Encoded clips slightly warped.

[jweathers7]

I have taken a few tiny clips from an MPEG2 video where I needed to add some transition effects, decoded the tiny clips, added my effect, re-encoded them, and then joined them back together with the main video.

When I used HCEnc for the the encoding process, I noticed upon playback on my DVD player and Widescreen TV that the on the joining parts of the re-encoded clips, the video seemed slightly different in a way perhaps attributable to encoding quality. Now I am not so sure.

As a test, I used TMPEnc instead to encode the clips and used the best quality that I could get. When I played the video back on my DVD player, something still seemed off when the video crossed the boundary from untouched video to re-encoded video. When I stepped across this boundary using the frame advance on my player, I noticed that it looks like the video is ever so slightly compressed along the horizontal dimension during the 2 seconds of re-encoded video. The compression is so subtle that it would not be noticeable accept for the fact that when you move across the boundary frames you can directly observe it since the neighbouring frame on the untouched side is almost identical and yet not compressed.

I noticed this effect before when using Womble MPEG Video Wizard to create a transition between two clips. I thought doing the process of creating the transition by hand might avoid this bizarre problem.

Does anyone have any idea what is going on here? There are no blackbars on the sides of the re-encoded video which might suggest that the compression along the horizontal is not uniform but rather distributed to be the greatest in the center.

[jweathers7]

Here's an update from my own investigations. When I took the problem area and decoded it again and then generated individual frame BMP files from it, I discovered that there actually are small black bars being added to the sides of my re-encoded clip although they weren't visible on my TV.

So why is TMPEnc (and other encoders as well) compressing the clip vertically and adding these black bars?

I'm making a 720x480 AVI from VirtualDub and then encoding as a 16:9 NTSC 720x480 MPEG2.

I've attached low quality JPEGs of a problem frame (Frame 88) and a normal neighboring frame (Frame 87).

[Mug Funky]

that's strange. i've never encountered something like this. all i can think of is something's happening either in decoding (unlikely) or during the adding of transitions. if the footage isn't being interpreted as the right aspect, your NLE program might be squishing the sides inward in an attempt to preserve the aspect ratio it thinks you need.

try re-export the transitions with all aspect settings in your NLE (interpret settings, project settings, etc) set to "square pixels", and then re-encode with HC.

that's all i can think of while the attachments are waiting to approve.

i do segment re-encodes all the time (seriously, like a couple a day. you'd be horrified how many glitches are to be found on master tapes), and i've never had a problem of this sort. granted, i do everything with avisynth, DGdecode and whatever software encoder suits the mood. there's no colourspace conversion and no fiddling around with aspect ratios, which is where i suspect the problem is coming from.