Mutate to Polar coordinates

[LuisAlfredo92]

It'd be nice to have a filter in image.Mutate() to map the image to polar coordinates, like this:

[JimBobSquarePants]

What's generating that output in your sample?

[LuisAlfredo92]

The Polar Coordinate filter in GIMP, there's also one like that in Photoshop

Sadly I couldn't find the source code of that filter so I don't know how it works

[JimBobSquarePants]

The GIMP plugin has a bunch of options which I haven't worked out but here's a basic implementation.

using (Image<Rgba32> image = Image.Load<Rgba32>(@"C:\Users\james\Downloads\polar-in.jpg")) // Load your image
{
    int width = image.Width;
    int height = image.Height;
    var center = new PointF(width / 2f, height / 2f);
    float maxRadius = Math.Min(center.X, center.Y);

    using (Image<Rgba32> polarImage = new(width, height))
    {
        polarImage.Mutate(ctx =>
        {
            ctx.ProcessPixelRowsAsVector4((pixelRow, offset) =>
                {
                    for (int x = 0; x < pixelRow.Length; x++)
                    {
                        // Map to polar coordinates
                        float dx = x - center.X;
                        float dy = offset.Y - center.Y;
                        float radius = MathF.Sqrt(dx * dx + dy * dy) / maxRadius;

                        if (radius <= 1f) // Only process pixels within the circle
                        {
                            float theta = MathF.Atan2(dy, dx) - MathF.PI / 2; // Subtract π/2 for 90-degree clockwise rotation

                            // Normalize theta to range from 0 to 1
                            theta = (theta + MathF.PI) / (2 * MathF.PI);
                            while (theta < 0)
                            {
                                theta += 1; // Wrap around if negative
                            }

                            // Map radius and theta back to source coordinates
                            int sourceX = (int)(theta * width);
                            int sourceY = (int)(radius * height);
                            sourceX = Math.Clamp(sourceX, 0, width - 1);
                            sourceY = Math.Clamp(sourceY, 0, height - 1);

                            pixelRow[x] = image[sourceX, sourceY].ToScaledVector4();
                        }
                    }
                }, PixelConversionModifiers.Scale);
        });

        polarImage.Save(@"C:\Users\james\Downloads\polar-out.jpg"); // Save the transformed image
    }
}

[LuisAlfredo92]

OMG how did you do it so fast?
It works perfect 🤯

[JimBobSquarePants]