Added support of APNG

[hoshi-sano]

What is this?

I added APNG support to ChunkyPNG.
You can decord/encord the Animated PNG file.
Also you can R/W access to the frame images's pixels like you do for the ChunkyPNG::Canvas.
Low level access to APNG-specific chunks is also possible.

refs:

• Possible to use ChunkyPNG for animated PNGs (APNG)?  #116
• mozilla wiki - APNG Specification
• Animated PNG demos

What did I do?

I made it possible for ChunkyPNG::Chunk module to parse acTL/fcTL/fdAT chunk types.
And I added the following three classes.

• ChunkyPNG::Animation - class to represent APNG images
• ChunkyPNG::Frame - class to represent each frames that construct the animation
• ChunkyPNG::AnimationDatastream - represents the internal structure of an APNG

Usage

I will show some samples.
Since APNG images are included, please view the following samples with APNG supported browsers (Safari/Firefox).

Read APNG file

input:

require 'chunky_png'

animation = ChunkyPNG::Animation.from_file('apng-sample.png')
puts "width: #{animation.width}px"         #=> width: 200px
puts "height: #{animation.height}px"       #=> height: 200px
puts "num_frames: #{animation.num_frames}" #=> num_frames: 8
puts "num_plays: #{animation.num_plays}"   #=> num_plays: 0 (infinite loop)

Modify and Export

input:

output:

require 'chunky_png'

animation = ChunkyPNG::Animation.from_file('apng-sample.png')
ads = animation.to_datastream
ads.each_chunk do |c|
  next unless c.is_a?(ChunkyPNG::Chunk::FrameControl)
  c.delay_den = c.delay_den / 2
end
ads.save('out_1.png')

Create animation from multiple images

input:

output:

require 'chunky_png'

frame_attrs = { delay_num: 1, delay_den: 10 }
animation = nil
(0..7).each do |i|
  name = '%03d' % i
  frame = ChunkyPNG::Frame.from_file("{name}.png", frame_attrs)
  if i == 0
    animation = ChunkyPNG::Animation.from_frame(frame)
  else
    animation.frames << frame
  end
end
animation.save('out_2.png')

Create animation from multiple images 2

In addition to above, prepare a default image for users of unsupported browsers.

input:
above 000-007.png and

output:


(left: supported browser, right: unsupported browser)

require 'chunky_png'

frame_attrs = { delay_num: 1, delay_den: 10 }
animation = ChunkyPNG::Animation.from_file('error.png')
(0..7).each do |i|
  name = '%03d' % i
  frame = ChunkyPNG::Frame.from_file("#{name}.png", frame_attrs)
  animation.frames << frame
end
animation.save('out_3.png')

Create animation from single image

input:

output:

require 'chunky_png'

radius = 10
ref_x = 40
ref_y = 40
animation = ChunkyPNG::Animation.new(200, 200, ChunkyPNG::Color::WHITE)
base_attrs = { delay_num: 1, delay_den: 10, dispose_op: 2, blend_op: 1 }
0.step(359, 20) do |degrees|
  radian = degrees * Math::PI / 180
  x_offset = ref_x + (radius * Math.cos(radian)).round
  y_offset = ref_y + (radius * Math.sin(radian)).round
  each_frame_attrs = base_attrs.merge(x_offset: x_offset, y_offset: y_offset)
  frame = ChunkyPNG::Frame.from_file('face.png', each_frame_attrs)
  animation.frames << frame
end
animation.save('out_4.png')

Who use this?

Uhmm... it's difficult.
However, as far as I know the following events have occured recently.

1. Added APNG support to Chromium https://codereview.chromium.org/2618633004/
2. Message app LINE allows creators to make animation stickers in APNG format since Jun 2016.
3. CLIP STUDIO PAINT supported exporting animation as APNG since ver1.6.3 (released Oct 2016).

2 and 3 may not affect other than Japan.
LINE's market share is biased toward Japan (https://www.similarweb.com/blog/worldwide-messaging-apps , https://techcrunch.com/2012/12/04/global-messaging-market/).
CLIP STUDIO PAINT is made in Japan, many of the users will be Japanese.

Nevertheless, I expect that APNG will become widespread in the near future.

Finaly

I hope it will be the basis for discussion.

However, I know that you want to keep this library small.
So, even if this Pull-Request is rejected I will think your decision is reasonable and I will respect it.

Thank you.

[wvanbergen]

This is really cool! Hoping to do a full review of this - I have a bunch of things on my plate right now, but hopefully before the end of the week.

[daniel-illi]

What's the status of this pull request?
We could use this feature.

[kaspergrubbe]

This could be cool to have merged.

[wvanbergen]

Hello, thanks again for the contribution. I want to apologize for the long delay on this PR.

I have decide to close this PR without merging. The reason for this is that I am maintaining this gem in my limited spare time, and have to be careful about how much support load I take on.

For this reason, this gem is in "maintenance" mode:

• I am considering this gem feature complete. As a result I will be very hesitant to accept new functionality and API surface to this gem. Specifically in this case: I know nothing myself about APNG, which will make it painful for me to support this after it is merged.
• I am still; happy to work on (or accept contributions for) things like bug fixes, performance improvements, code cleanup, security issues, etc.

For new functionality, I would suggest making them available as separate "plugin" gems (e.g. chunky_png-apng) in this case. If anything in the gem needs to be refactored to support this kind of extensibility, I am happy to accept PRs for that.

I will be updating the README of this project to include this message.