I am using after effects CC
I import an AVCHD 1920x1080p 50p 28 Mbps clip , then I use the effects details preserving upsacle in order to upscale it to 4K.
The problem occurs when I try to export it to h264 mp4 at 4k maintaining the 16/9 ratio 4096x2304 with a bitrate of 50 Mbps , an error message says "Output size will be resized from 4096x2304(1,00 PAR) to 1636x2304 (2,50 PAR) to meet format constraints.".
How can I export my original clip in a 4K 16/9 ratio with the effect detail preserving upscale ?
It's because you don't understand mpeg encoding. H.261 or is very picky about frame sizes and only a few are legal. This is because MPEG is encoded in even numbered blocks of pixels. You must use the standard frame sizes. If this means your footage is cropped or letterboxed then that is what has to be.
A quick Google search well give you the specifications for H.261 or encoding.