the following conditions are met: AVIF is activated for your project (contact our sales team); output image resolution is under the threshold (currently 1 MPx); image/avif MIME type is supported by the client.
WebP is used when image/webp is one of the supported MIME types.
JPEG is used when the original image is fully opaque.
PNG is used when the source image has an alpha channel with non-opaque pixels.

Note: auto works when you use primary Uploadcare storage (not S3 Bucket) and the default Uploadcare CDN domain ucarecdn.com.

400x301 png 116Kb
Transparent 400x301 jpeg 16Kb
Opaque 400x301 webp 15Kb
Transparent, size is equal
to the opaque one.

HTML5 image auto format example

You can use <picture> tag to render WebP image on client when possible. Place <img> within <picture> and add <source> with type="image/webp".

<picture>
  <source srcset="//ucarecdn.com/:uuid/:operations/-/format/webp/" type="image/webp"/>
  <img src="//ucarecdn.com/:uuid/:operations/-/format/jpeg/"/>
</picture>html

Browsers that support WebP will load this image version, while others will display JPEG or PNG instead.

Quality

-/quality/:value/

Sets output quality for lossy formats (JPEG, WebP, and AVIF). Since actual settings vary from codec to codec and, more importantly, from format to format, we provide five simple tiers and two automatic values consistent across different formats and other settings and suit most cases of image distribution.

A higher quality level will typically result in a larger output file. However, setting the quality level higher than the original level of the uploaded image won’t increase your file size.

normal — the default behavior when no quality operation is applied. The reasonable quality for 1x pixel density.
smart — adjusts compression and format automatically to preserve visual quality while minimizing the file size. See the detailed explanation below.
smart_retina — similar to smart, yet optimized for double pixel density.
better — can be used to render relatively small and detailed previews. ≈125% file size compared to normal.
best — can be used to deliver images close to their pristine quality (e.g., for artwork). ≈170% file size.
lighter — useful when applied to relatively large images to save traffic without significant quality loss. ≈80% file size.
lightest — highest compression ratio for high pixel ratio. ≈50% file size.

Smart compression

Smart quality modes (-/quality/smart/ or -/quality/smart_retina/) analyze input images using content-aware algorithms to determine maximum compression that won't cause noticeable visual artifacts.

Prior to compression, a smart mode selects an output image format. It can be PNG for images with flat colors (like graphs, charts or web graphics). You can define format explicitly while using smart and smart_retina. In this case, the image format won't be adjusted, but the compression will. For example:

-/quality/smart/-/format/jpeg/

Optimizing for high pixel ratios

A great approach for high pixel densities is to increase images resolution and reduce quality at the same time. Compared to just increasing quality, images will look clearer on all screens with nearly the same file size. To adjust quality, you can use lighter and lightest presets, or we can adjust quality more precisely for this case on the per-image basis with smart_retina preset.

1x best 16Kb
Blurry on retina. 1.5x lighter 14Kb 2x lightest 12Kb
Perfect for all screens.

1x smart 9.6Kb
Optimized size for 1x. 2x smart_retina 17Kb
Optimized size for retina displays.

Progressive JPEG

-/progressive/yes/
-/progressive/no/

Returns a progressive image. In progressive images, data are compressed in multiple passes of progressively higher detail. This is ideal for large images that will be displayed while downloading over a slow connection allowing a reasonable preview after receiving only a portion of the data. The operation does not affect non-JPEG images; does not force image formats to JPEG.

Baseline loading.

Progressive loading.

Strip meta information

-/strip_meta/all/
-/strip_meta/none/
-/strip_meta/sensitive/

The original image often comes with additional information built into the image file. In most cases, this information doesn't affect image rendering and thus can be safely stripped from the processed images to reduce image weight. Currently, you can only keep EXIF meta information. Other storage, such as XMP or IPTC, will always be stripped when creating a new processed image version.

all — the default behavior when no strip_meta operation is applied. No meta information will be added to the processed file.
none — uses the EXIF from the original file. The orientation tag will be set to 1 (normal orientation).
sensitive — uses the EXIF from the original file but skips geolocation. The orientation tag will be set to 1 (normal orientation).

If you need to restrict user access to EXIF information, please protect your images from editing.

Animated images optimization

Video files are much smaller than GIFs, without noticeable quality loss. Their delivery to end users is much faster. Gif to video operation converts animated image files, such as GIF, WebP, and HEIC, to video and transforms them on the fly. Learn more about animated images optimization.