Support for decoding incomplete PNG files

[Poker-sang]

I hope ImageSharp to decode incomplete PNGs, in order to use ImageSharp for progressive decoding of PNGs.

Right now decoding incomplete PNGs throws an exception: “IDAT block corrupted”. Maybe changing the behavior to end decoding would implement this feature.

[JimBobSquarePants]

It really depends on the level of corruption. Can you share some I can experiment with?

[Poker-sang]

It's easy to find examples, where picking a random PNG and truncating it from the middle of the file, and then using Image.LoadAsync will encounter this exception. However Skia, WIC can loads this half of the image normally

SixLabors.ImageSharp.InvalidImageContentException:“CRC Error. PNG IDAT chunk is corrupt!”

In addition, for randomly truncated images, ImageSharp can read WebP, JPG, GIF, etc., and will not throw exceptions

original PNG	truncated PNG

[JimBobSquarePants]

Thanks…. If they can, we should be able to. I’ll have a look asap.

[JimBobSquarePants]

Actually @Poker-sang have you tried the options?

https://docs.sixlabors.com/api/ImageSharp/SixLabors.ImageSharp.Formats.Png.PngCrcChunkHandling.html

[Poker-sang]

Thank you! I'm sorry I didn't see that option. Also I didn't find PngCrcChunkHandling in the source code, it seems to have migrated to SegmentIntegrityHandling, is this a change that hasn't been released yet?

[JimBobSquarePants]

No worries! It’s a large API!

Yeah, the main branch is the upcoming v4 build. You want release/v3.1.x for current releases.

For V4 the approach was normalised across all decoders for increased security.

[Poker-sang]

Thanks. Integrity is much easier to handle as a unified API than judging PNGs individually and using PngDecoder, I'm looking forward to the v4 release!