APNG (Animated Portable Network Graphics)

Although APNG does not count as an official PNG extension, it is backward compatible to the basic format. An application program that supports PNG (but not APNG) is still able to play an APNG file, but in this case, just a single image can be seen, not the complete animation.

From a technical point of view, the APNG data stream largely corresponds to the standard PNG data stream whose structure can be found in the official image format specifications. However, the Animated Portable Network Graphics format includes three additional types of information blocks (chunks) that describe the animation and provide additional frame data.

• Animation control chunk (acTL): This block informs the parser that the PNG is animated. It shows the number of frames that make up the animation and also describes how often the animation should be played before it stops.
• Frame control chunk (fcTL): The frame control chunk precedes each frame contained in the APNG animation. It contains information, the most important of which is the display time of the frame.
• Frame data chunk (fdAT): Frame data chunks are structurally equivalent to the standard PNG image data blocks (IDAT), but are additionally marked with a sequence number. The respective number is also automatically assigned to the preceding fcTL block, so that the different frames can be brought back into the correct sequence at any time.

In order to create your own APNGs, you first need suitable software. This software must either be able to merge several PNG files into a single APNG file or convert foreign file formats into PNG and animated PNG. As far as these functions are concerned, you have the choice between classic desktop programs and web applications. In the following, we present APNG Assembler and the animated PNG Maker from ezgif.com as solution examples of both types.

You can use the MIT and zlib-licensed command line tool APNG Assembler on all common platforms. Windows users even have a graphical user interface at their disposal, which can be downloaded free of charge from the official website. If you make use of this option, start the tool by double-clicking on the file “apngasm_gui”.

In order to create your own APNGs, you need suitable image material in PNG format: The software does not offer functions for converting images, animations, or even videos in other formats like JPEG, GIF, or MPEG. Once you have the desired images ready, simply drag and drop them into the “Input files” field:

Once you have added all the PNG images you want to include in the animation, you can specify the setup of this animation using the options below:

• Playback settings: In the playback options, you can set how often the animation should be repeated (without limitation by default). To do this, just uncheck “Play indefinitely” and enter the appropriate number. You can also select “Skip the first frame” option if you want the first frame in the animation to be skipped.
• Compression settings: With zlib, 7zip, and Zopfli, APNG Assembler provides three different algorithms for compressing the animation. In the compression settings, you can select one of these algorithms. You can also activate or deactivate the optimization of the color palette and color types using the tool.
• Delays – all frames/selected frames: Under “Delays” you can set the time interval between the individual frames of the animation. You can either make a general setting for all frames or set the distance frame by frame.

Once you have adjusted your settings, create the APNG file by clicking on “Make Animated PNG.” You can then view the result with any playback program that supports the format, with your browser, for example. 

The Animated PNG Maker on the ezgif.com website is partly based on the previously presented command line and desktop tool APNG Assembler. For this reason, it works very similarly, even if you cannot start the service directly, but have to call it with the browser of your choice. In the first step, you also add the desired image files in this tool. Unlike APNG Assembler, you are not limited to the PNG format: The web service also allows you to implement images in JPG, WebP, or GIF format. Simply click on “Browse,” select the files of your choice, and click on “Upload.”

Using the settings known from APNG Assembler (delay, playback options, and compression), you then set the basic animation setup. With “Make APNG!”, you can finally create your own APNG and save it locally.

It is not always practical to assemble a new animation from individual image files. For example, it is not uncommon to have a small video clip ready that you want to convert into an APNG animation. You can use “Video to APNG” to upload video files of up to 120 megabytes in size and then convert them into an APNG animation. Supported formats include MP4, WEbM, AVI, MPEG, FLV, MOV, and 3GP.

The website also provides a converter for converting GIF animations into animated PNG. With this tool, you can save up to 35 megabyte GIFs on your hard disk in the alternative animation format. You can also convert GIFs from platforms like Giphy by entering their URL in the line “OR paste image URL” and clicking “Upload.” In both cases, start the conversation process by clicking “Convert to APNG!” after uploading. Before doing so, you can adjust and optimize the original in a number of ways, such as changing the playback speed, adding text and effects, or using additional compression methods.