Video convert using ffmepd exc

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C#

Hello guys,


i try to convert other format video file (large file 2 GB) into flv file.
My ffmepg command is below

string cmd = " -i \"" + filePath + "\\" + fileName + "\" -f flv -vcodec copy -acodec copy -s 1280×720 \"" + filePath + "\\" + fileName.Remove(fileName.IndexOf(".")) + ".flv" + "\"";

but generated flv size 0 kb. but thats code is running for the small file size.

One more Question
After video convert i want to cut the first frame of video file so i used following command.

string imgargs = " -i \"" + filePath + "\\" + fileName.Remove(fileName.IndexOf(".")) + ".flv" + "\" -ss 0 -vframes 1 -vcodec mjpeg -f image2 \"" + filePath + "\\" + fileName.Remove(fileName.IndexOf(".")) + ".jpg" + "\"";

Same problem. for small size convertor is running successfly but lager file it not able to convert into flv..

thanks in advance
govind