[HELP] how to crop 720×480 to 1920×1080 without stretch?

[Selur]

so in option1, the picture quality will loss some effective coverage? because 720x480 crop to 720x360, in especial it will crop top and bottom, not left and right black side.

yes, but if your input image has black bars crop all of them and if you want to archive a resolution add back black bars using letterbox.

in option2, is it need to check "Custom" in letterbox? and letterbox is used for some custom crop?

Letterbox does not crop. It adds borders to the active image to archive a given resolution.
read: https://en.wikipedia.org/wiki/Letterboxi...filming%29
By default Hybrids letterbox the source to center the active image, if you want to have a centered image use 'custom' otherwise don't.

Cu Selur