Skiasharp Read and Modify Pixels - Improve performance

[mikepasoglou]

I am trying to use C sharp and Skiashap to implement image processing in a Blazor webassebly app.
The code below is trying to implement edge detection. The result of the code below is quite good , it needs improving , but this is not my point.
The code below is my first try.
`canvas.Clear(SKColors.White);
// USerskbitmapNEW has been loaded earlier.
SKBitmap image = new SKBitmap(USerskbitmapNEW.Width, USerskbitmapNEW.Height);
for (int i = 0; i < image.Width; i++)
{
for (int j = 0; j < image.Height; j++)
{
SKColor c1 = USerskbitmapNEW.GetPixel(i, j - 1);
SKColor c2 = USerskbitmapNEW.GetPixel(i, j + 1);
int xval = 0;
int yval = 0;
xval = (int)(c1.Red * .3 + c2.Blue * 0.11 - (c2.Red * .3));
yval = (int)(c2.Red * .3 + c2.Green * .29 + c2.Blue * 0.11);
if (xval > yval )
{
SKColor c = SKColors.Black;
image.SetPixel(i, j, c);
}

                }
            }
            canvas.DrawBitmap(image, myframe);

My problem is that the methods : SKbitmap.GetPixel(i, j); and SKbitmap.SetPixel(i, j, color); are extremely slow when use them with images bigger than 200kb size.
In contrast to the above methods( SKBitmap .GetPixel and SKBitmap .SetPixel) , when you apply image various filters , for example , var cf = SKColorFilter.CreateTable(null, null, colorTable, colorTable); .... skiasharp is very fast.

My second try to improve performance , is to use SKPixmap class insted of the method above. So my code looks like the snipset below.
`canvas.Clear(SKColors.White);

            SKBitmap inputBitmap = USerskbitmapNEW.Copy();
        SKBitmap outputBitmap = new SKBitmap(inputBitmap.Width, inputBitmap.Height);

            SKPixmap inputPixmap = inputBitmap.PeekPixels();
            SKPixmap outputPixmap = outputBitmap.PeekPixels();
            
            for (int y = 1; y < inputPixmap.Height - 1; y++)
            {
                for (int x = 1; x < inputPixmap.Width - 1; x++)
                {
                    SKColor c00 = inputPixmap.GetPixelColor(x - 1, y - 1);
                    SKColor c01 = inputPixmap.GetPixelColor(x - 1, y);
                    SKColor c02 = inputPixmap.GetPixelColor(x - 1, y + 1);
                    SKColor c10 = inputPixmap.GetPixelColor(x, y - 1);
                    SKColor c11 = inputPixmap.GetPixelColor(x, y);
                    SKColor c12 = inputPixmap.GetPixelColor(x, y + 1);
                    SKColor c20 = inputPixmap.GetPixelColor(x + 1, y - 1);
                    SKColor c21 = inputPixmap.GetPixelColor(x + 1, y);
                    SKColor c22 = inputPixmap.GetPixelColor(x + 1, y + 1);

                    float gx = c00.Red * -1 + c02.Red + c10.Red * -2 + c12.Red * 2 + c20.Red * -1 + c22.Red;
                    float gy = c00.Red * -1 + c01.Red * -2 + c02.Red * -1 + c20.Red + c21.Red * 2 + c22.Red;

                    float magnitude = (float)Math.Sqrt(gx * gx + gy * gy);
                    SKColor color = magnitude > 128 ? SKColors.Black : SKColors.White;

                    // Set the output pixel color 
                    //outputPixmap.SetPixel(x, y, color);   //   HERE is my problem 
                   
                }
            }
		canvas.DrawBitmap(outputBitmap, myframe);`

In that case the problem is that SKPixmap does not have any method to set the output pixel color accordingly to Skbitmap.Setpixel(x,y,color), so the above code using SKPixmap cant not implement.

My questions for this discussion are:
Is there any other way , using SKPixmap class , to set and modify pixels of the image?

In general for Skiasharp. Is there any other way to read and modily pixels instead of Skbitmap.SetPixel ?
( which is working but with bad performance.)

Thanks in advance
Michalis