Aspect ratios - not sure I'm doing it right

[DRP]

I'm not 100% sure I've got the aspect ratio calculation maths right. Could somebody please check this and tell me if this is right?

Video is PAL DVD rip
The VOBs are 720x576
PowerDVD reports video aspect ratio @ 16:9
There is 12 pixels of letterboxing top & bottom of the image

What I want to do is convert the video to standard PAL SVCD but at the same time crop it to display at an aspect ratio of 22:15 (1.467:1)

Here's what I've done:

576 x 1.778 = 1024 = MPEG true width
576 - (2 x 12) = 552 = MPEG true height
Video aspect ratio = 1024 / 552 = 1.85:1

To find out how much to crop left & right to bring it back to 1.467:1 ratio I did the following:

552 x 1.467 = 810 = MPEG true width after cropping
1024 - 810 = 214 = Amount to crop, left + right

But this value 214 is taken from the true (stretched) MPEG width of 1024, so to find out the crop value from the MPEG width of 720...

720 / 1024 x 214 = 150 = Amount to crop, left + right in TMPGEnc's crop filter.

Is that correct?

[Jukka]

Ok, never done an SVCD conversion but have done many VCD conversions.

Now as far as I am aware (S)VCD do not (officially) support widescreen, so what are we trying to achieve here?

Is it:

(a) a cropped film for a 4:3 TV that isn't letterboxed?
(b) a 16:9 film letterboxed for 4:3 TV?
(c) a 16:9 SVCD that displays correctly on a widescreen TV and 'skinny' on a 4:3?

Depending on what you want there will be different methods:

What you need to do is play around with the 'Video Arrange Method' and try to imagine how it will look on your TV (and this depends on whether you are Wide or Normal) and the clipping settings.

I had problems with this trying to convert a widescreen SVCD (yes I know - non standard) to a normal DVD (see https://www.videohelp.com/forum/viewtopic.php?t=178909&highlight= ) although this is the reverse (nearly) of what you are trying to do.

Most of the time 'Full Screen, Keep Aspect Ratio' does the trick.

Good luck.

Jukka

[DRP]

Ok, never done an SVCD conversion but have done many VCD conversions.

Now as far as I am aware (S)VCD do not (officially) support widescreen, so what are we trying to achieve here?

Is it:

(a) a cropped film for a 4:3 TV that isn't letterboxed?

Yep. That one. The only exception being that I still want it letterboxed, a bit. Full 16:9 ratio is too small for this particular movie on the screen I need to watch it on, so I choose to display it at a 22:15 ratio instead which is a compromise between 16:9 & full screen 4:3 display.

What I want to be able to work out correctly is how much to crop to get this ratio out without going down the 'fat' & 'skinny' people road.

Most of the time 'Full Screen, Keep Aspect Ratio' does the trick.

Yeah I know, but I also want to take into consideration the (considerable, in my TV's case) overscan, so I am using the Centre (custom size) option to do this. I know how to work this bit out for myself however, so no help is required here providing I can work out the clipping numbers properly.

[FOO]

You guys need to find DJrumpy. He's a maniac on the subject

[DRP]

You guys need to find DJrumpy. He's a maniac on the subject

I've PMed DJrumpy for advice. Thanks.

[DJRumpy]

Sorry for the delay. I've been out on vacation.

From the sounds of this, you want to convert a movie from anamorphic widescreen (2.35:1), to 16:9 (1.77:1). The aspects your giving are not used here in NTSC land, but I'm assuming they are the equivelent of ours.

Forgive me if I spew up a few NTSC numbers, but I'll convert them to PAL in the end.

Since your video will need to be cropped at the sides and enlarged, we'll need to work off the height to find the new width. First we convert your videos image area from PAL to NTSC. Since it's a 2.35:1 video, your vertical image area would be about 432 pixels high. We multiply by .8334 to convert that PAL vertical to NTSC.

432 * .8334 = 360 NTSC Vertical

Your video's true height is 360 pixels high and 852 pixels wide (ntsc).

480 * 1.77 Widescreen = 852 true width
852 / 2.35 = 360 rounded

To convert this to 1.85 in NTSC land, we would simply reduce the full width by enough pixels to change the aspect from 2.35 to a 1.85 aspect. To get this, we multiply the video's height (image area only!), times the desired aspect ratio. In this case, 1.85 (or whatever aspect you want..).

