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chapster
Aspect ratio
on Aug 6, 2007 at 1:59:35 pm

I have compressed videos that are at 720 x 480 and set my size to 540 x 360. I'm starting to read that this will cause distortion to flv files. I don't seem to see distortion happening? Am I going blind or just not seeing something. How I came up with this size was by drawing a 720 x 480 shape in Photoshop and then scaling that down proportionately to 540 which give me 360 in height. I thought the maintain aspect ratio in Sorenson squeeze would give me the same dimensions seen that I did scale down proportionately. But it doesn't it gives me 540 x 405. I'm a little confused with this, is it bad to not stick to 540 x 405 or can I stray to 540 x 360?


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Rich Rubasch
Re: Aspect ratio
on Aug 6, 2007 at 2:14:55 pm

Since flass video will be viewed on a compute monitor (most likely) it should be viewed with square pixel dimensions. 720 x 480 DV video is non-square pixels. The square pixel equivalent would be 720 x 534.

By choosing "Maintain Aspect" the encoder believes you want to maintain the non-square pixel dimensions and scales down accordingly.

Using 720 x 534 as the square pixel equivalent to your source clip, you can multiply the dimensions by .75 to get in the ballpark to what you want the Flash video to be.

720 x .75 = 540
534 x .75 = 400.5 (round to 400)

If you want your horizontal frame to be 540 pixels, I would set the vertical size to 400 pixels (with "Maintain Aspect" not checked) and you will have a non distorted image in the final Flash clip.

Did this help?

Rich Rubasch
Tilt Media


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chapster
Re: Aspect ratio
on Aug 6, 2007 at 2:20:09 pm

Thanks Rich for this detailed response. The problem is the layout that was given to me has to stay at 540 x 360. The strange thing is I'm not seeing distortion. By scaling it to 540 x 360 isn't this still keeping the scale at a fixed proportion at least in Photoshop it is?

Thanks for your help.


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chapster
Re: Aspect ratio
on Aug 6, 2007 at 3:22:38 pm

Ok so I did a test I compressed one video at 540 x 400 and the other at 540 x 360 both from the same source DVD 720 x 480. To my eye there is no difference between the 2. I see know distortion. If all my files are going to be 720 x 480 to start with can I get away with using 540 x 360 or do I have to stick with 540 x 400

Thanks,


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Craig Seeman
Re: Aspect ratio
on Aug 6, 2007 at 6:06:41 pm

Try 512x384 (ideal), 528x396, 544x408 if you can go over a bit (and maybe crop).

Rich pointed at one way to do the math. Here's another:
720x480 non square is also 640x480 square pixels.
You can think of moving up or down frame size by 32x24.

Because of the way most codecs work when they encode in blocks, keeping things divisible by 16 evenly is a good rule. At the very least the number should be divisible by 4 or 8.

Here's an example:
4x3 (odd number not good)
8x6 (6 is not evenly divisible by 8 or 4)
16x12 (12 is divisible by 4 but not 8 or 16)
32x24 (it's 4x3 ratio and both numbers are divisible by 8, that's good). So this is a pretty good base unit of measure.
64x48 (this is ideal since both numbers are divisible by 16 evenly).

Keeping these ratios in mind, divisible by 16 best, 8 good, 4 ok, you'll find that:
512x384 is divisible by 16.
544x408 is divisible by 8.
528x396 is divisible by 8.

540x400 is close to 544x408 so the distortion might be too small for your eye to notice. You could go with that and crop a few pixels so it doesn't distort instead.

BTW, you might see some sites use
240x180 (divisible by 8) for their smallest frame size and others use 256x192 (divisible by 16). 320x240 is divisible by 16.

For those who want to look at this another way. With 720x480 non square which is 640x480 square, you can get target frame size by:
64 by 48 multiplied by n (best frame sizes)
32 by 24 multiplied by n (best and good frame sizes)
16 by 12 multiplied by n (best, good and "passable" frame sizes)

I hope I haven't muddied the waters too much but I thought the above might explain some ways to figure out frame sizes that don't distort and work well for encoders.


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Rich Rubasch
Re: Aspect ratio
on Aug 7, 2007 at 1:56:02 am

More detail....see if I can get my typos under check...

The true 4 x 3 equivalent to 720 x 480 is in fact 720 x 540. However DV video has 6 less lines than D1 NTSC (720 x 486) so you just can't stretch it up to 540 vertical lines...that's where 534 comes in. But 720 x 534 is not in fact true 4 x 3, so it is not a true downscale to 640 x 480. I use cropping in Cleaner or other encoder to remove lines to create the 4 x 3, then do the scaling.

