Anamorphic film to anamorphic DVD

ChrisStephens · 01-09-2007, 07:58 PM

Why is it that a anamorphic film gets unsqueezed in telecine only to be squeezed again for the DVD ?

It would make more technical sense to make one geometric conversion ?

ChrisStephens · 01-09-2007, 08:34 PM

Geometry conversions are lossy, just FYI.

Allan Jayne · 01-09-2007, 10:33 PM

As far as I know, the scanning of a film in the telecine machine yielding a video signal of the full pixel count desired is a one stage geometric process. Whatever content is on the film is desired is matched up with the scanner mechanism depending on whether or not the film is anamorphic and whether or not the DVD to be pressed is to be anamorphic.

Let's say we are putting through a 1.85:1 anamorphic film. If we adjust the telecine so that the entire film frame is covered by the telecine scanner (more correctly scanning a little past the top and bottom edges), we immediately get a correct 16:9 or 1.78:1 optimized anamorphic video signal. Although all anamorphic DVD's are optimized for 1.78:1, anamorphic films are more often optimized for 1.85:1 so the correct telecine setting is not exactly the film frame.

Let's say we are putting through a 1.85:1 hard matted (prerecorded black bars) spherical (non-anamorphic) film. If we adjust the telecine to scan the entire film frame we get a 1.85: letterboxed video signal. If we adjust the telecine to scan just the picture portion (actually a wee bit more on top and bottom) we get a correct 16:9 optimized anamorphic video signal.

No more scaling loss occurs in any of the above scenarios compared with the others.

Whether the desired part of the film image is optically stretched or zoomed to fit over a fixed scanning window or whether the scanning is optically zoomed or stretched (no change in pixel count) to cover the desired part of a fixed size projected film image depends on the make and model of telecine. After the scanning is done, any extraneous material captured (such as the bottom edge of the frame above or top edge of the frame below) is blacked out in terms of scan lines or pixels.

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ChrisStephens · 01-10-2007, 01:44 AM

So your saying its aquired during telecine in the correct geometery for 16:9 presentation ?

Is this still true when doing 4K files ?

Stephen_J_H · 01-10-2007, 09:17 AM

Alan's comments are more or less correct, except for one thing: there is no such thing as an anamorphic 1.85:1 film. This is typically called "flat" in the industry, as it is acquired without any optical anamorphic process. 2.35:1 is known as "scope" and is the only anamorphic acquisition/projection process in film. When both of these processes are acquired for telecine, it is done in the 16:9 frame as this is native for HD captures (2K, 4K, whatever). This is why the HD discs for old 4:3 films are "pillarboxed" (black bars on the sides) and why 2.35:1 films exhibit smaller letterboxing bars on 16:9 displays.

It is very important not to confuse anamorphic DVD with anamorphic film. Anamorphic DVD is a digital process whereby the pixels in an "enhanced" disc are interpreted as square and so when a DVD player is incorrectly set up for the display type, it yields an image of full resolution with 33% anamorphosis or "squeezing". This is why it is imperative that your DVD player is set up correctly for the display. When a DVD is flagged as anamorphic, it instructs the DVD player to read the disc according to the display. If the output device is 16:9, no conversion is necessary, as the 16:9 display reads the pixels as rectangular and displays in the correct AR; if the display is 4:3, it implements an algorithm that removes every 4th line of horizontal resolution so that the picture does not appear "squeezed." HD discs do not require this conversion, as the native AR is 16:9.

I suppose it would be theoretically possible to reduce optical anamorphosis to compensate for the 16:9 squeeze, but it would be a purely academic exercise.

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Patrick McCart · 01-10-2007, 01:29 PM

According to the Terminator 2: Extreme Edition liner notes, they scanned the 2.35:1 image at 1.78:1 (image is stretched to take advantage of 100% of pixels) on a 1080p transfer. For actual viewing formats (DVD, HD-DVD, BluRay), the image is re-scaled to the correct 2.35:1... but it's not lossy since it started out with more pixels than the final HD-DVD (even though both are 1080p). I'm not sure if this is standard for 2.35:1 films... but I like the idea of not wasting space on the original transfer.

cafink · 01-10-2007, 01:49 PM

Quote:

Originally Posted by Patrick McCart

but it's not lossy since it started out with more pixels than the final HD-DVD (even though both are 1080p).

But that is a lossy conversion, isn't it? The original scan had all 1080 lines of resolution dedicated to the image, while the properly re-scaled version has fewer lines of resolution dedicated to the image (since some of the 1080 lines are dedicted to the black letterboxing bands). This is a loss of resolution and necessarily entails a loss of detail.

Edit: Re-reading your post, you probably meant that scanning the image at 1.78:1 then down-scaling it to 2.35:1 doesn't result in a loss of detail compared to simply scanning it at 2.35:1 to begin with. In that case, you are quite correct. I apologize for the confusion.

MarkHastings · 01-10-2007, 04:40 PM

Anamorphic DVD's are really not squished at all.

Quote:

Originally Posted by Stephen_J_H

Anamorphic DVD is a digital process whereby the pixels in an "enhanced" disc are interpreted as square and so when a DVD player is incorrectly set up for the display type, it yields an image of full resolution with 33% anamorphosis or "squeezing".

This is correct. Anamorphic DVD's contain the 16x9 image in a 720x480 square pixel format.

When the DVD is not flagged for anamorphic (i.e. 4x3), each square pixel is converted into a 0.9:1 ratio which produces a 4x3 image on your TV.

When 'anamorphic' is flagged on the DVD, the square pixels are converted into a 1.2:1 pixel ratio. It's the same number of pixels as with 4x3 material, it's just that the pixels are elongated.

So basically, the image isn't technically 'squished' onto the DVD, meaning no resolution is lost. Changing the size of the pixels does not yield resolution loss.

In basic terms, while it appears that the image was shrunk down to fit into DVD aspect, it's more like folding a piece of paper in half. The paper is now smaller, but no paper loss has occurred.

ChrisWiggles · 01-10-2007, 07:47 PM

The way the original film was filmed (anamorphically or not) does not have bearing on whether the DVD is encoded anamorphically or not. Any possible loss in scaling or stretching the telecine or whatnot PALES in comparison to what is getting put on the DVD. Basically keep the whole idea of whether a film is anamorphic and whether the encode is anamorphic, separate. They are not related in any way.

And it's good to see that you're still around Chris, hope things are well with you!