BitmapSource.CopyPixels -what's a good value for stride?

You can use stride = pixel_size * image_width value. For example, for RGBA bitmap with 100 pixel width, stride = 400.

Some applications may require special line alignment. For example, Windows GDI bitmaps require 32-bits line alignment. In this case, for RGB bitmap with width = 33, stride value 33*3=99 should be changed to 100, to have 32-bits line alignment in destination array.

Generally, you should know destination array requirements. In there are no special requirements, use default pixel_size * image_width.

var stride = ((bitmapSource.PixelWidth * bitmapSource.Format.BitsPerPixel + 31) / 32) * 4;

or

var stride = ((bitmapSource.PixelWidth * bitmapSource.Format.BitsPerPixel + 31) >> 5) << 2;