WriteableBitmap b = new WriteableBitmap(Weight*frame_size,Height*frame_size);
// WriteableBitmap uses BGRA format which is 4 bytes per pixel.
byte[] imageArray = new byte[b.PixelHeight * b.PixelWidth * 4];
for (int i = 0; i < imageArray.Length; i += 4)
{
    imageArray[i] = 0; // Blue
    imageArray[i + 1] = 0;  // Green
    imageArray[i + 2] = 255; // Red
    imageArray[i + 3] = 0;
}
//Open a stream to copy the image contents to the WriteableBitmap's pixel buffer
using (Stream stream = b.PixelBuffer.AsStream())
{
    await stream.WriteAsync(imageArray, 0, imageArray.Length);
}

SoftwareBitmap outputBitmap = SoftwareBitmap.CreateCopyFromBuffer(b.PixelBuffer,BitmapPixelFormat.Bgra8,b.PixelWidth,b.PixelHeight);