Gifs sometimes get optimized incorrectly

[SigmundurMorkore]

Prerequisites

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

ImageSharp version

v2.1.3

Other ImageSharp packages and versions

ImageSharp.Web 2.0.2

Environment (Operating system, version and so on)

Linux and Mac

.NET Framework version

6

Description

When doing any processing on some gifs, the output isn't correct.

Example 1
This is the original gif:

When sending it through ImageSharp.Web with any query parameters, even inconsequential ones, the output gif is extremely slow (with rmode=crop):

Example 2
This is the original gif:

When sending it through ImageSharp.Web with any query parameters, even inconsequential ones, the output gif has the first "V" in the smaller text almost turned into a "M" (with rmode=crop):

Steps to Reproduce

1. Run a gif through ImageSharp.Web
2. It may or may not have things that are incorrect

Images

[JimBobSquarePants]

I think the issue here is that we always encode the GraphicsControlExtension even though it is optional. I should be able to code up a workaround to prevent this.

It's actually the opposite. We've been ignoring the extension if the delay is 0.

Here's a version using my local changes.

[SigmundurMorkore]

@JimBobSquarePants awesome :D

Is the weird artifacting (around the "V") in the second example a separate issue?

[JimBobSquarePants]

Ah... Didn't spot that. Separate issue. Not sure what the cause is there....Yet

[JimBobSquarePants]

OK. The issue there is that the image contains both global and local color tables but we only support one or the other.

BTW. I wouldn't currently recommend resizing/cropping multi-frame gifs in this manner with ImageSharp. Unfortunately, a current architectural limitation requires that all frames are the same size so each frame is padded if smaller to match. This can lead to multi-frame images being much larger following processing than the input.

[SigmundurMorkore]

Aah I see, makes sense 👍

Is it possible for ImageSharp to implement support for gifs with both kinds of color tables?
I may want to look into a tool that can convert gifs with both global and local color tables to just use one or the other.

Thanks for the info on resizing/cropping!

[JimBobSquarePants]

It's absolutely possible. Working on a fix just now. Might take me a few days though.

[JimBobSquarePants]

Got some cleanup and investigation to do but this is major progress

[SigmundurMorkore]

That looks perfect! ❤️