SK Matrix and Regular Canvas Operations

[silenuz]

I have a series of canvas translations and a rotation that when applied individually to the canvas results in the correct placement of the image, but when I create the SKMatrix with the same data the result is not the same. I believe it is to do with the way the rotation is created by the matrix, but could be wrong. Anyway so I set up some variables, for this example I'll use hard coded variables.

        int locationX = 0;
        int locationY = 256;

        int centerX = 256;
        int centerY = 128;

        int drawX = -128;
        int drawY = 0;
        var radians = MapTester.ToRadians(90);

When I use canvas directly with these values, as in the following code example the result is a blue rectangle in the lower left corner:

        // draw a blue rectangle in the lower left corner of the image
        canvas.Translate(locationX, locationY);
        canvas.Translate(centerX,centerY);
        canvas.RotateRadians(radians);
        canvas.Translate(drawX, drawY);
        canvas.DrawPicture(RectangleTile.Picture);
        canvas.Restore();
        
        // Draw a black square in top left corner to show canvas was restored
        paint.Color = SKColors.Black;
        canvas.DrawRect(0, 0, 7, 7, paint);
        canvas.Save();

When I try to create a Matrix with the same values and pass a reference to the DrawPicture method it doesn't work:

        // Attempt adding a red rectangle on top of the above blue rectangle
        SKMatrix matrix = SKMatrix.CreateIdentity();
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(locationX,locationY));
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(centerX, centerY));
        matrix = matrix.PostConcat(SKMatrix.CreateRotation(radians));
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(drawX, drawY));
        canvas.DrawPicture(TestTile.Picture, ref matrix);
        canvas.Save();

The resulting image only has the blue rectangle and not the one drawn using the matrix.

In case it makes a difference here is the method in it's entirety:

    public void DrawMatrixFailExample()
    {
        SKImageInfo imageInfo = new SKImageInfo(512, 768);
        SKSurface surface = SKSurface.Create(imageInfo);
        SKCanvas canvas = surface.Canvas;
        canvas.Clear(SKColors.Azure);

        SKPaint paint = new SKPaint();
        paint.Color = SKColors.Blue;
        paint.Style = SKPaintStyle.Fill;

        SquareTile.Draw(canvas);
        canvas.Save();
        
        int locationX = 0;
        int locationY = 256;

        int centerX = 256;
        int centerY = 128;

        int drawX = -128;
        int drawY = 0;
        
        var radians = MapTester.ToRadians(90);
        
        // draw a blue rectangle in the lower left corner of the image
        canvas.Translate(locationX, locationY);
        canvas.Translate(centerX,centerY);
        canvas.RotateRadians(radians);
        canvas.Translate(drawX, drawY);
        canvas.DrawPicture(RectangleTile.Picture);
        canvas.Restore();
        
        // Draw a black square in top left corner to show canvas was restored
        paint.Color = SKColors.Black;
        canvas.DrawRect(0, 0, 7, 7, paint);
        canvas.Save();
        
        // Attempt adding a red rectangle on top of the above blue rectangle
        SKMatrix matrix = SKMatrix.CreateIdentity();
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(locationX,locationY));
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(centerX, centerY));
        matrix = matrix.PostConcat(SKMatrix.CreateRotation(radians));
        matrix = matrix.PostConcat(SKMatrix.CreateTranslation(drawX, drawY));
        canvas.DrawPicture(TestTile.Picture, ref matrix);
        canvas.Save();
        
        using (var data = surface.Snapshot())
        using (var pngImage = data.Encode(SKEncodedImageFormat.Png, 100))
        {
            string output_file = output_folder + Path.DirectorySeparatorChar + "matrix-rotation-fail-example.png";
            using (var stream = File.OpenWrite(output_file))
            {
                pngImage.SaveTo(stream);
            }
        }
    }