How to resize vob file using VirtualDub and maintain correct aspect ratio

[nebbish_2112]

OK, I've been learning alot lately about resolution vs. aspect ratio. But I am still a little confused.

I have a vob file which in GSpot gives me info as attached. My goal is to use DivX compression option within virtualdub to attain a 600 kbps DivX video for viewing on a 480X320 PDA screen size. I have the option to use resize filter or resize option in the DivX settings.
If I understand this correctly, these are just resolution numbers. They don't correspond to any aspect ratio in particular.

My question is, I don't know what to input for resize numbers. I tried setting resize filter in VirtualDub to 480X320, but GSpot reported DAR of 1.500 (3:2) and SAR of 1.500 (3:2). I than input resize of 480X270. GSpot reported DAR=1.778 (16:9) and SAR=1.778 (16:9).

Does the 16:9 flag completely get lost here , and does resize of 480X270 really achieve a 1.7777778 aspect. I thought these were resolutions and not related to aspect ratio. Would a 2.35:1 film need resize of 480X204? It's almost like I'm resizing only the movie, and not the black bars. Wouldn't I want max resolution of 480X320? It's like I'd want resolution of 320 height, because I'm now losing resolution setting height at only 270, or 204, right?

[jagabo]

Divx uses square pixels so the display aspect ratio is the same as the relative frame dimensions. For 16:9 video: 480 * 9 / 16 = 270

Divx will be a little happier with dimensions that are even multiples of 16. So use 480x272.

I'm assuming here that your player will maintain that aspect ratio on playback. If it enlarges the video to full screen you'll have to add black bars to letterbox the frame.

[nebbish_2112]

jagabo, this is starting to make sense. my player does play the movie at the correct aspect ratio. but another question... the GSpot example in my original post states a SAR of 1.500 (3:2). The PAR of 1.185 takes care of the computer monitor pixels to properly display the 16:9 frame?

My original VOB has black bars on top and bottom. Are these black bars figured into the 720X480 frames? If I resize to 480X270, the 270 height will include the video and black bar information, and my player will add 25 to the top and bottom of the frame? If this is the case, aren't the black bars taking up useless information, and extra file size? Wouldn't it make more sense to crop the video and eliminate the black bars altogether? Is just the video portion responsible for a 16:9 aspect ratio?

If you're tempted, is it a coincidence that:

1/1.185 = 0.84388185654008438818565400843882

than

0.84388185654008438818565400843882 X 320 = 270.04

which is what the resize height of my DivX movie should be?

Sorry for all the questions, but eager to learn properly.

[jagabo]

jagabo, this is starting to make sense. my player does play the movie at the correct aspect ratio. but another question... the GSpot example in my original post states a SAR of 1.500 (3:2). The PAR of 1.185 takes care of the computer monitor pixels to properly display the 16:9 frame?

Yes. Basically the VOB files have a flag that tells the DVD player that the video should be displayed as 16:9 or 4:3.

My original VOB has black bars on top and bottom. Are these black bars figured into the 720X480 frames? If I resize to 480X270, the 270 height will include the video and black bar information, and my player will add 25 to the top and bottom of the frame?

Yes.

Is just the video portion responsible for a 16:9 aspect ratio?

No, the black bars are part of the 16:9 picture. DVD only supports two aspect ratios, 4:3 or 16:9. Anything other than that will have black bars added to fill up the frame.

If this is the case, aren't the black bars taking up useless information, and extra file size?

Yes. But they consume very little bitrate.

Wouldn't it make more sense to crop the video and eliminate the black bars altogether?

You can trim the black bars off if you want. It's faster and works best if you crop first then resize. But then you have to figure out exactly what dimensions to resize to. The easiest way to figure it out is to resize first then crop, then note the dimensions of the remaining video. Say you resize to 480x270 and then crop away the letterbox bars, leaving 480x204. Now you know that if you crop the black bars first you need to resize what's left to 480x204.

If you're tempted, is it a coincidence that:

1/1.185 = 0.84388185654008438818565400843882

than

0.84388185654008438818565400843882 X 320 = 270.04

which is what the resize height of my DivX movie should be?

I have to think about this a bit but I'm too tired right now. I'm pretty sure it's not a coincidence and that it's simply another way of approaching the problem.

[nebbish_2112]

Thank you, this is the information I need. I experimented cropping the black bars and preserving the black bars, and the difference in file size was very insignificant, along the lines of .09% difference. I think I will keep the bars intact as I than have the convenience of dragging the file in GSpot and the DAR being a true 16:9 frame.

This leads me to yet another question. As you stated, the black bars figure into the 16:9 aspect ratio. When I experimented cropping out the black bars, the aspect ratio of the video content itself was nowhere near 16:9. It was a very odd size ratio of 12:5. Shouldn't the video content be 16:9? If I watch the VOB on my widescreen TV, there would be no black bars visible. How can this be if the video content has a ratio of 12:5 and my TV a ratio of 16:9?

On a side note, how does the DVD player know how to not display the black bars of such content, even though they are there and figure into the 16:9 content?

Sincerely,
The more I learn, the less I know

[jagabo]

This leads me to yet another question. As you stated, the black bars figure into the 16:9 aspect ratio. When I experimented cropping out the black bars, the aspect ratio of the video content itself was nowhere near 16:9. It was a very odd size ratio of 12:5. Shouldn't the video content be 16:9?

