Rasterizing artifacts when rendering line with DrawLines

[FrancoFun]

Description

When plotting a graph using DrawLines, if the curve crosses the same y coordinate multiple times, some unwanted horizontal lines are being rendered.

Steps to Reproduce

Here's code producing the issue displayed in the image below. Artifacts are particularly visible on the bottom right corner.

public static void DrawLinesTest()
{
    int width = 1000;
    int height = 500;
    using (var image = new Image<Rgba32>(width, height))
    {
        image.Mutate(imageContext =>
        {
            imageContext.BackgroundColor(Rgba32.White);
            int pointCount = 1234;
            var line = Enumerable.Range(0, pointCount).Select(x => x * width / (float)pointCount).Select(x => new PointF(x, (0.5f + (float)Math.Sin(x / 3) / 2) * height)).ToArray();
            imageContext.DrawLines(new Rgba32(255, 0, 0), 1, line);
        });

        using (var file = File.Create("DrawLinesTest.png"))
        {
            image.SaveAsPng(file);
        }
    }
}

System Configuration

• ImageSharp version: 1.0.0-beta0002
• Other ImageSharp packages and versions: Drawing version 1.0.0-beta0002
• Environment (Operating system, version and so on): VS2017, Windows 10
• .NET Framework version: .net Core 2.0 console application
• Additional information: Saving the image as bmp didn't change anything

[zati-]

Same problem here, workaround is to disable antialiasing

[antonfirsov]

Might be related to (or actually the same issue as) SixLabors/ImageSharp#572 and SixLabors/Shapes/#42.

@tocsoft is my supposition right?

[tocsoft]

yeah, looks like its going to be the exact same root issues.

[woutware]

This is indeed a duplicate of SixLabors/ImageSharp#572.

I've verified it now works ok after the fix, see attached image.

[JimBobSquarePants]

Closing now as in nightlies

[antonfirsov]

Shouldn't be fixed yet. SL.ImageSharp.Drawing is still referencing shapes 1.0.0-ci0018.

We still need a PR updating shapes.

[JimBobSquarePants]

I thought the PR was in no?

[antonfirsov]

@JimBobSquarePants only in SL.Shapes: SixLabors/Shapes#43

We need to update the reference to have these issues closed.

[antonfirsov]

Fixed with SixLabors/ImageSharp#596 in beta4.

[jjxtra]

Any plans to put fix in a nuget package?

[JimBobSquarePants]

@jjxtra It was back in May 2018.

https://www.nuget.org/packages/SixLabors.ImageSharp/1.0.0-beta0004

Are you still seeing issues with either beta6 or the latest nightly builds from MyGet?

[jjxtra]

Yes with beta 6. Rendering us state lines onto an image, I am seeing horizontal gray lines if anti alias is on.