Emerge finds all large images in your app, including ones in asset catalogs (iOS), and determines if their size could be reduced or updated to more optimized image formats.

iOS

Image formats and compression

On iOS, Emerge will show an optimized image insight for any image whose size can be reduced by more than 4kb through lossy compression (iOS) or converted to HEIC format.

Emerge starts by setting the compression level of each image to 85, and flags any with significant size reductions as optimizable. If the image is not already in HEIC format, and your app targets iOS 12 or later, Emerge will run the same analysis but with the image conversion to HEIC. The larger of these savings will be used to compute your total estimated size reduction, but both are displayed in the details row for an image:

For brevity in the UI we will only list the image with the highest scale of an image set that needs to be optimized. For example, our insight might flag a 3x image as needing to be optimized, but it's possible the 2x and 1x variants also need to be optimized.

Optimize your images

Once you decide to optimize an image, you can use command-line tools such as Imagemin or a GUI like ImageOptim to export a compressed image. To convert an image to HEIC on Mac open it in Preview and choose File -> Export then select HEIC from the format dropdown.

Android

Lossless WebP format

Emerge finds all instances of a PNG or JPEG file in your resources or assets directory in your APK(s) (split APKs if you uploaded an AAB). Emerge then runs a comparison of the original PNG/JPEG image to the lossless WebP converted version of the image, and if there is a reduction in app size between the two, Emerge will suggest using the lossless WebP and include the potential size savings of using the WebP image.

Note: Based on your minSdkVersion, Emerge will recommend lossless WebP conversion if the minSdkVersion is >= 18. If < 18, Emerge will skip any assets with Alpha, as those are not supported below 18.

Converting an image to WebP

Emerge allows you to automatically download the lossless WebP image directly from our insights. Download from the link in the insight and replace your file locally with the optimized image!

Alternatively, you can use Android Studio to convert a JPEG/PNG/BMP/static GIF to WebP, or cwebp, a command-line tool, to convert your images manually to WebP.

With these tools, you can specify additional lossy compression parameters to obtain further size savings, but at the tradeoff of image quality.