Problems encoding low-contrast bright scenes

[padeen]

I've got a xvid/avi video of work being done in the Artic, and I'd like to burn it to DVD so I can free up some disk space.

I'm having trouble with scenes set outdoors in sunlight on snow. The sunlight isn't strongly bright as it is winter in the artic and the sun is low on the horizon, but it results in lowish contrast and everything is white or light grey, ground and sky.

The first attempt lost quite a lot of detail in the background. The horizon (snow meets greyish sky) was almost completely lost into bright white, and detail in the middle distance like small shrubs and shadows indicating bumps and contours in the snow was lost. The foreground workers drilling holes was fine.

The bit rate on the original xvid is about 1000kbps and I set the encoding mpeg2 bitrate to be about 2500kbps (in HCEnc) to try and make sure detail wasn't lost, but unsuccessfully.

At this point, I wondered if changing the matrix might help this unusually lit video. I had used mpeg, so changed to dede's six-of-nine. It brought some of the detail out so it was better, but still not nearly like the original. So perhaps the matrix doesn't make much difference.

I was officially stumped now. I would have thought mpeg at 2500kbps would be fine for transcoding from xvid at 1000kbps. Anyone got any bright ideas?

Thanks for any help,

[poisondeathray]

Why not burn it as is (i.e. an .avi data file)? Standalone DivX/Xvid players are not that expensive these days (~$40 USD). You would lose no quality from your original, and use less space

[padeen]

Thanks for the reply. Usually I do burn it as avi and watch it like that on the DVD player.

Originally I wanted a DVD so my elderly father could watch it. He has trouble working a remote, so a simple Play button seemed easiest.

Then I noticed the loss of detail and became curious. So now it's become more of an exercise in finding out more about encoding. I clearly don't understand it as well as I'd like in things like the relationship between bit rate in xvid and mpeg for the same quality, and the relationship between matrices and bit rate and quality.

[guns1inger]

To preserve existing image quality when going from Xvid to mpeg-2 usually requires a bitrate that is around 3 - 4 times that of the source. Personally, I believe more is often required. 2500 for DVD is very low, regardless fo source. Normally you would only use a bitrate this low if you were encoding at half-D1 resolution.

How long is the video ?

[padeen]

The video's only about 45 minutes. I'll try bumping up the bitrate, didn't realise mpeg needed 3 - 4 times xvid. I thought 2 x would be ample

Any ideas on a better matrix? Six-of-nine did seem to help, but I seem to recall it was designed for bitrates lower than 4000kbps.

thanks for the help,

[jagabo]

The first attempt lost quite a lot of detail in the background. The horizon (snow meets greyish sky) was almost completely lost into bright white, and detail in the middle distance like small shrubs and shadows indicating bumps and contours in the snow was lost. The foreground workers drilling holes was fine.

Using more bitrate or a different DCT matix will not help you. You need to work with an editor that doesn't convert to RGB. When converting YV12 to RGB most programs use the rec.601 matrix which increases contrast. Very bright and very dark areas will lose all detail.

If you're using HcEnc you must be using AviSynth. You can use AviSynth to reduce the contrast of your source so that all the luma values fall between IRE 0 (y=16) and IRE 100 (y=235). A good tool for checking levels in AviSynth is VideoScope().