Interlacing is not supported

[JimBobSquarePants]

From @Bartmax on May 6, 2016 4:25

When using a Png file I get this error:

ImageProcessorCore.ImageFormatException: Interlacing is not supported

some png works but some doesn't.

The code is here:
https://github.com/JimBobSquarePants/ImageProcessor/blob/abbf63f863635d4df1a04d322eccd425db7e82db/src/ImageProcessorCore/Formats/Png/PngDecoderCore.cs#L385

What does this mean? is this a limitation of the library or am I doing something wrong or a bug?

Copied from original issue: JimBobSquarePants/ImageProcessor#379

[JimBobSquarePants]

Hi @Bartmax

It means exactly that. Png files can be interlaced but it is strongly recommended not to do that as it hurts compression, increasing filesize.

See

• http://stackoverflow.com/a/14124340/427899
• http://blog.codinghorror.com/getting-the-most-out-of-png/

Interlaced png files are generally uncommon so I haven't gone out of my way to support them cos it's complex. I could maybe add support in the future though as there is good reference material to base my code on so if you could share a failing image that would be really useful.

[JimBobSquarePants]

From @Bartmax on May 6, 2016 4:39

Thanks for the fast reply.

Here is a sample file that is failing. I have 3 out of 10 that seems to be interlaced :(

[JimBobSquarePants]

No worries. I'll branch off and try to tinker with it. Fingers crossed it's not too difficult.

[JimBobSquarePants]

This is tricky stuff...

I've been looking at the spec and also the Golang implementation.

https://www.w3.org/TR/PNG/
https://golang.org/src/image/png/reader.go

Reading up it looks like I need to decode each scanline 8 times which will require some significant refactoring here.

https://github.com/JimBobSquarePants/ImageProcessor/blob/7d28a87716e4b7c2dea73bc0fb804873d30372f6/src/ImageProcessorCore/Formats/Png/PngDecoderCore.cs#L242

I could really do with someone who has done this before stepping up and adding the code.

[JimBobSquarePants]

From @MarcosMeli on August 29, 2016 16:28

@JimBobSquarePants
Just a little question here. There is an option to use the .Net native image load implementation and convert the resulting Bitmap to the format you need ?

ie. try to use custom decode for the pngs but, if interlaced, so use Bitmap constructor: https://msdn.microsoft.com/en-us/library/z7ha67kw(v=vs.110).aspx

Thanks

[JimBobSquarePants]

From @dlemstra on August 29, 2016 16:59

The library should build on .NET Core where we don't have the Image class of System.Drawing. Adding it to the encoder should be the way to go.

[JimBobSquarePants]

From @MarcosMeli on August 29, 2016 17:50

@dlemstra Thanks for clarification, I understand now.