Pixel Aspect Ratios and Photoshop CS

Maestro wrote on 1/4/2004, 9:06 PM

I know the pixel aspect ratio discussion has been driven wholeheartedly into the ground several times, but I found something out playing with Photoshop CS that I wanted to see if anyone has any insight on:

In the past, you had to create Photoshop documents for video in 720x528 (720x534 produced small vertical black bars on the ends). When brought into Vegas, if the PAR was left at 1.0, Vegas correctly resized the document. So far no surprises.

Now the new Photoshop CS has these video templates that are 720x480, but have a built-in PAR of 0.9091. So in theory, if you create something with this template, import it into Vegas, and tell Vegas that it has a PAR of 0.9091, it should match the 720x528 file, right?
Wrong.

If you compare the 720x528 and the Photoshop CS documents side-by-side on the timeline, the CS file has more horizontal stretch than the 528. They should look the same. I have "Maintain Aspect Ratio" checked for both files, and nothing in Pan & Crop set.

I'm just trying to figure out which is correct, because there's a problem somewhere. Anyone with any ideas I'd love to hear them!

-Brent

Comments

rmack350 wrote on 1/4/2004, 10:17 PM

It's kind of tough because I don't have CS yet. I'll make some assumptions though:

A template of 720x480x0.9091 is probably intended to show you the image as if it had square pixels. So if you capture a frame to the clipboard and then paste it into this document it should look like 1.0PAR (even though it's not)

If you paste an image from your still camera into the same document? Well... photoshop should pad it out so that it can be a 0.9091PAR image. This means it would need to add pixels horizontally-or something even more complex.

My guess is that Photoshop doesn't do this. Maybe it's only tweaking the display of the image. Try comparing the image in two docs-one with the template and one without.

I suspect that dropping a 1.0PAR image into this 720x480 image isn't really appropriate.

Rob Mack

farss wrote on 1/4/2004, 10:24 PM

I'm really guessing here but it does seem mighty strange. The PAR is basically irrelevant, it only relates to how something is displayed right.
So as you said CS is just letting you see the frame as it would look on a TV monitor. Maybe when you save the file you need to switch the PAR back to unity.

I usually start out by exporting a frame from the TL and opening that in PS and then delete the layer if not needed, bit of a no brainer that way, image always comes back into Vegas 100%.

rmack350 wrote on 1/4/2004, 10:36 PM

Yeah, we're on the same page here. The template probably takes a 0.9091PAR image and makes it appear to be 1.0 PAR.

If so, and you then drop a 1.0 piece of media onto it, what happens? Most likely it would just appear to be squeezed horizontally.

So, if you are working on a 720x480x0.9091 image then anything you paste into it should first be made 0.9091. And then pasted.

There are reasons to work in 720x480x0.9091. Specifically, if I have a shot of a circuit board and want to tweak it in Photoshop I'd do my frame grab to the clipboard and paste it into a photoshop doc. The reason here is that converting it down to 655x480 destroys some of the fine details. Especially on things like cpu socket pinholes.

I think if you're going to use this template you need to be really clear about what is happening. Better to just leave it alone.

Rob Mack

rmack350 wrote on 1/4/2004, 10:51 PM

A quirk with vegas-It resizes images to fit into the frame. Sometimes this can be misleading, as it is in the case of a 720x534 image.

720x534 is equivalent to 655x486. The 486 dimension is often used in MJPEG systems. It's probably what you get with an SDI interface. The extra 6 pixels represents the retrace time from the bottom right to the top left of frame. DV25 omits these lines, the assumption being that since they don't carry any useful image data they aren't needed. It doesn't mean that the time doesn't exist, just that it isn't recorded in a DV25 file.

So, Vegas' resizing of a 720x534 image to something less than 720 px wide is misleading. It ought to be displayed with the extra 6 pixels bleeding off the top and bottom of frame.

I'd prefer it if Vegas could be set to not resize stills. But if you drag more than one still into the timeline it should ask what you want to do: leave as is, size to fit frame, or size to fill frame. Currently it just sizes to fit frame.

Rob Mack

Maestro wrote on 1/4/2004, 10:56 PM

I know that CS says that the 0.9091 PAR setting is for display purposes. So viewing a 720x480x0.9091 image in Photoshop, you're seeing the image as it would appear on a TV (and in Vegas)--horitzontally scaled. But you can set the PAR for the document to 1.0, and it again looks just like a regular 720x480 image.

I did find out (the one test I didn't bother to run last night) that dropping a 720x480x0.9091 file and a 720x480x1.0 file in Vegas, and telling Vegas that both documents had a PAR of 0.9091 resulted in equal images.

Ugh. Just when I thought I had a handle on all this PAR stuff...

-Brent

farss wrote on 1/5/2004, 5:36 AM

This makes perfect sense to me. There is nothing in the file that defines PAR as far as I know. There is for example nothing in a DV file or on a camera tape that says "Hi I'm 4:3 or 16:9" About the only place this is supposed to happen is in broadcast, they are supposed to set a flag somewhere in the frame that some TVs will read and adjust the AR.

So 720 x 480 pixels is just that , the PAR is solely at the discretion of the display device. That's why you have to define it in the project and tell the preview window to Simulate Device Aspect Ratio.

farss wrote on 1/5/2004, 5:40 AM

You can set Vegas to not resize stills. Just turn off Stretch video to fill frame, do not letterbox.

rmack350 wrote on 1/5/2004, 7:21 AM

Have to jump in the car right now but I'll hunt for it in a bit. Right now I can't find it-although I remember seeing the settings.

Rob

rmack350 wrote on 1/5/2004, 9:16 AM

Okay, I've found this setting as a checkbox when rendering. However, with this set a 1600x1200 image is still resized to 640x 480 (in a an NTSC DV project). I'd rather it be left at 1600x1200 (or better yet, have an option offered when I drop it on the timeline).

I understand the reasoning. The assumption is that any time you drop a still into Vegas you probably want it sized to fit into the frame. But this can be very confusing if you drop in an image that is almost, but not quite, 655x480.

I've never thought that this was a good default behavior with no feedback whatsoever. A better model would be a dialog that pops up offering a range of useful choices. The dialog could have a checkbox to choose to always use that choice. You'd have to uncheck it in preferences.

Rob Mack

pelladon wrote on 1/5/2004, 9:30 AM

On page 173 of the Vegas 4 manual it describes the pixel dimensions for still frames:

655x480 for NTSC
787x586 for PAL

both take into effect the pixel aspect ratio.

So if you use these numbers in any paint program, vegas will fit it without distortion in your DV project.

rmack350 wrote on 1/5/2004, 9:36 AM

Or any multiple of those numbers, if you like. However, taking stills from Vegas that have been sized down to 655x480 will cause a loss of detail horizontally. This routinely happens if you save a still from the preview window instead of copying the frame to the clipboard.

Oh and it's probably a typo. PAL size should be 576 high.

Rob Mack

pelladon wrote on 1/5/2004, 10:04 AM

Yup that was a typo,

should read 787x576 PAL DV. Sorry about that.

These values were recommended for importing images.