What's wrong with sine?

Question

Mathematics

I wanted to make a similar wave effect :



For this I used the following algorithm:

for (int y = 0; y < img.Height; ++y)
{
    Span<Rgba32> pixelRowSpan = img.GetPixelRowSpan(y);
    for (int x = 0; x < img.Width; ++x)
    {

        int y1 = Convert.ToInt32(y + 20.0 * Math.Sin(x / 32.0));

        if (y1 >= img.Height)
            y1 = img.Height - 1;
        if (y1 < 0)
            y1 = 0;

        var sourcePixel = img.GetPixelRowSpan(y1)[x];

        pixelRowSpan[x] = sourcePixel;
    }
}

But instead of the desired result, I get this:


Original Image :


Thanks Wataru!
Original Image :


Result :

Answer 1

You change the image in place by walking over it from top to bottom. When your sine shifts the pixels down, you overwrite all the pixels in the topmost column.
It is necessary either to copy with changes to a new image, or to change the direction of the passage depending on the sign of the sine.