apng

APNG golang library

This apng package provides methods for decoding and encoding APNG files. It is based upon the work in the official “image/png” package.

See apngr for an APNG extraction and combination tool using this library.

NOTE: The decoder should work for most anything you throw at it. Malformed PNGs should result in an error message. The encoder currently doesn’t handle differences of Image formats and similar and has not been tested as thoroughly.

If a regular PNG file is read, the first Frame of the APNG returned by DecodeAll(*File) will be the PNG data.

Types

APNG

The APNG type contains the frames of a decoded .apng file, along with any important properties. It may also be created and used for Encoding.

Signature	Description
Frames []Frame	The stored frames of the APNG.
LoopCount uint	The number of times an animation should be restarted during display. A value of 0 means to loop forever.

Frame

The Frame type contains an individual frame of an APNG. The following table provides the important properties and methods.

Signature	Description
Image image.Image	Frame image data.
IsDefault bool	Indicates if this frame is a default image that should not be included as part of the animation frames. May only be true for the first Frame.
XOffset int	Returns the x offset of the frame.
YOffset int	Returns the y offset of the frame.
DelayNumerator int	Returns the delay numerator.
DelayDenominator int	Returns the delay denominator.
DisposeOp byte	Returns the frame disposal operation. May be apng.DISPOSE_OP_NONE, apng.DISPOSE_OP_BACKGROUND, or apng.DISPOSE_OP_PREVIOUS. See the APNG Specification for more information.
BlendOp byte	Returns the frame blending operation. May be apng.BLEND_OP_SOURCE or apng.BLEND_OP_OVER. See the APNG Specification for more information.

Methods

DecodeAll(io.Reader) (APNG, error)

This method returns an APNG type containing the frames and associated data within the passed file.

Example

package main

import (
  "log"
  "os"
  "github.com/kettek/apng"
)

func main() {
  // Open our animated PNG file
  f, err := os.Open("animation.png")
  if err != nil {
    panic(err)
  }
  defer f.Close()
  // Decode all frames into an APNG
  a, err := apng.DecodeAll(f)
  if err != nil {
    panic(err)
  }
  // Print some information on the APNG
  log.Printf("Found %d frames\n", len(a.Frames))
  for i, frame := range a.Frames {
    b := frame.Image.Bounds()
    log.Printf("Frame %d: %dx%d\n", i, b.Max.X, b.Max.Y)
  }
}

Decode(io.Reader) (image.Image, error)

This method returns the Image of the default frame of an APNG file.

Encode(io.Writer, APNG) error

This method writes the passed APNG object to the given io.Writer as an APNG binary file.

Example

package main

import (
  "image/png"
  "os"
  "github.com/kettek/apng"
)

func main() {
  // Define our variables
  output := "animation.png"
  images := [4]string{"0.png", "1.png", "2.png", "3.png"}
  a := apng.APNG{
    Frames: make([]apng.Frame, len(images)),
  }
  // Open our file for writing
  out, err := os.Create(output)
  if err != nil {
    panic(err)
  }
  defer out.Close()
  // Assign each decoded PNG's Image to the appropriate Frame Image
  for i, s := range images {
    in, err := os.Open(s)
    if err != nil {
      panic(err)
    }
    defer in.Close()
    m, err := png.Decode(in)
    if err != nil {
      panic(err)
    }
    a.Frames[i].Image = m
  }
  // Write APNG to our output file
  apng.Encode(out, a)
}

Custom Compression

A custom compression writer can be used instead of the default zlib writer. This can be done by creating a specific apng Encoder and assigning a construction function to the CompressionWriter field:

enc := apng.Encoder{
	CompressionWriter: func(w io.Writer) (apng.CompressionWriter, error) {
		return NewCustomZlibWriter(w)
	},
}
enc.Encode(out, a)