APNG support

[Poker-sang]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Implement APNG decoding and encoding based on PNG

[CLAassistant]

All committers have signed the CLA.

[JimBobSquarePants]

Thanks for this, I really appreciate your efforts!

I've tried to offer guidance as to what need to change. Please let me know if anything doesn't make sense.

[Poker-sang]

Thanks for your review! It is rigorous and easy for me to understand. I'm sorry for accidentally formatting some code that confused you (e.g. x++ to ++x). Now it's all reverted.

[JimBobSquarePants]

@Poker-sang Sorry this is taking so long to merge. There's a bit of a backlog we need to shift first to ensure there are no functional conflicts.

[Poker-sang]

It's all right. Take your time please.

[JimBobSquarePants]

@Poker-sang Thanks again for your patience. Following the merge of #2485 there's a few conflicts here. Would you be able to update your PR to include the updates from main? If not, I can have a try.

[Poker-sang]

I can have a try. But I'm sorry I can't handle it for I'm busy these days. If you're not in a hurry you can wait a few days for me. ☺️ Or you can handle it the way you want if it doesn't bother you.

[JimBobSquarePants]

I'm having a go at merging main back into this PR now.

[JimBobSquarePants]

@Poker-sang Can you give me collaboration access to your fork? There's an open issue with Git LFS which prevents me pushing images to your fork without those permissions.

git-lfs/git-lfs#5001

[Poker-sang]

Sure. When I'm cloning this repo lately, Filtering Content always takes me a long time. Is it caused by too many test images?

[JimBobSquarePants]

Sure. When I'm cloning this repo lately, Filtering Content always takes me a long time. Is it caused by too many test images?

I'm not sure what you mean by Filtering Content.

[Poker-sang]

I'm not sure what you mean by Filtering Content.

[JimBobSquarePants]

Ah yeah, there's a lot of LFS files.

You should only be cloning the repo 1 time really though and using feature branches for your PRs. Every time someone with a fork clones the repo it actually uses up my LFS bandwidth and ends up costing me money if I go over my pre-purchased allowance.

[Poker-sang]

Oh I see. I'll pay attention to it.

[JimBobSquarePants]

I’ve started adding the tests locally.

[Poker-sang]

APNG Specification says:

If the default image is the first frame:

Sequence number    Chunk
(none)             `acTL`
0                  `fcTL` first frame
(none)             `IDAT` first frame / default image
1                  `fcTL` second frame
2                  first `fdAT` for second frame
3                  second `fdAT` for second frame
....

If the default image is not part of the animation:

Sequence number    Chunk
(none)             `acTL`
(none)             `IDAT` default image
0                  `fcTL` first frame
1                  first `fdAT` for first frame
2                  second `fdAT` for first frame
....

Seems like a simple and not very useful feature, do we need to implement it? Says adding an option in the Encoder to exclude the first frame.

[JimBobSquarePants]

I think that just means the first frame control is optional. The code I have locally handles that.

[JimBobSquarePants]

I'm mostly happy with this now. All the test images I've thrown at it pass with flying colors. Will review again in the morning to be sure.

[Poker-sang]

I am very happy to hear that. I am so grateful that you have helped me so much!❤

[JimBobSquarePants]

I'm happy with this now. Thanks @Poker-sang for a great addition to the library!