allowed by PNG chunk ordering rules. Frames utilize the same bit depth, color type, compression method, filter method, interlace method, and palette (if any) as the default image. The PNG specification was designed with future extensions in mind. An application reading a PNG file is supposed to simply ignore any chunks which it does not understand. This is the reason why APNG is backwards compatible. Existing applications just recognize the first frame and ignore the additional animation chunks. Compression and Optimization A number of optimization techniques make APNG files as small as possible: Inter-frame optimization utilizing alpha-blend and alpha dispose operations, smaller than the full-size subframes, dirty transparency, color type and color palette optimizations, and various compression options: zlib, 7-Zip, Zopfli. Derived formats Animated stickers for Signal are APNG with some restrictions. Support A server-side library exists that allows web browsers that support the canvas tag, but do not support APNG, to display APNGs. Examples of such browsers include Microsoft Edge and Internet Explorer 9. See also Multiple-image Network Graphics WebM WebP References External links The APNG specification at mozilla.org's Wiki APNG patch for libpng Mozilla's APNG Demo Site Directory of APNG Software and Developers resources APNG tests for browsers supporting APNG 1.0 AJAX loading animations in APNG APNG consideration in the Chromium development group Animated graphics file formats Graphics file formats Computer-related introductions in 2004 Mozilla Open file formats Portable Network Graphics