Imagesharp: GIF Artifacts

[TodesBrot]

Prerequisites

• [x] I have written a descriptive issue title
• [x] I have verified that I am running the latest version of ImageSharp
• [x] I have verified if the problem exist in both DEBUG and RELEASE mode
• [x] I have searched open and closed issues to ensure it has not already been reported

Description

This specific GIF:

shows artifacts when loaded using ImageSharp. When loading the GIF using ImageSharp and saving it again, the resulting file looks like this:

Steps to Reproduce

Load the GIF with ImageSharp.

System Configuration

• ImageSharp version: beta 2
• Other ImageSharp packages and versions:
• Environment (Operating system, version and so on): Windows 10
• .NET Framework version: 4.5.2
• Additional information:

[TodesBrot]

I just noticed the second GIF file also appears to be faster than the first one.

[JimBobSquarePants]

@TodesBrot I can't replicate this in the current master. Here's the output from there. Can you give it a try from the repo.

[TodesBrot]

@JimBobSquarePants I have noticed some weird behavior. When I create a new project and load the image, it loads just fine, but in my old project it has the buggy output. However, if I move the line that loads and saves the file to the beginning of the project, it works fine. It seems like something in my project changes something somewhere that causes the bug to occur. I'll have to do some more investigating.

[JimBobSquarePants]

Thanks, let us know how you get on.

[TodesBrot]

@JimBobSquarePants Okay, now this is a weird one. Loading this specific image:

and saving it again before loading the GIF leads to the corruption shown above. Additionally, loading it and not saving it beforehand leads to a different corruption.

Code example: (The GIF is called "Kumin.gif" here, and the image above is called "Icon.png")

Image.Load("Kumin.gif").Save("Kumin ImageSharp 1.gif");

Image<Rgba32> icon = Image.Load("Icon.png");

Image.Load("Kumin.gif").Save("Kumin ImageSharp 2.gif");

icon.SaveAsPng(new MemoryStream());

Image.Load("Kumin.gif").Save("Kumin ImageSharp 3.gif");

Kumin ImageSharp 1.gif looks just fine:

Kumin ImageSharp 2.gif has purple corruptions:

Kumin ImageSharp 3.gif has the blue corruptions from my first post:

Note: The GIF file has to be loaded again each time, as the corruptions occur during the decoding, not the encoding.

[TodesBrot]

It seems like loading and saving any 256x256 image causes this bug. I created one with only blue and green stripes in it, and it caused blue and green stripes to appear.

[JimBobSquarePants]

I think this might be an array pooling thing as the LZW decoder doesn't use clean arrays to read from.

Would you be able to alter the code locally in GifDecoderCore to clear the array before decoding? Also the global color table?

[antonfirsov]

@JimBobSquarePants I had the same idea

[JimBobSquarePants]

@TodesBrot I've actually just pushed a branch that should fix this for you, could you please dl, build it and test against it?

[JimBobSquarePants]

@TodesBrot Versioning has gone out of whack for our CI server (Gitversion bug) so SixLabors.ImageSharp.1.0.0-ci0030 will contain the fix.