Correct size Resolution for AVI

[Sliztzan]

I want to convert a Pal DVD 720 x576 25fps to a Divx Avi playable only on a computer. I will keep the same frame rate. I just need to know what resolution to re-size it too please. This isn't a high quality DVD source, so perfection is not needed. For Avi I normally use 656 x480 but with this odd resolution I'm not sure what to use and gspot doesn't give the size to use.
Thank you.

[JohnnyMalaria]

Leave as-is and let your DVD player software take care of it.

BTW, 720 x 576 isn't odd (!) - more of the planet uses it than 720 x 480

[Sliztzan]

true, but it's odd for me, I'm in the USA. I will be using a 1:1 pixel for computer playback. But your answer is not the one I'm looking for. Thanks anyway, I'm not making a DVD.

[Ai Haibara]

Don't change the resolution. Your computer shouldn't have any problems playing a 720x576/25fps AVI, and any changing of the resolution will require you to re-encode the video further, which will degrade the quality of the video even more.

[Sliztzan]

I'll be re-encoding the video anyway. There is much noise, audio distortion, and color issues. I'm using TmpgEnc Xpress 4

[Ai Haibara]

Yes, from the original VOB/MPEG to AVI. But scaling the resolution down would be an even bigger hit on quality.

[Sliztzan]

Well after cropping it's coming out to 690 x 568.

[manono]

To paraphrase JohnnyMalaria, more of the planet resizes when encoding for AVI than lets the player do it at playback. However, to know how to resize it we need to know first if the DVD is 4:3 or 16:9.

[Ai Haibara]

Odd resolutions shouldn't really be a problem if you're only going to play the video on a computer, though some software might still complain about it. If you ever plan to convert it to another format, or make a DVD from it, though, then there will be problems.

[Sliztzan]

To paraphrase JohnnyMalaria, more of the planet resizes when encoding for AVI than lets the player do it at playback. However, to know how to resize it we need to know first if the DVD is 4:3 or 16:9.

Right, It's Pal 4:3

[manono]

OK, then after cropping it as you said, you can resize to 640x480.

[Sliztzan]

Odd resolutions shouldn't really be a problem if you're only going to play the video on a computer, though some software might still complain about it. If you ever plan to convert it to another format, or make a DVD from it, though, then there will be problems.

Not a problem. I own the DVD.

[jagabo]

If it's 4:3 DAR and you want square pixel encoding use any 4:3 ratio frame size. 704x528, 640x480, 576x432, 512x384, 448x336, 384x288, 320x240, 256x192...

Keep in mind that some codecs have specific requirements for the frame size. Many require integer multiples of 2, 4, 8, or 16. Most will not allow odd (as in odd vs. even) frame sizes. Multiples of 8 or 16 are best for MPEG family codecs. The above listed frame sizes are all mod 16.

With Divx I would use nothing less than Mod 4. Some decoders will have problems with Mod 2 or odd frame sizes.

And as others have pointed out, for computer playback there's no reason you can't leave it at 720x576 and just set the DAR flag to 4:3. Some set-top Divx/DVD players won't respect the DAR flag, especially older ones.

[Sliztzan]

How to determine if it is 4:3 Dar? I don't understand DAR

[manono]

You already told me it was PAL 4:3. Did you not understand what you were saying?

[Sliztzan]

I mentioned what I wasn't clear about. I'll repeat again that I don't understand DAR. I've never seen that term. I just call them 4:3.

[guns1inger]

DAR - Display Aspect Ratio - The aspect ratio that will be used on playback. 4:3 and 16:9 are examples of Display Aspect Ratios.

Display Aspect Ratio is important because the actual aspect ratio of the image may not match. For example, PAL DV is 720 x 576, which is actually 5:4 as a pure aspect ratio. However this same file can display as either 4:3 or 16:9. It dies this by using non-square pixels (which therefore have a Pixel Aspect Ratio different to 1:1) and setting a DAR flag to tell the player how to display the results. DVD works in the same manner.

Divx is different. Generally Divx/Xvid files are encoded with square (1:1 PAR) pixels. Players understand this without having to do any thinking - just display what is there. Smarter players will also check to see if a DAR has been set, and will use that for playback. This means you can encode a video pixel foe pixel and still have it play back correctly. While every player I have used observes and uses the DAR flag in Divx/Xvid files, many do not. I believe many of the Philips models don't, for example. To be on the safe side, Divx/Xvid files should be resized to have a 1:1 PAR so DAR is not an issue.

Finally, if you don't understand a term, do a little homework. There is a Glossary link in the menu on the left of this page, and google will usually fill in the gaps.

[jagabo]

In my experience, Philips players before the 5960 generation didn't respect the DAR flag. The 5960 and newer players do.

[Sliztzan]

DAR - Display Aspect Ratio - The aspect ratio that will be used on playback. 4:3 and 16:9 are examples of Display Aspect Ratios.

Display Aspect Ratio is important because the actual aspect ratio of the image may not match. For example, PAL DV is 720 x 576, which is actually 5:4 as a pure aspect ratio. However this same file can display as either 4:3 or 16:9. It dies this by using non-square pixels (which therefore have a Pixel Aspect Ratio different to 1:1) and setting a DAR flag to tell the player how to display the results. DVD works in the same manner.

