I made a mosaic with the HTTP interface mosaic wizard.
The background resolution is 1600x1200, same as my screen and I divided it in 2 raws and columns so i get 4 800x600 screens.
But when i input some 16:9 videos (1920x1080 for instance) their aspect ratio is stretched to fit 800x600, so i would like to crop them left and right to keep the original proportions (I don't want black bars either).