360 x 1.85 = 666

Crop the video's width from 852, down to 666. This should give you an image area of 666 x 360. Again, this assumes only the image area, and not any letterboxing.

At this point, you can simply crop your video to these new dimensions, and resize to the standard sizes for your target aspect. To convert this to PAL, you would simply multiply the 360 vertical by 1.2 to convert it back to PAL like so:

360 x 1.2 = 432

You PAL movie should be cropped to 666 x 432, and then resized to a 1.85:1 aspect ratio, with appropriate letterboxing. Note that we haven't cropped anything from the vertical (except for letterboxing of course ).

For PAL SVCD, you would need to resize this into a 4:3 MPEG aspect ratio, which makes this a tiny bit more complicated. Now that we have our 1.85 cropped video, we can resize it to a standard SVCD 4:3 resolution with letterboxing. Since a 4:3 display aspect is about 640 pixels wide, then we know that a 1.85 video should be 344 pixels high (rounded to the nearest 4).

480 x 1.333 display ratio (4:3 TV) = 640 Display Width.
640 / 1.85 Aspect = 344

344 is your new NTSC SVCD 4:3 height (ntsc). Simply convert this to PAL:

344 x 1.2 = 412 (rounded to nearest 4)

To recap:

Crop from 852 down to 664 width ( 664 x 432 )
Resize to 480x412
Add Letterboxing to fill the 412 to the full 576 vertical height

[DRP]

Sorry for the delay. I've been out on vacation.

No worries. Thanks for your time in replying

From the sounds of this, you want to convert a movie from anamorphic widescreen (2.35:1), to 16:9 (1.77:1).

No, not quite, but you do have the right idea. The movie I've got is actually 1.85:1 ratio and I want to convert it down to 22:15 (don't ask) ratio. It really isn't worth me explaining how I came up with wanting 22:15 ratio but if you can just accept that as being what I want then that'd be great

Since your video will need to be cropped at the sides and enlarged, we'll need to work off the height to find the new width. First we convert your videos image area from PAL to NTSC. Since it's a 2.35:1 video, your vertical image area would be about 432 pixels high.

Excuse me, but I don't understand how you got 432 for being the equivalent height in NTSC. My video is 576 high with 12 pixels of letterboxing black border top & bottom. That makes for an image height of 576-24 = 552. Surely the NTSC equivalent of that would be 0.8333 x 552 = 460??

We multiply by .8334 to convert that PAL vertical to NTSC.

Understood and agreed.

432 * .8334 = 360 NTSC Vertical

Sorry, but I just don't understand this.

Your video's true height is 360 pixels high and 852 pixels wide (ntsc).

I would make it 853x460

480 * 1.77 Widescreen = 852 true width
852 / 2.35 = 360 rounded

Yep. Understood. That's what I did too. The only difference is that the original uncropped movie is a 1.85 ratio film, not 2.35 so that's where the error is coming in.

To convert this to 1.85 in NTSC land, we would simply reduce the full width by enough pixels to change the aspect from 2.35 to a 1.85 aspect. To get this, we multiply the video's height (image area only!), times the desired aspect ratio. In this case, 1.85 (or whatever aspect you want..).

360 x 1.85 = 666

Crop the video's width from 852, down to 666. This should give you an image area of 666 x 360. Again, this assumes only the image area, and not any letterboxing.

Yeah that's what I thought and what I've done. One more question though, the crop filter in TMPGEnc, which is what I use to do the cropping, takes pixels away from the MPEG (or VOB) dimensions *without* taking into consideration the display aspect ratio "stretch" doesn't it? So... when using TMPGEnc to do the cropping, it's not quite so simple as just taking (852-666) / 2 from each side is it? That number is taken from the stretched MPEG dimension of 852 rather than from the video aspect ratio of the VOB file which is 720. So the amount to crop in TMPG actually needs to be 720/852 x {(852-666) / 2} from each side isn't it?

Please confirm this is right. This is what I'm not sure about.

At this point, you can simply crop your video to these new dimensions, and resize to the standard sizes for your target aspect. To convert this to PAL, you would simply multiply the 360 vertical by 1.2 to convert it back to PAL like so:

360 x 1.2 = 432

You PAL movie should be cropped to 666 x 432, and then resized to a 1.85:1 aspect ratio, with appropriate letterboxing. Note that we haven't cropped anything from the vertical (except for letterboxing of course ).

