having custom implementation of BitmapSource is useless in following case:

source bitmap 20000x25000 pixels 24bpp

rendering to Visual as 200x200pixels

because next components after custom BitmapSource in WIC pipeline can asks for data only by using CopyPixels, there is no way how to tell to back to custom bitmap that destination resolution is only 200x200 and that can be used pre-resampled source instead of hugh original of custom bitmap....