Skisharp ColorMatrix Invert black/white

Question

I try to draw an image and after the first drawing i want invert the color of the drawing. If the background of my new square is black i need white and if it white i need black. In my example i draw 3 squares and make a offset of 10 pixel in the x.

Unfortunately, it does not produce the wanted result.

using var skBitmap = new SKBitmap(100, 40);
using var skCanvas = new SKCanvas(skBitmap);
skCanvas.Clear(SKColors.White);

var color = SKColors.Black;
float[] invertMatrix = {
    -1.0f,  0.0f,  0.0f,  0.0f,  255.0f,
    0.0f,  -1.0f,  0.0f,  0.0f,  255.0f,
    0.0f,  0.0f,  -1.0f,  0.0f,  255.0f,
    0.0f,  0.0f,  0.0f,  1.0f,  0.0f
};

using var skPaint = new SKPaint();
skPaint.Color = color;
skPaint.Style = SKPaintStyle.Fill;

skCanvas.DrawRect(10, 10, 20, 20, skPaint);

skPaint.ColorFilter = SKColorFilter.CreateColorMatrix(invertMatrix);

//move +10 in x
skCanvas.DrawRect(20, 10, 20, 20, skPaint);

//move +10 in x
skCanvas.DrawRect(30, 10, 20, 20, skPaint);

Answer 1

I have found a solution for my problem unfortunately without a matrix.

using var skBitmap = new SKBitmap(100, 40);
using var skCanvas = new SKCanvas(skBitmap);
skCanvas.Clear(SKColors.White);

var color = SKColors.Black;

using var skPaint = new SKPaint();
skPaint.Color = color;
skPaint.Style = SKPaintStyle.Fill;

skCanvas.DrawRect(10, 10, 20, 20, skPaint);

DrawInvertRectangle(20, 10, 20, 20, skBitmap);
DrawInvertRectangle(30, 10, 20, 20, skBitmap);

private static void DrawInvertRectangle(int x, int y, int width, int height, SKBitmap skBitmap)
{
    using var skPaint = new SKPaint();
    skPaint.Color = SKColors.Black;
    skPaint.Style = SKPaintStyle.Fill;

    using var skBitmapInvert = new SKBitmap(skBitmap.Width, skBitmap.Height);
    using var skCanvas = new SKCanvas(skBitmapInvert);
    skCanvas.DrawRect(x, y, width, height, skPaint);

    for (var row = 0; row < skBitmapInvert.Height; row++)
    {
        for (var column = 0; column < skBitmapInvert.Width; column++)
        {
            var pixel = skBitmap.GetPixel(column, row);
            var pixelInvert = skBitmapInvert.GetPixel(column, row);
            if (pixelInvert.Alpha == 255 && pixelInvert.Blue == 0 && pixel.Blue == 255)
            {
                skBitmap.SetPixel(column, row, SKColors.Black);
            }
            if (pixelInvert.Alpha == 255 && pixelInvert.Blue == 0 && pixel.Blue == 0)
            {
                skBitmap.SetPixel(column, row, SKColors.White);
            }
        }
    }
}