For PAL SVCD, you would need to resize this into a 4:3 MPEG aspect ratio, which makes this a tiny bit more complicated. Now that we have our 1.85 cropped video, we can resize it to a standard SVCD 4:3 resolution with letterboxing. Since a 4:3 display aspect is about 640 pixels wide, then we know that a 1.85 video should be 344 pixels high (rounded to the nearest 4).

480 x 1.333 display ratio (4:3 TV) = 640 Display Width.
640 / 1.85 Aspect = 344

344 is your new NTSC SVCD 4:3 height (ntsc). Simply convert this to PAL:

344 x 1.2 = 412 (rounded to nearest 4)

To recap:

Crop from 852 down to 664 width ( 664 x 432 )
Resize to 480x412
Add Letterboxing to fill the 412 to the full 576 vertical height

Thanks. I've got this last bit under control and I'm sure I'm doing this bit correctly (including an allowance for TV overscan as well).

DRP

[DJRumpy]

The confusion has to do with the fact that I was assuming your video is a 2.35:1.

No, not quite, but you do have the right idea. The movie I've got is actually 1.85:1 ratio and I want to convert it down to 22:15 (don't ask) ratio

Not a problem. Your target aspect ratio isn't an issue. Just divide 22 by 15 to get the decimal value of that aspect ratio. 22 / 15 = 1.466

Excuse me, but I don't understand how you got 432 for being the equivalent height in NTSC.

Remeber that those numbers were based on a 2.35 PAL video. The 432 vertical is the Image Area Only of a PAL video using a 2.35:1 aspect ratio. Letterboxing is not part of what I refer to as the video's "image area". Since your source video is 1.85:1, then the image area would be 460 NTSC, or 552 PAL.

432 * .8334 = 360 NTSC Vertical


Sorry, but I just don't understand this.

This is where I converted the 2.35 PAL vertical to NTSC. Since your source is 1.85, and not 2.35, then it would look like so:
552 * .8334 = 460

I would make it 853x460

You should never use odd numbers for your video, and you should always make your numbers divisible by 4, and even better, by 8. I usually round to the nearest 4. It sounds like your on the right path though. Just some confusion as to your sources aspect ratio on my part.

Yeah that's what I thought and what I've done. One more question though, the crop filter in TMPGEnc, which is what I use to do the cropping, takes pixels away from the MPEG (or VOB) dimensions *without* taking into consideration the display aspect ratio "stretch" doesn't it?
...
That number is taken from the stretched MPEG dimension of 852 rather than from the video aspect ratio of the VOB file which is 720. So the amount to crop in TMPG actually needs to be 720/852 x {(852-666) / 2} from each side isn't it?

Hmm..That's a good question. I don't know if the author stretches the video out to it's full height or not and I didn't take into account that you were using TMPGenc to do your cropping (I'm an AVISynth user ). (:edit: I just checked..the tool does not take into account the aspect width of the video, so yes, you should should adjust accordingly :edit:). You should be able to tell by the resolution reported. If it doesn't report 852 or somesuch as the full width, then yes, you would have to take in the MPEG's physical width into account.

I can't help you with overscan, as that depends entirely on your TV, and how much overscan you prefer. Now that I know you have a 1.85:1 film, the numbers would look like this:

1.85:1 image area only (no letterboxing) for PAL = 552 Vertical
Convert to NTSC: ( 552 * .8334) = 460 Vertical Image Area (NTSC)
Find 1.466 Aspect Width: 460 * 1.466 = 672 (rounded to the nearest 4)
720/852 x ((852-672) / 2) = 76

[DRP]

That's cool. thanks for your input. I wanted someone to check me out because I just had this funny feeling that I wasn't doing something right. After I had done all these calculations, the final result looked very slightly stretched vertically (tall thin people syndrome) to me. Not a lot mind you, but just very slightly.

It's very hard to tell though just by looking at it. Your brain tends to compensate very quickly to distortions, such that the longer you look at them, the more normal they appear when they are only very slight.

Without an exact reference of known dimensions on the screen, it can be impossible to tell for sure.

Thanks for your time.