Should I resize the image?

[nomad32]

Hi,

I have an NTSC SVCD I am encoding to DivX 5 and am not sure whether I should resize the image. The origianl frame is 480x480 and the image size is 480x204. When I encode, should I resize this to 640x272 or leave it as 480x204?

I tried it on a 5-second shot and the 640x272 was 1.6MB compared to the 1.2MB of the 480x204 video.

Can someone advise me which way is best?

Cheers

[ookzDVD]

Enlarging resolution is not a good idea, imho.

[bb]

I'd resize to 480x208, since 204 is not divisible by 16.

:eek:

bb

[Gren]

i'd leave it at the current res - enlarging isn't going to increase quality

@bb
do u REALLY need to have your res divisible by 16?

[grug2k]

When you play an SVCD, the 480x480 resolution is stretched to 640x480 for playback. As such, I'd encode it at 640x272, simply because you'll want the extra horizontal resolution.

As your image is going to be stretched regardless when you play it (unless you like looking at tiny 480x??? windows), you should just encode it at 640x272 and then drop your screen res to 640x480 when you watch it so you're not stretching the movie twice.

Encoding it stretched and playing it at 640x480 seems to provide better quality than stretching it during playback.

[bb]

@Gren:
When encoding to DivX5 you should always encode to resolutions that are divisible by 16 because of the pink/green bug. You might drop this constraint as soon as we get a corrected DivX5.xx ...

For compatibility reason it's also recommended to use a horizontal res which is divisible by 32 (Matrox problem).

bb

[DJ Bobo]

As bb said, go for 480x208 (using BilinearResize since you're resizing in only one direction)

[ftack]

I would follow grug2k here. I definitely would go for 640x272 in order not to loose to many vertical resolution.

[ftack]

Sorry, I gave this a second thought. SVCD is encoded at 480 times 480, but at playback time stretched to a 4/3 aspect ratio. So with "frame size is 480x480 and the image size is 480x204", you probably mean that the image is letterboxed. I'd go for an encode with no loss in resolution: I'd crop off the black borders and I'd enlarge horizontally as to get the correct aspect ratio. In this case, we would need to resize the cropped image to 640 x 204, but in order to have a height divisible by sixteen, you could cheat and resize to 208 (you won't notice it) or leave two tiny 2 pixel black lines at the top and the bottom of the image.

[sonni_kuba]

Enlarging is never a good idea. You will not gain any extra detail by encoding at 640x272.
Instead, you will use up bits that could have gone to encoding a detailed picture at 480x208, to create a washed-out blurry picture at 640x272, furthermore, if you want to constrain yourself to a certain filesize, you increase the risk of macroblocks appearing.
Think of it as trying to create a 256kb/s MP3 from one that was originally encoded at 96kb/s.

[Nazgul]

While it's true that you won't gain any detail from enlarging to 640x272, you also need to keep in mind that shrinking to 480x208 will cause you to lose some data, since even if it wasn't letterboxed material you'd have to shrink it to 480x360 to get the aspect ratio right.

I had this issue recently with several SVCD's I was encoding that were letterboxed. I was trying to decide between 480x208 and 640x272. I ended up using 480x208 because of file-size issues. I could actually use the same bitrate at 640x272 and I got image quality that I was quite happy with, but the files were just a few megs bigger than what I wanted. With 480x208, they were just right. :)

[sonni_kuba]

Re: Losing some data/quality by resizing to 480x208

I agree completely.

I believe that you should be able to encode at 480x360, and then use The Playa to play back the video with a custom aspect ratio to correct the picture.

[Nazgul]

Originally posted by sonni_kuba
Re: Losing some data/quality by resizing to 480x208

I agree completely.

I believe that you should be able to encode at 480x360, and then use The Playa to play back the video with a custom aspect ratio to correct the picture.

Why would you want to do that? If you're going to use the Playa to set the aspect ratio, why shrink the file?

[sonni_kuba]

Perhaps, I was not clear while making my point.

I meant to say that in case quality preservation was of great concern, AND you wanted a smaller file size, then the best solution would be to crop the black borders and encode the movie at 480x360 (aspect ratio would be completely off) and then when viewing the movie, the aspect ratio would have to be manually adjusted. The Playa offers this option.

Personally, I would never follow this protocol, but instead wanted to suggest an option.

[ftack]

| Enlarging is never a good idea. You will not gain any extra detail by encoding at 640x272.

I don't agree, sonni_kuba. You are indeed not gaining detail (you never do, the source is the most you've got), but you are not going to loose detail because you're not reducing the vertical dimension.

It depends on your priorities. If you want to retain maximum quality, you should not reduce any dimensions. Henceforth the need to expand horizontal resolution to obtain a correct aspect ratio. If file size versus quality is of concern, it may be aceptable to go for 480x208, but this *is* inevitably at the expense of detail. For valuable, good quality video material, I'd definitely go for 640x272 and a larger file size.

| Think of it as trying to create a 256kb/s MP3 from one that was originally encoded at 96kb/s.

This is not a correct analogy here. The vertical resolution of the source *is* 272 pixels, so the final vertical resolution is not larger than the original. Is is only in the horizontal direction that indeed you need to 'waste' bandwidth. If the AVI file format would include aspect ratio information (as a DVD does), I would't resize at all and follow your other suggestion, i.e., correct aspect ratio at play time (but I am too lazy to manually set the aspect ratio for every file I want to play :-).

[grug2k]

Actually, after reflection, I'd have to change my opinion...
If you have a player that can force a custom aspect ratio, just encode it at 480x272 and then force the player to output at 2.35:1.

(I was working under the assumption that my player would only force to 1:1, 4:3 or 16:9, and therefore encoding at 640x272 was better than encoding at 480x480 (with black bars) or 480x208.)

[nomad32]

OK. Thanks for all the input guys :):) its really helpful.

As a result, I'm going to try a few different methods. Once with no resizing but altering the aspect ratio when palying and once with resizing to 640x272. Which filter should I use for this resize from 480x272 to 640x272? I have been using Precise Bicubic (A=1.00), is this right?

Cheers