Full-frame Encoding?

[h00z]

There has been a bit of discussion and debate in these forums and others about encoding widescreen movies in a full-frame format. The general concensus always seems to be "Don't waste bitrate on black bars!" How much bitrate are we really wasting though? Has anyone even tested it to see? Well... I did.

I started with a reasonable 6m 15s clip taken from the (1.85:1) widescreen movie, "Tenchi Forever". To make it fair, I cropped and resized the video (just like I normally would) to a size of 512x272. For my full-frame test, I used the exact same clip and added the black bars to the top and bottom (via VirtualDub's SmartResize filter) for a final resolution of 512x384.

Doing a two-pass encode at identical bitrates would have been counter-productive since the two encodes would've been almost identical in size. Instead I decided to use a one-pass quality based encode for each video... I let DivX v5.0.1 decide what bitrate to use for the best quality (Quantizer of 4 for each encode).

How did it turn out? Not as drastic as you might think. The full-screen (512x384) file ended-up at 21.9mb final size while the widescreen (512x272) file was 21.1mb. Cropping the black bars only saved me 3.7% on the final file size.

What does this mean? This means that we aren't wasting as much bitrate as we thought if we decide to encode the video with the black bars. That will be good news to people with an Xcard, and people who are trying to encode with future set-top compatability in mind.

I also found some other interesting benefits to having the full-frame. Mostly due to the way subtitles are displayed. Here are a couple of snapshots I took to illustrate my point:

http://www.hooz.org/misc.images/divx.stuff/widescreen.jpg

http://www.hooz.org/misc.images/divx.stuff/full-frame.jpg

What do you guys think?

[Koepi]

I think we need every single bit and thus throwing away the possible quality gain for "just 4% more space needed"-method is hurting.
Why not crop it? Because it's easier not to setup cropping? Because it's easier to transcode to (S)VCD? I don't see reasons for this.

That's what I think of it, you asked for it ;)

Regards,
Koepi

[h00z]

I've seen (here and other places) people asking about the Xcard and its lack of support for "anamorphic" DivX. Meaning it must be encoded fullscreen or the image will be stretched to fit fullscreen and consequently distorted.

I also have a sneaking suspicion that any set-top DivX players we will see in the (near) future will act the same way... Forcing people to encode at fullscreen resolutions. I just wanted to show that you don't lose much of anything when you do, and I think the proof is there in the tests.

Look at it this way... If you're encoding at a bitrate of 1000 (easy number) with widescreen, the equivalent bitrate for fullscreen would be ~1040 to achieve the same quality. That's a drop in the bucket if you ask me. Especially for a 2CD rip.

[Sven Bent]

i never have seeen the idea of baying af dvd-play or divxstadnalonme player.
when you cna build af mini pc to do the same job and i cna always be upgradet to newer formats

with a micro atx mothebaord and a nice looking minicase and some good equipmente you can get better pictures quality of of min pc then a regular dvd player

jsut remmeber the you want to have either ATI og matrox tv-out

[neo_sapien]

Can someone confirm that set-top DivX players will only be able to play full screen movies? My entire collection is cropped. That would truly, truly blow goats if it were the case.

[Head Hunter]

@h00z
There isn't much of a change in filesize when the borders are added after the cropping was initially done, and were probably multiples of sixteen. Try redoing you test, but adding an extra pixel to the top, and take one away from the bottom. You should see more of a bitrate increase due to the black, being inside a macroblock with picture actually in it.

[Mahimahi]

Hi Sven,

Can you give me a hint were I can find something written about that mini pc you're talking about?

Do you think it can be configured in such a way that it starts playing without the need of a keyboard? (Just switch it on, insert the CDROM and enjoy)

[dragoman]

Hi,

An alternative to a set-top DivX player is a laptop with Tv-out....my new Sony Vaio with the ATI Radeo 7500 Mobile graphics card plays divx flawlessly on any TV I've tried it on....and it also encodes pretty quick too....

dragoman

[Mahimahi]

Hi Dragoman,

Priceticket of such a thing?

[neo_sapien]

Anyone?

Will future set-top players be unable to play movies that have been cropped correctly? Do I need to start encoding my 640x272 movies at 640x480 by using AddBorders with 104 on the top and bottom?

I would REALLY appreciate some input here, preferably from someone who has some knowledge into the official MPEG-4 specs regarding sizes that will go into the set-top box general design.

[dragoman]

@mahimahi

I paid about 2200 for this particular model. However, I am sure there are other laptops out there for cheaper than have a TV-out feature, you just have to look around.

I personally wanted the 15" screen, the faster processsor, and more RAM.

dragoman

[DmitryR]

The only thing I can say: black boxes do not affect bandwidth at all if their height is a multiplier of 16 and picture height is a multiplier of 16 (so the coordinate of the upper line of the lower box is multiplier of 16). In other way it can seriously impact quality or size. I recently forgot to crop 2-5 pixel border and got 150% the size of the cropped version.

[oddball]

For widescreen TV owners black bars on DiVX = pain the butt. Zooming the image can result in degraded picture quality depending on how your TV does it etc. Using progs like DiVXG400 which can auto crop black bars can help but is imperfect. Why encode the black bars at all? Not needed. The issue with the XMPEG card is a single instance thus far. If you have a decent TV out card it will put the black bars back in on 4:3 TV's anyhow if outputting correctly (And on widescreen TV's if wider than 16:9).

To me black bars on DiVX equals only one thing. Lazy assed ripper.

[Maccara]

Besides, if you look at the sample pictures, you would notice that the widescreen version was not properly cropped. 1 pixel of black remains in almost all sides.

That's what's eating the bandwidth; not the full bars.

Make a new test cropping 1 pixel into the picture and you'll notice a much bigger difference. (keep the picture in multiple of 16 pixels, though, or you'll waste bandwidth again)

(caveat: I haven't tested this myself, but this is what most people are saying, so I'd suggest you try it out yourself rather than take my word for it ;)

Rgds,
Maccara

[MaXiMuS]

What is the point of XMPEG card ?

any TV out card will do the job (if ya have celeron or better cpu)

[h00z]

Nevermind. You people are missing the point.

I actually did crop 1 pixel into the picture (VirtualDub displays a "shadow" around the preview window for some reason) and then added the black bars... Effectively adding a step to my encoding process.

But anyway... Forget I said anything. The people who actually need this information will hopefully find it useful.