square pixel and 4:3
by Pau Perez
on
May 29, 2008 at 8:49:07 am
Hi everyone. I suppose most of you work with NTSC, but I think I would have the equivalent doubts with that format as I have with PAL.
This is the point: Isn't PAL supposed to look 4:3 in it's final square pixel look in computer?
I mean, when I capture, the video has a flattened 768x576 look because of the non square pixel I suppose, although it says it is 720x576. So, looking flat it is 4:3. Now, when I compress it for example with the Flash encoder, it gains the non flatenned square pixel look and becomes really 720x576. But 720x576 is not 4:3. So the only way of having 4:3 in the computer is having it flattened?
Re: square pixel and 4:3 by Kevin Camp on May 29, 2008 at 7:59:06 pm
if you view a non-square pixel image on a computer screen in ae, it will look distorted. you can enable the pixel aspect ratio correction in the preview window, but then you get annoying jaggies (it's just in the preview, it doesn't effect you render, but i find them annoying to look at).
if you want to see your footage in the correct aspect ratio and undistorted, you can place your footage into a square pixel preset (for pal, the 768x576 preset). ae should then correctly compensate for the pixel aspect ratio mismatch and make the footage appear like it does on tv.
note, that you may need to scale it back to standard pal dimensions/settings to use for tv. you can do this by placing the square pixel pal comp into a standard pal comp. if the destination is a computer screen (via web or cd/dvd) then you should be fine.
the nice thing about pal is that you don't have to worry about interlacing when doing this... in ntsc if you do this you will normally be affecting the vertical scale and thus the footage need to be deinterlaced first (not that that is hard, but if you don't you'll have problems).
Re: square pixel and 4:3 by Pau Perez on May 30, 2008 at 7:54:50 am
Thanks Kevin,
Well, the point is that's what I did. I made a composition with the standards of the PAL square pixel (768x576). Then I placed the footage, which fits perfectly tot that composition but looks distorted (short and fat). So I go to the project window and click to Interpret footage and choose square pixel, so it will display it in square pixel. And what it does: it stretches the image to 720x576 so it will look correctly non-distorted. But then, it's no longer 4:3 (it's inside a 4:3 composition but it doesn't cover all the composition).
So I don't understand if that's a problem with me or that's supposed to be normal. But then, if a PAL square pixel composition is 768x576, why is my captured video looking 720x576 in square pixel? That's the oposite of what I understood it should be.
Re: square pixel and 4:3 by Darby Edelen on May 30, 2008 at 6:31:26 am
If you want PAL footage to look 4:3 on a square pixel display then you'll need to use some process to stretch the footage the same way a PAL display would (which i believe gives you about 768x576). You can do this in After Effects by creating a square pixel PAL composition (728x576) and placing your non-square pixel footage/comp into it.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
Re: square pixel and 4:3 by Pau Perez on May 30, 2008 at 8:01:03 am
But then, if a PAL square pixel composition is 768x576, why is my captured video looking 720x576 in square pixel (when I interpret the footage in the project window and tell it to be square pixel? That's the oposite of what I understood it should be. The PAL square pixel composition and the PAL square pixel captured video should be the same size, no?? I don't understand it.
Re: square pixel and 4:3 by Dave LaRonde on May 30, 2008 at 5:53:23 pm
[Pau Perez]"I don't understand it."
After Effects does the understanding for you.
If you import standard-definition, 720x576 PAL video, AE knows that it is to have a certain pixel aspect ratio. If it doesn't, you can change it in the Interpret Footage settings.
If you use an SD PAL comp preset, it knows that it should be a certain pixel aspect ratio when rendered. By toggling the pixel aspect ratio compensation switch near the bottom of the comp window, AE will display the comp in either:
• square pixels, which looks a little wide, but nice & smooth, or
• in rectangular pixels which is in 4x3, but looks aliased
rest assured that when AE renders, the comp will be in the proper pixel aspect ratio, and everything will look nice and smooth.
If you put a SQUARE-pixel image into an SD PAL comp, AE will know what to do with the square pixels. It's very good at it.
Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA
Re: square pixel and 4:3 by Pau Perez on Jun 2, 2008 at 3:26:27 pm
Ok, that's clear, but let's get out of AE for one minute. Imagine any TV channel, like NBC. They have videos on the Internet with what they broadcasted on TV. What they broadcasted on TV is 4:3, and so is what they have on the web, when being a square pixel it should look like NTSC (3:2 I think). How do they manage? They cut? They stretch? The image doesn't look stretched at all so I suppose they cut...
Re: square pixel and 4:3 by Darby Edelen on May 30, 2008 at 8:10:54 pm
[Pau Perez]"The PAL square pixel composition and the PAL square pixel captured video should be the same size, no?? I don't understand it."
Is your captured video square pixel? I doubt it. PAL devices use 720 x 576 pixels, but they are stretched horizontally on display so that they appear to be 768 x 576 square pixels (they are not, they are still 720 x 576 rectangular pixels).
The encoded video has a 5:4 frame aspect ratio, but it is displayed on a PAL device with a 4:3 frame aspect ratio. The 720 x 576 pixels on a square pixel device (i.e. computer) when displayed without preview correction will appear with the 5:4 frame aspect ratio.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
Re: square pixel and 4:3 by Pau Perez on May 31, 2008 at 10:15:36 am
No, my captured video is not square pixel. But I have to turn it square pixel in order to see it correctly for the computer, right? I want its natural proportions.
So, the point is, it is impossible to have square pixel PAL videos in 4:3 without squeezing or cutting. The natural look of the PAL video in a computer is 5:4, right? It's just that I had understood that PAL on a square pixel device would be 768x576, but not by squeezing the image. I suppose that I'll have to cut a part of it on top or bottom to fit the 4:3.
I just don't understand why we have a different aspect ratio for non square pixel devices and for pixel devices. Imagine that you want to broadcast something and later have it on the Internet too. Broadcasted it will be 4:3, but on the Internet it will be 5:4 unless you deform or cut the image.
Re: square pixel and 4:3 by Darby Edelen on May 31, 2008 at 3:46:35 pm
[Pau Perez]"I just don't understand why we have a different aspect ratio for non square pixel devices and for pixel devices. "
It's the same encoded aspect ratio, for native PAL devices the resolution is always 720x576.
Only in the cases where you want to see what those 720x576 encoded pixels will look like when displayed would you need to scale the image (yes scale it, stretch it) out to 768x576.
However, this is not the way I would recommend to work, instead I would recommend placing your non-square footage into a matching non-square comp and enabling pixel aspect ratio correction preview (it looks like a small screen with a double sided arrow running horizontally through it, it's along the bottom of the comp preview pane). That way you are maintaining the same correct 720x576 encoded frame throughout the workflow.
To reiterate: unless you know what you're doing, it's best to always maintain the 720x576 resolution of the footage.
Whether the image is being displayed on a square pixel or non-square pixel device will determine whether those 720x576 pixels will appear to be 5:4 (square pixel device) or 4:3 (PAL device). We do not use different aspect ratios whether working on a square or non-square device, we just have to account for the different ways in which the same pixels are treated.
When viewing non-square footage on square pixel devices, most applications provide you a way to 'preview' the way the footage will look on a non-square device and AE is no exception. This will allow you to see approximately what it will look like on a PAL device.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
Re: square pixel and 4:3 by Pau Perez on Jun 1, 2008 at 2:29:07 pm
Thanks for your long explanation. But I have a general doubt: Imagine any TV channel, like NBC. They have videos on the Internet with what they broadcasted on TV. What they broadcasted on TV is 4:3, and so is what they have on the web, when being a square pixel it should look like NTSC (3:2 I think). How do they manage? They cut? They stretch? The image doesn't look stretched at all so I suppose they cut...
Re: square pixel and 4:3 by Darby Edelen on Jun 2, 2008 at 8:28:38 pm
[Pau Perez]"How do they manage? They cut? They stretch? The image doesn't look stretched at all so I suppose they cut..."
The image in it's native 720x480 3:2 resolution would look stretched on a square pixel device. NTSC TVs due to their 0.9 pixel aspect ratio stretch the image down to about 90% of it's 'square pixel' width on display (to about 640x480) so NBC must stretch their 720x480 footage down to 640x480 (approx.) for display on a square pixel device.
Darby Edelen Lead Designer Left Coast Digital Santa Cruz, CA
