Animated Portable Network Graphics

APNG is an extension of the PNG format, adding support for animated images. It is intended to be a replacement for simple animated images that have traditionally used the GIF format, while adding support for 24-bit images and 8-bit transparency. APNG is a simpler alternative to MNG, providing a spec suitable for the most common usage of animated images on the Internet.APNG is backwards-compatible with PNG; any PNG decoder should be able to decode the first frame of an APNG and treat it as a normal single-frame PNG.

PNG Structure

An APNG stream is a normal PNG stream as defined in the PNG Specification, with two additional chunks describing the animation and providing additional frame data. The first frame of an animation, frame 0, is encoded as a normal PNG. This frame is what decoders that do not understand the APNG chunks will display.

The size of the first frame defines the boundaries of the entire animation; hence, if extra space will be needed for later frames that is unused in the first frame, the first frame should be appropriately padded with fully transparent regions.

To be recognized as an APNG, an aCTL chunk should appear the stream before any IDAT chunks. If an aCTL appears after an IDAT chunk, a decoder must ignore it and treat the stream as a single-frame PNG. The aCTL structure is described in the next section.

A fCTL chunk must also appear before IDAT, providing frame information for the first frame encoded in the PNG stream’s IDAT chunks, known as frame 0.

Subsequent frames are encoded in aDAT chunks, containing information about placement and rendering of a frame in fCTL chunks, as well as frame data encoded as normal PNG streams. The full layout of the aDAT chunk is described in section

aCTL: The Animation Control Chunk

Note: For purposes of chunk descriptions, an “unsigned int” shall be a 32-bit unsigned integer in network byte order; a “signed int” shall be a 32-bit signed integer in network byte order; an “unsigned short” shall be a 16-bit unsigned integer in network byte order; a “byte” shall be a 8-bit unsigned integer.

The aCTL chunk is an ancillary chunk as defined in the PNG Specification. It must appear before an IDAT chunk within a valid PNG stream.

Format:

byte
0 num_iterations (unsigned int) Number of times to loop this APNG. 0 indicates infinite looping.

num_iterations indicates the number of iterations that this animation should play; if it is 0, the animation should play indefinitely. If nonzero, the animation should come to rest on the final non-skipped frame at the end of the last iteration.

aDAT: The Animation Data Chunk

The aDAT chunk contains a stream of images to be used as frames for the animation. Any aDAT chunks must follow any IDAT chunks.

Format:

byte
0 sequence_number (unsigned int) Sequence number of this aDAT chunk, starting with 0
4 remaining stream data

The sequence_number shall begin with 0 for the first aDAT in the PNG stream, and increase by one with each subsequent PNG stream. The length of the stream data is determined by the length of the aDAT chunk, minus the size of the sequence_number. If only one aDAT chunk is present in the PNG stream, all frames must be encoded within its data section. If more than one aDAT chunk is present, the first chunk (that is, the chunk with sequence_number 0) must have empty stream data to allow the decoder to check for the most common out-of-order errors by looking at the sequence_number of the next frame. All aDAT chunks must be adjacent in the PNG stream.

The stream data within the aDAT consists of a stream of PNG chunks, with each frame beginning with a fCTL chunk, followed by an IHDR, and then containing normal PNG image chunks followed by one or more IDAT chunks providing the data for the frame. No IEND is to be written for these embedded streams; instead, a fCTL chunk indicates that the frame has ended and provides control information for the next frame. For the final frame of the image, its end is indicated by an end of the aDAT data section, and a PNG chunk after the aDAT that is not another aDAT. (??? Do we want an explicit fEND or something here?)

Each frame must be of the same color type as the parent stream. Each frame’s region, as defined by its width and height (specified in the IHDR) and its x and y offset (specified in the fCTL, described below) must lie within the first image canvas.

Encoders are strongly encouraged to use a single aDAT chunk whenever possible, to remove the chance of out of order chunks. Should a decoder receive an APNG stream with missing or out of order aDAT chunks, it is under no obligation to attempt to reorder the chunks and may treat that case as an error condition.

fCTL: The Frame Control Chunk

Format:

byte
0 x_offset (unsigned int) X position at which to render this frame
4 y_offset (unsigned int) Y position at which to render this frame
8 delay_num (unsigned short) Frame delay fraction numerator
10 delay_den (unsigned short) Frame delay fraction denominator
12 render_op (byte) Type of canvas area disposal to be done after rendering this frame

The delay_num and delay_den parameters together specify a fraction indicating the delay after the current frame, in seconds. If the denominator is 0, it is to be treated as if it were 100 (that is, delay_num then specifies 1/100ths of a second). If the the value of the numerator is 0 the decoder should render the next frame as quickly as possible, though viewers may impose a reasonable lower bound on the delay.

The frame is rendered within the region defined by the x_offset and y_offset from the fCTL, and the width and height from this frame’s IHDR. For frame 0, the x_offset and y_offset fields must be 0. Should parts of the region fall outside the canvas defined by frame 0, rendering is to be clipped to that canvas.

The render_op parameter specifies contains flags describing how the frame is to be disposed before rendering the next frame; it also specifies whether the frame is to be alpha blended into the current canvas content, or whether it should completely replace its region in the canvas. Valid render_op flags are:

bit
+——————————-+
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+————-|—|—|—|—|-+
| | +—+—+—— bits 0-2: dispose_op
| |
| +—————— bit 3: APNG_RENDER_OP_BLEND_FLAG
|
+———————- bit 4: APNG_RENDER_OP_SKIP_FRAME

Bits 5 through 7 are reserved and must be set to 0. Valid values for dispose_op are:

value:
0 APNG_RENDER_OP_DISPOSE_NONE
1 APNG_RENDER_OP_DISPOSE_BACKGROUND
2 APNG_RENDER_OP_DISPOSE_PREVIOUS

* APNG_RENDER_OP_DISPOSE_NONE: no disposal is done on this frame before rendering the next; its contents are left on the canvas. This is the default.
* APNG_RENDER_OP_DISPOSE_BACKGROUND: the frame’s region is to be cleared to the background color. If no bKGD chunk is specified, the result is fully transparent black (r, g, b, and a all 0).
* APNG_RENDER_OP_DISPOSE_PREVIOUS: the frame’s region is to be reverted to the previous contents.

APNG_RENDER_OP_BLEND_FLAG may be added to any of the above disposal operations. If this flag is not set, all color components of the frame, including alpha, overwrite the current contents of the frame’s canvas region. If the BLEND_FLAG is set the frame should be composited onto the canvas based on its alpha, using a simple OVER operation:

Csrc - component of pixel in the frame
Asrc - alpha component of a pixel in the frame
Cdst - component of pixel in the canvas region
C - resulting component value to be rendered in the canvas region

C = Csrc * Asrc + Cdst * (1 - Asrc)

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