Resolution Loss with Gnot Encoding

[marc999]

I started making DivX encoded movies with Gknot about 5 months ago. One thing I have noticed with all them is a huge loss in resolution compared to the original DVD. By resolution, what I mean is size of the video. If I playback a DVD on my computer using PowerDVD, the regular size of the window is huge. If I then select full screen, it doesn't have much to expand and thus it looks great still. However, my DivX movies are very different. In Gknot, I use the largest resolution I can without going over 100 % in the W-Zoom or
H-Zoom categories (or only slighlty over, such as 103 %). An example of one of the more common resolutions I end up selecting is: 736 x 304. Its regular 100 % size (and every other movie I encoded) is much smaller and doesn't look near as good when expanded to full screen. I realize part of the loss of resolution (as compared to DVDs resolution of 720 x 480) is because of the cropping, but that doesn't explain why the DivX movie and original movie should be any different in size when played back on my computer. Once cropped, the size of each video should still be the same (since we aren't cropping out the video, just the black bars). The only difference should be that the AVI doesn't have the black bars on the top and bottom of the video like the DVD version does. Just in care you are wondering I use the highest possible quality settings: 2 800 MB CDs for a high bit rate, 2 pass, no Pro features (such as BiDirectional Encoding) and the Sharp Bicubic Resize Filter. So is there any way to fix this, so that my AVI file has the same video size as the original DVD but just without the black bars on the top and bottom? Any help would be greatly appreciated. Thanks for reading my novel.

[jggimi]

While all NTSC DVDs are 720x480, they are designed for display in one of two display aspect ratios: 4:3 or 16:9. Letterboxing is used to recreate the original film aspect ratio for widescreen discs.

You could actually create DivX videos from DVD without any resizing, you would need an .avi player capable of setting the aspect ratio to 4:3 or 16:9, and you would have to very carefully restrict cropping values to ensure legal resolutions are passed to the codec.

The subjective differences you encounter with resized DivX may be due to the type of resizing you're doing; for example, you may want to try switching from neutral bicubic to sharp bicubic or Lanczos resizing, and see if that makes a difference in your results.

Lastly, some of the DivX Pro features can make a significant difference in perceived detail. Bidirectional frames (B-Frames) in particular are worth the small effort of a DivX Pro installation. In addition, I've found some improvements using light psychovisuals, particularly with content that includes scenes with water.

[marc999]

Thanks for your reply jggimi.
I think from reading your reply and other stuff, what I want to do is encode a DivX file WITHOUT cropping or resizing. But Gknot will not let me do it. What I want to do is keep my input resolution the same as my output resolution (i.e. 720 x 480). But even chosing disable for crop under the resolution tab in Gknot, I still can't select 720 x 480 output resolution. The closest thing Gknot gives me is 720 x 400 and it STILL has black bars, which totally baffles me. Any suggestions to help me get Gknot to do this ?

[hakko504]

On resolution tab, set Display AR to 1.5 (NTSC, 1.25 for PAL) and w-modul to 16. Then use cropping to get rid of the black bars, and comment out the resize in the final .avs

You can't encode MPEG-2 as 2,35:1 so you have to add black bars to make it fit 16:9 or 4:3, so that's why you get 704x400 AND have black bars.

[jggimi]

To eliminate resizing (and possibly cropping), you can edit the .avs before submitting the encoding job. Either comment out or remove the lines from the .avs script. Just click "edit" instead of "Save & Encode" on the filter options window that pops up after pressing "Save & Encode" on the display window.

You will not be able to edit a compressibility check job and submit it in this fashion, and would instead have to set up a compressibility check manually through Virtual Dub. If you're interested, I wrote up a procedure in this link (http://forum.doom9.org/showthread.php?s=&threadid=37945).

[marc999]

Thanks Hacko and jggimi !!
With the helpful info from both of your replies, I think now I can do what I want. I noticed as soon as I set Display AR to 1.5 I could select 720 x 480 output resolution !! I'll try everything tonight, and hopefully all will go well. Hacko I noticed you suggested to crop out the black bars. Is there an advantage to cropping out the black bars? For some reason I seem to think it may cut down on the final size of the .avi, but I figure not by much. Please correct me if I'm wrong.

[hakko504]

Thing is, that the sharp edge between the black area and the video is very hard to encode and requires quite a lot of bitrate. Tests with fixed quantizers (Q=2) have shown that you can gain 2 to 8 % of bitrate by cutting the black area. (Do a search, I know there is at least one thread in the General forum about this.)

[marc999]

Very interesting info (about the encoding between the black bars and the video) Hacko.

Well I tried it last night and by the time I cropped out the black bars (after setting the Display AR to 1.5 and starting with 720 x 480) I ended up with a resolution of 704 x 352, which isn't really better than what I was getting before (by following the Doom9 guide of chosing Input Pixel Aspect Ratio -> NTSC anamorphic [16:9]). However, knowing about setting Display AR to 1.5 is still a very valuable piece of information for me to know. One day, when I feel justified in spending the money, I will use a computer to play back DivX files on my TV. However, I don't think a TV would be able to display an image of 704 x 352. So I could always encode without cropping or resizing (unfortunately with a bitrate penalty of 2 to 8 per cent as you pointed out Hacko) and pass it on to the TV at a standard resolution. The TV then has an internal scaler which can upconvert from 480p to 540p, which would hopefully add to the PQ (picture quality).

Another thing I learned from doing this experiment last night, is that I am not losing any resolution wiht Gknot. Well, first of all the paragraph above explains that, but another thing happened this morning when I checked my DivX file. I went to compare it to the DVD ripped file, and for some reason my computer was running unusually slow. This slow performance allowed me to see what PowerDVD does when a .vob file is launched. It started out at about the same size as my DivX rips. Then it went one size bigger, than another size bigger. So all I can conclude from this is that PowerDVD not only plays back the DVD, but it scales the video to a higher resolution. Another thing that leads me to believe this, is that when I preview the .vob file in DVD2AVI, it is again pretty much the same size as my final DivX file, and of course the .vob file is at 720 x 480.

[jggimi]

I'm glad you've figured out what's been going on. But if I read your latest comment correctly .... was your original complaint, then, rooted in your not selecting full screen playback with your .avi player of choice?

[marc999]

jggimi,

To answer your question, my original problem was not that I didn't select full screen in my .avi player. Even after learning what I have from this post, I still find myself with the problem that the quality in full screen mode is lacking. But that is on a computer monitor, which is so accurate that slight imperfections really stand out. Maybe on a TV it would look great in full screen. My original problem comes from comparing apples to oranges. I was comparing the default resolution of the .vob file to the default resolution of the .avi file. The .avi player just plays it back at it's native Gknot encoded resolution. The .vob player (PowerDVD) on the other hand, takes the DVD at 720 x 480 and scales it to a higher resolution, even when you don't select full screen mode. Since the .vob file was bigger, I assumed with cropping, resizing, etc, I was losing very significant resolution the Gnot. But everything is relative. It's not that my Gnot .avi was significantly smaller than the original 720 x 480 on the DVD, it's that the DVD file was being played back at a higher (larger) resolution than 720 x 480 by PowerDVD. It's kinda hard for me to put in words but hopefully this makes sense to you.