I also use the multiple of 16 if I can...usually I can get there, otherwise I use the multiple by 4 method.

The key is that if you know the encode is going to look a lot better if you stick to these basic rules, you have apretty good argument for your client why you are suggestng to tweak the frame size a bit. Usually a player frame can be adjusted in a small way to make it work.

Good luck,

Rich Rubasch
Tilt Media


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Craig Seeman
Re: Aspect ratio
on Aug 7, 2007 at 4:28:04 am

[Rich Rubasch] "The true 4 x 3 equivalent to 720 x 480 is in fact 720 x 540."

There's much discussion as to what "true" is when it comes to aspect ratio. I'll point to this well researched article for those who don't mind video engineering math.

http://lipas.uwasa.fi/~f76998/video/conversion/
One might look at this chart and the "notes" section too.
http://lipas.uwasa.fi/~f76998/video/conversion/#conversion_table
also this
http://www.lurkertech.com/lg/pixelaspect.html



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Danny2007
Re: Aspect ratio
on Aug 7, 2007 at 9:25:06 am

ON2, the makers of the VP6 CODEC used in Flash recommend frame sizes divisible by 8.



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chapster
Re: Aspect ratio
on Aug 7, 2007 at 1:59:28 pm

Thanks to all for you're replies. This has become very technical not really the intent. I presumed it was a little easier than this but I guess not.

Danny2007 you mentioned ON2 says the frame sizes has to be dividable by 8 does this mean I could use something like
544 x 360 or am I getting the math completely wrong.
544/8=68
360/8=45

Thanks,


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Craig Seeman
Re: Aspect ratio
on Aug 7, 2007 at 3:51:38 pm

Please do reread my first post

32x24 (it's 4x3 ratio and both numbers are divisible by 8, that's good). So this is a pretty good base unit of measure.

32 by 24 multiplied by n (best and good frame sizes).

Both the height and width would be evenly divisble by 8. The relationship between the two is 4 to 3.


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Rich Rubasch
Re: Aspect ratio
on Aug 7, 2007 at 4:46:28 pm

...AND 3 divided by 4 = .75.

So if 544 will be your horizontal resolution, then you would use 408 for your vertical resolution. Both are divisible by 8 and 544 by 408 is an exact .75 ratio or 4 x 3.

Rich Rubasch
Tilt Media


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chapster
Re: Aspect ratio
on Aug 8, 2007 at 2:21:51 am

Ok I got the math of it now. I saw this site today which is presenting quicktime video which is not your usual size and the video does not look distorted. Maybe someone can shed some light as to why.
The size seems to be 600 x 416. Now I presume it should be 600 x 450 in order to keep the aspect ratio correct.
[URL="http://www.jacobandkole.com/artists/36_jenny/index6.html"]http://www.jacobandkole.com/artists/36_jenny/index6.html[/URL]


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chapster
Re: Aspect ratio
on Aug 8, 2007 at 2:26:58 am

Oops sorry about the link here it is again.
QuickTime Link .




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chapster
Re: Aspect ratio
on Aug 8, 2007 at 4:16:47 am

I just downloaded the file and the size is 600 x 400 which is not keeping to the set aspect ratio. if it was it should be 600 x 450.
So can anyone explain why they are not having any distortion?

Thanks for all the help on here.


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Danny2007
Re: Aspect ratio
on Aug 8, 2007 at 8:58:13 am

As far as I'm concerned all those clips are distorted, squashed, squished or whatever you want to call it. Nobody has faces that fat where I come from!. (I'll ignore the rest of the terrible encoding 'mistakes' for now....)

That said, you can make a clip any size/shape you want, from perfectly square to wildly rectangle without distortion. You do this simply by cropping out that desired shape from the original frame.

DL


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chapster
Re: Aspect ratio
on Aug 8, 2007 at 1:32:12 pm

So to clear me up is DVD 720 x 480 16:9 ratio or 1:5 ?



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Danny2007
Re: Aspect ratio
on Aug 8, 2007 at 2:22:35 pm

Neither - DVD 720x480 is 4:3, using non-square pixels, if were using square pixels it would be 1.5:1

Check out these helpful guides:
http://www.doom9.org/index.html?/aspectratios.htm
http://lipas.uwasa.fi/~f76998/video/conversion/
http://en.wikipedia.org/wiki/Aspect_ratio_(image)

http://www.3ivx.com/support/par.html
http://en.wikipedia.org/wiki/Pixel_aspect_ratio


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