Hi Shinji,
There are two reasons why you are observing this:
Firstly, the screen preview helpers (eg IDeckLinkDX9ScreenPreviewHelper) are by design for small screen rendering by dropping every second line. We have a FAQ posted on the developer site for developers who want to generate their own full-screen preview helpers.
https://www.blackmagicdesign.com/support/faq/59024 - Full screen preview with DeckLink screen preview helpers
Secondly, you may want to try resizing your preview window to 1 pixel less than the captured frame size (ie 1279 x 719). Because the Direct3D top-left vertices is from (-0.5,-0.5), it will try to render from a frame of size 1281 x 721, then you will see some aliasing due to non-integral upscaling.
There is a good reference on Windows Dev Center on Direct3D rasterization rules:
https://docs.microsoft.com/en-us/window ... tion-rulesRegards
Cameron