Divx is different. Generally Divx/Xvid files are encoded with square (1:1 PAR) pixels. Players understand this without having to do any thinking - just display what is there. Smarter players will also check to see if a DAR has been set, and will use that for playback. This means you can encode a video pixel foe pixel and still have it play back correctly. While every player I have used observes and uses the DAR flag in Divx/Xvid files, many do not. I believe many of the Philips models don't, for example. To be on the safe side, Divx/Xvid files should be resized to have a 1:1 PAR so DAR is not an issue.

Finally, if you don't understand a term, do a little homework. There is a Glossary link in the menu on the left of this page, and google will usually fill in the gaps.

Thanks for the explanation. I read the glossary, but I still didn't understand what it meant. The term itself has little use.

[jagabo]

Generally, when people talk about aspect ratio in video they mean the display aspect ratio (shape of the displayed picture). But when you are dealing with video files you have several aspect ratios to consider: the Storage Aspect Ratio (SAR, frame dimensions), the Pixel Aspect Ratio (PAR, the shape of individual pixels), and the Display Aspect Ratio (DAR, the final shape of the picture). Mathematically the relationships is:

DAR = SAR * PAR

In a Divx/Xvid file, when you set the DAR flag, you are telling the player "no matter what the frame size, display the picture with this particular aspect ratio." So any frame size can be displayed at any aspect ratio you specify. If you set the PAR flag instead, the player will calculate the proper display aspect ratio using the above equation. If you specify square pixels the display aspect ratio will equal the storage aspect ratio. For example a 640x480 frame will display 4:3, a 480x480 frame will display as a square image, a 640x360 frame will display as 16:9.

[PuzZLeR]

Without even thinking, all you need is the following equation, and a little algebra:

Width / Height = DAR / PAR

But, to answer your question, I too will vote for 640x480 with PAR = 1 (1:1), since the destination is AVI. You can encode to 720x576 but would need PAR = 16:15 to avoid distortion (plug it into the equation and solve the unknown to get this ).

But, even though OpenDML AVI is a modified AVI container to accept anamorphic PAR (non 1:1), the AVI container in general still doesn't get much respect for it.

AVI suffers from what I call SPAS ("Square Pixel Assumption Syndrome"). Many playback devices will simply ignore the PAR flag, assume it's 1:1, and play it distorted if it's otherwise.

All three of my DivX Certified stand-alone players ignore it. They will even ignore it in .mpg files if I rename them to .avi, yet still play the video... strange...

I actually went to Graduate School with the creator of ODML AVI (no bull!). I should ask him what he thinks of SPAS next time I talk with him...

[jagabo]

I don't think the Divx/Xvid DAR/PAR setting is saved in the ODML chunk. It's private data in the MPEG4 stream.

[PuzZLeR]

I don't think the Divx/Xvid DAR/PAR setting is saved in the ODML chunk. It's private data in the MPEG4 stream.

You raise a good point. Yes indeed, the PAR value is specified within the stream itself. But with ODML AVI, unlike vanilla ice cream AVI, you can specify it at the container level as well (much like MKV).

I may also need to restate - although AVI/ODML-AVI suffer from SPAS, it's the ignorant decoder itself that has the disease - it's decoding wrong.

[jagabo]

with ODML AVI, unlike vanilla ice cream AVI, you can specify it at the container level as well

Yes, I know the ODML chunk can include AR information but I've never seen a program do so. I'm not sure if any players respond to it either. The good news is that most of the better players will respond to Divx/Xvid PAR/DAR flags. MPCHC, VLC, KMPlayer...

[PuzZLeR]

For the sake of discussion, I just had a word with Kamiwa, my brilliant colleague, over at the DivX forums - developer of DivXMuxGUI - he says there is a PAR flag definitely in the video stream for anamorphic DivX encodes, but we are currently looking for tools to investigate what actually does occur at the container level, but we are both up in arms about which to use for ODML AVI - gee, I gotta have a talk with the Creator himself when I'm back in town (where I graduated in Montreal).

And yes, a decoder/player that doesn't read the PAR flag, at any level, is NOT a good decoder. Unfortunately, this group still includes a few too many even today (including my current WMP, an older version that I should update soon).

[Sliztzan]

I had someone send me a message telling me that I should resize to 560x460. How would one come up with that size? What are valid frame sizes for 4:3 Pal at 25fps?

[guns1inger]

For Divx any resolution that is 4:3 with a 1:1 PAR (although it supports other PARs) where the X and Y sizes are even numbers (and preferably divisible by 16 for efficiency) are acceptable. Of course the lower you go, the crappier it will look when you blow it up on your TV.

[jagabo]

Mod 16 4:3 frame sizes:
704x400
640x480
576x432
512x384
448x336
384x288
320x240
256x192
192x144
128x96
64x48

[manono]

I had someone send me a message telling me that I should resize to 560x460.

Since you've already told us you want an AVI resized to some 1.33:1 ratio (a 1:1 PAR), and since you've already given us the crop values, the person that told you that is flat-out wrong because he suggested creating a 1.22:1 video. Also, the height is only Mod4.

[Verify]

For a FULL discussion of "Determining Aspect Ratios and Resolutions" see: http://forum.videohelp.com/topic174200.html