No, the black bars are included in the 16:9 DAR. What you have is a ~2.4:1 movie on a 16:9 DVD. As mentioned earlier, DVD only supports two aspect ratios, 4:3 and 16:9.

If I watch the VOB on my widescreen TV, there would be no black bars visible. How can this be if the video content has a ratio of 12:5 and my TV a ratio of 16:9?

With any movie that's wider than 16:9 you should see the black bars on a widescreen TV. Unless you're using a stretch feature that stretches the image vertically or a zoom feature that zooms in and cuts off the sides.

On a side note, how does the DVD player know how to not display the black bars of such content, even though they are there and figure into the 16:9 content?

The DVD player doesn't know how to display the content without the black bars. It displays the full 16:9 image with black bars included. There are some players that have a zoom/stretch capability.

[nebbish_2112]

Thank you so much for your response. I have experimented with a few different VOB formats. As expected GSpot lists them all as 16:9. But when cropping the black bars I get aspect ratios all over the map. I am guessing there are many different aspect ratios for films.

What I want to find now is a true 16:9 ratio VOB so I can see for myself there are no black bars.

[jagabo]

http://www.imdb.com/SearchRatios?1.78%20:%201

[nebbish_2112]

JAGOBO,

You read my mind, thank you for the link

Thanks again for all your input. This board continues to be a wealth of information.

[jagabo]

Glad to help out.

[Cornucopia]

Here's the other thing,
Gspot is reading the SAR in "dumb" mode. IOW, it's doing this:

1. Count/read horizontal pixels ...720
2. Count/read vertical pixels...480
3. Simple compute of SAR... (H/V = SAR)...1.5
4. Read DAR flag value...16:9 (aka 1.7888)
5. Simple compute of PAR... (DAR/SAR=PAR)...1.185

But Gspot should know better.

The video image size of 720x480 uses (should) 8pixels per side as padding (black). So the true horizontal image resolution is 704 and this is the number that should be used in the calculations.
Plus, for SD video, you've really only got 1 real screen format and that's 4:3.
For NTSC (525/60) 4:3, the PAR is 10/11 or .90909090909
So a 4:3 DAR, with a PAR of .909090909 is going to have a vertical size of 480, and (working backwards) should have a horizontal size of 704 (or 720 if padded).

Similarly with 16:9 [ANAMORPHIC] SD video,
For NTSC (525/60) 16:9, the PAR is 40/33 or 1.2121212121212
So a 16:9 DAR, with a PAR of 1.212121212 is going to have a vertical size of 480, and (working backwards again) should have ... a horizontal size of 704(720 if padded)! AGAIN!

So when Gspot sees h=720 and v=480, and DAR of 4:3 (aka 1.33333), it should CORRECTLY see the PAR as .90909090909 and the SAR should be something like 1.333333 also (unless one always assumes that SAR is "supposed" to be uncompensated).
Similarly with 16:9, when Gspot sees h=720 and v=480, and DAR of 16:9 (aka 1.77777778), it should CORRECTLY see the PAR as 1.212121212 and the SAR should again be something like 1.7777778 also (barring similar false assumptions).
Clearly, there will have to be a smarter way of computing SAR, PAR and DAR for them to be truly correct and worthy of use with other programs.

WRE your source, my guess is that you've got a ~2.35 movie encoded as 16:9 widescreen DVD.
IIWY, I'd:
1. Crop the 8pixel pads to 704x480
2. Smart Resize to square-pixelled 16:9, or 853x480
3. DeInterlace, if needed (however you prefer)
4. Crop the black 2.35AR black padding, to ~853x360 (120 off top+bottom each)
5. Assuming your PDA doesn't support DivX Anamorphically-encoded video, resize to ~480x202 and pad with black for letterboxing (59 on top and bottom each) to fill the 480x320 screen (I --ALWAYS-- prefer adding in generated solid black if letterboxing is unavoidable, as every bit counts especially at PDA bitrates)
6. Encode to DivX.

Scott

[nebbish_2112]

I'm back. In order to maximize real estate on my 480X320 screen size, I desire to eliminate the black bars certain movies. I had experimented doing a crop in virtualdub on a 720X480 4:3 vob. No resizing was done. The crop was set for 720X400. No compression was set and I saved as a uncompressed avi.

The result was a 720X400 file, but aspect ratio is incorrect. I view the original VOB side by side with the cropped video in VLC media player, and the cropped video is much wider.

I than took the 720X480 VOB, added no crop or resize, saved it as uncompressed avi, and the same thing occured. GSpot reported the aspect ratio as 1.5 (3:2).

I venture to guess this is happening because the 4:3 flag gets lost in any processing I do. How do I crop and resize the 720X480 vob file an still maintain my desired aspect ratio?

[nebbish_2112]

I GOT IT!! Had to resize first and then crop. I was trying the reverse.

I really appreciate all your help (Jagabo and Cornucopia). You guys were of invaluable assistance. I'm a real stickler for doing something the correct way, and do ask a LOT of questions. But I like to learn why things are done and for what reason. I like to try and see the whole picture.

Once again, thank you for everything!